This paper describes detail principle operation MC34063 A78S40 switchi
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AN920/D Theory Applications MC34063 mA78S40 Switching Regulator Control Circuits
This paper describes detail principle operation MC34063 A78S40 switching regulator subsystems. Several converter design examples numerous applications circuits with test data included. INTRODUCTION MC34063 A78S40 monolithic switching regulator subsystems intended converters. These devices represent significant advancement ease implementing highly efficient simple switching power supplies. switching regulators becoming more pronounced over that linear regulators because size reductions equipment designs require greater conversion efficiency. Another major advantage switching regulator that increased application flexibility output voltage. output less than, greater than, opposite polarity that input voltage. PRINCIPLE OPERATION order understand difference operation between linear switching regulators must compare block diagrams step-down regulators shown Figure linear regulator consists stable reference, high gain error amplifier, variable resistance series-pass element. error amplifier monitors output voltage level, compares reference generates linear control signal that varies between extremes, saturation cutoff. This signal used vary resistance series-pass element corrective fashion order maintain constant output voltage under varying input voltage output load conditions. switching regulator consists stable reference high gain error amplifier identical that linear regulator. This system differs that free running oscillator gated latch have been added. error amplifier again monitors output voltage, compares reference level generates control signal. output voltage below nominal, control signal will high state turn gate, thus allowing oscillator clock pulses drive series-pass element alternately from cutoff saturation. This will continue until output voltage pumped slightly above nominal value. this time, control signal will turn gate, terminating further switching series-pass element. output voltage will eventually decrease below nominal presence external load, will initiate switching process again. increase conversion efficiency primarily operation series-pass element only saturated cutoff state. voltage drop across element, when saturated, small dissipation. When cutoff, current through element likewise power dissipation also small. There other variations switching control. most common fixed frequency pulse width modulator fixed on-time variable off-time types, where on-off switching uninterrupted regulation achieved duty cycle control. Generally speaking, example given Figure does apply MC34063 A78S40.
Linear Control Signal Error Linear Regulator Error Vout Voltage Vout
Gated Latch
Voltage
Digital Control Signal
Switching Regulator
Figure Step-Down Regulators
Semiconductor Components Industries, LLC, 2006
May, 2006 Rev.
Publication Order Number: AN920/D
AN920/D
GENERAL DESCRIPTION MC34063 series monolithic control circuit containing active functions required converters. This device contains internal temperature compensated reference, comparator, controlled duty cycle oscillator with active peak current limit circuit, driver, high current output switch. This series specifically designed incorporated step-up, step-down voltage-inverting converter applications. These functions contained 8-pin dual in-line package shown Figure A78S40 identical MC34063 with addition on-board power catch diode, uncommitted operational amplifier. This device 16-pin dual in-line package which allows reference noninverting input comparator pinned out. These additional features greatly enhance flexibility this part allow implementation more sophisticated applications. These include series-pass regulation main output derived second output voltage, tracking regulator configuration even second switching regulator. FUNCTIONAL DESCRIPTION oscillator composed current source sink which charges discharges external timing capacitor between upper lower preset threshold. typical charge discharge currents respectively, yielding about ratio. Thus ramp-up period times longer than that ramp-down shown Figure upper threshold equal internal reference voltage 1.25 lower approximately equal 0.75 oscillator runs continuously rate controlled selected value During ramp-up portion cycle, Logic present input gate. output voltage switching regulator below nominal, Logic will also present input. This condition will latch cause output Logic "1", enabling driver output switch conduct. When oscillator reaches upper threshold, will start discharge Logic will present input gate. This logic level also connected inverter whose output presents Logic reset input latch. This condition will cause low, disabling driver output switch. logic truth table these functional blocks shown Figure output comparator latch only during ramp-up initiate partial full on-cycle output switch conduction. Once comparator latch, cannot reset latch will remain until begins ramping down. Thus comparator initiate output switch conduction, cannot terminate latch always reset when begins ramping down. comparator's output will Logic when output voltage switching regulator above nominal. Under these conditions, comparator's output inhibit portion output switch on-cycle, complete cycle, complete cycle plus portion cycle, multiple cycles, multiple cycles plus portion cycle.
Drive Collector Switch Collector
Latch
Sense Comparator Inverting Input Comp
Switch Emitter
1.25 Reference Regulator
Timing Capacitor
Ground MC34063
Noninverting Input
Timing Capacitor
Sense
Driver Collector Diode Anode
Comp
Latch
Output
Inverting Input
Noninverting Input
mA78S40
Figure Functional Block Diagrams
Upper Threshold 1.25 Typical
Lower Threshold 0.75 Typical Charge Discharge
Figure Voltage Waveform
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Diode Cathode
Output
Switch Emitter
1.25
Switch Collector
Inverting Input
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Active Condition Timing Capacitor, Begins Ramp-Up Begins Ramp-Down Ramping Down Gate Inputs Latch Inputs Output Switch
Comments State Output Switch Switching regulator's output nominal (`B' change since before Ramp- Down. change even though switching regulator's output nominal. Output switch cannot initiated during Ramp-Down. change since output switch conduction terminated when went Switching regulator's output went nominal during Ramp-Up (`B' Partial cycle output switch.
Ramping Down Ramping
Ramping
Switching regulator's output went nominal (`B' during Ramp-Up. change since cannot reset latch. Complete on-cycle since before started Ramp-Up. Output switch conduction always terminated whenever Ramping Down.
Begins Ramp-Up Begins Ramp-Down
Figure Logic Truth Table Functional Blocks
Current limiting accomplished monitoring voltage drop across external sense resistor placed series with output switch. voltage drop developed across this resistor monitored Sense pin. When this voltage becomes greater than current limit circuitry provides additional current path charge timing capacitor This causes rapidly reach upper oscillator threshold, thereby shortening time output switch conduction thus reducing amount energy stored inductor. This observed increase slope charging portion voltage
Comparator Output
waveform shown Figure Operation switching regulator overload shorted condition will cause very short finite time output conduction followed either normal extended off-time internal provided oscillator ramp-down time extended interval result charging beyond upper oscillator threshold overdriving current limit sense input. This caused operating switching regulator with severely overloaded shorted output having input voltage grossly above nominal design value.
Timing Capacitor,
Output Switch
Nominal Output Voltage Level Output Voltage Startup Quiescent Operation
Figure Typical Operating Waveforms
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25°C Ichg, Charging Current (mA) Step-Down Vout 0.03 Ichg Idischg +Vin VCLS, Current-Limit Sense Voltage MC34063 A78S40
+Vin MC34063 A78S40
+Vout
+Vout
Figure Timing Capacitor Charge Current versus Current-Limit Sense Voltage
Step-Up Vout +Vin A78S40
Under extreme conditions, voltage across will approach cause relatively long off-time. This action considered feature since will reduce power dissipation output switch considerably. This feature disabled A78S40 only, connecting small signal transistor clamp. emitter connected base reference output, collector ground. This will limit maximum charge voltage across less than With current limiting, saturation storage inductor prevented well achieving soft startup. practice current limit circuit will somewhat modify charging slope peak amplitude each time output switch required conduct. This because threshold voltage current limit sense circuit exhibits "soft" voltage turn-on characteristic turn-off time delay that causes some overshoot. threshold defined where charge discharge currents equal value with shown Figure current limit sense circuit disabled connecting Sense VCC. system design flexibility, driver collector, output switch collector emitter pinned separately. This allows designer option driving output switch transistor into saturation with selected forced gain driving near saturation when connected Darlington. output switch typical current gain designed switch maximum collector-to-emitter, with peak collector current. A78S40 additional features on-chip uncommitted operational amplifier catch diode. high gain single supply type with input common-mode voltage range that includes ground. output capable sourcing sinking separate provided order reduce integrated circuit standby current useful power applications operational amplifier incorporated into main switching system. catch diode constructed from lateral transistor capable blocking will conduct current There however, "catch" when using
-Vout
Voltage-Inverting |Vout|
Figure Basic Switching Regulator Configurations
Because integrated circuit substrate common with internal external circuitry ground, cathode diode cannot operated much below ground forward biasing substrate will result. This totally eliminates diode from being used basic voltage inverting configuration Figure since substrate, common ground. diode considered only power converter applications where total system component count must held minimum. substrate current will about percent catch diode current step-up configuration about percent step-down voltage-inverting which common negative output. System efficiency will suffer when using this diode package dissipation limits must observed. STEP-DOWN SWITCHING REGULATOR OPERATION Shown Figure basic step-down switching regulator. Transistor interrupts input voltage provides variable duty cycle squarewave simple filter. filter averages squarewaves producing output voltage that level less than input controlling percent conduction time that total switching cycle time. Thus,
Vout Vout toff
MC34063/A78S40 achieves regulation varying on-time total switching cycle time. explanation step-down converter operation follows: Assume that transistor off, inductor current zero, output voltage Vout nominal
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value. output voltage across capacitor will eventually decay below nominal because only component supply current into external load This voltage deficiency monitored switching control circuit causes drive into saturation. inductor current will start flow from through and, parallel with rise rate V/L. voltage across inductor equal Vsat Vout peak current instant
Vsat Vout
off-time, toff, time that diode conduction determined time required inductor current return zero. off-time related ramp-down time cycle time network equal ton(max) toff minimum operating frequency
fmin ton(max) toff
on-time, turned off. magnetic field inductor starts collapse, generates reverse voltage that forward biases peak current will decay rate energy supplied voltage across inductor during this period equal Vout current instant
IL(pk) Vout
minimum value inductance calculated known quantities voltage across inductor required peak current selected switch conduction time.
Vsat Vout Lmin Ipk(switch)
Assume that during quiescent operation average output voltage constant that system operating discontinuous mode. Then IL(peak) attained during must decay zero during toff ratio toff determined.
Vsat Vout toff Vout Vsat Vout toff
This minimum value inductance calculated assuming onset continuous conduction operation with fixed input voltage, maximum output current, minimum charge-current oscillator. charge cycle delivered output filter capacitor must zero, output voltage remain constant. ripple voltage calculated from known values on-time, off-time, peak inductor current, output capacitor value.
Vripple(p-p)
where
toff
Note that volt-time product must equal that toff inductance value concern when determining their ratio. output voltage remain constant, average current into inductor must equal output current complete cycle. peak inductor current with respect output current
IL(pk) IL(pk) toff (Iout ton) (Iout toff) IL(pk)(ton toff) Iout (ton toff) NIL(pk) Iout
toff toff
Substituting yields:
(ton (toff toff
(ton toff)
peak inductor current also equal peak switch current Ipk(switch) since series. on-time, ton, maximum possible switch conduction time. equal time required ramp from lower upper threshold. required value determined using minimum oscillator charging current typical value oscillator voltage swing both taken from data sheet electrical characteristics table.
Ichg(min)
graphical derivation peak-to-peak ripple voltage obtained from capacitor current voltage waveforms Figure calculations shown account ripple voltage contributed ripple current into ideal capacitor. practice, calculated value will need increased internal equivalent series resistance capacitor. additional ripple voltage will equal Ipk(ESR). Increasing value filter capacitor will reduce output ripple voltage. However, point diminishing return will reached because comparator requires finite voltage difference across inputs control latch. This voltage difference completely change latch states about minimum achievable ripple output will feedback divider ratio multiplied
Vripple(p-p)min (1.5 Vref
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Voltage Across Switch Vsat Vsat IC(AVG) Diode Current ID(AVG) Inductor Current Iout Ipk/2 IC(AVG) ID(AVG) Capacitor Current
Voltage Across Diode
Switch Current
-Ipk/2
Capacitor Ripple Voltage
Vout Vout Vout
Figure Step-Down Switching Regulator Waveforms
This problem becomes more apparent step-up converter with high output voltage. Figures show different ripple reduction techniques. first uses A78S40 operational amplifier drive comparator feedback loop. second technique uses Zener diode level shift output down reference voltage.
Step-Down Switching Regulator Design Example
schematic basic step-down regulator shown Figure A78S40 chosen order implement minimum component system, however, MC34063 with external catch diode also used. frequency chosen compromise between switching losses inductor size. There will further discussion this other design considerations later. Given following: Vout Iout fmin Vin(min) 21.6 Vripple(p-p) 0.5% Vout mVp-p
Ipk/2 ton/2 Vripple(p-p)
+Ipk/2
Determine ratio switch conduction versus diode conduction toff time.
Vout Vin(min) Vsat Vout toff 21.6
toff/2
0.37
cycle time network equal ton(max) toff.
ton(max) toff fmin
cycle
Next calculate toff from ratio ton/toff toff using substitution some algebraic gymnastics, equation written toff terms ton/toff toff.
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equation
toff ton(max) toff
toff
0.37
Note that ratio ton/(ton toff) does exceed maximum 0.857. This maximum defined ratio charge-to-discharge current timing capacitor (refer Figure maximum on-time, ton(max), selecting value
(5.4
14.6
Since ton(max) toff
ton(max) 14.6
standard capacitor.
Comp 1N5819
Test Line Regulation Load Regulation Output Ripple Short Circuit Current Efficiency, Internal Diode Efficiency, External Diode*
Iout Iout 21.6 Iout Iout Iout
Figure Step-Down Design Example
1.25
Vout V/50
Conditions
Results 0.16% 0.28% mVp-p 45.3% 72.6%
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peak switch current
Ipk(switch) Iout
nominal output voltage programmed resistor divider. output voltage
Vout 1.25
With knowledge peak switch current maximum time, minimum value inductance calculated.
Vin(min) Vsat Vout Lmin ton(max) Ipk(switch) 21.6
divider current without affecting system performance. selecting minimum current divider equal
1.25
Rearranging above equation that solved yields:
Vout 1.25
value current limit resistor, Rsc, determined using current level Ipk(switch) when
pk(switch) Vsat Vout ton(max) Lmin
standard tolerance resistor chosen will also standard value.
1.25
0.33 pk(switch) 0.33
Using above derivation, design optimized meet assumed conditions. Vin(min), operation onset continuous mode output current capability will greater than Vin(nom) i.e., current limit will activate slightly above rated Iout STEP-UP SWITCHING REGULATOR OPERATION basic step-up switching regulator shown Figure waveform Figure Energy stored inductor during time that transistor "on" state. Upon turn-off, energy transferred series with output filter capacitor load. This configuration allows output voltage value greater than that input following relationship:
Vout Vout toff toff
2.86
This value have adjusted downward compensate conversion losses increase Ipk(switch) current varies upward. exceed maximum Ipk(switch) limit when using internal switch transistor. minimum value ideal output filter capacitor obtained.
Ipk(switch) (ton toff) Vripple(p-p)
Ideally this would satisfy design goal, however, even solid tantalum capacitor this value will have typical (equivalent series resistance) which will contribute ripple. ripple components phase, assumed conservative design. satisfying example shown, tantalum with selected. ripple voltage should kept value since will directly affect system line load regulation.
explanation step-up converter's operation follows. Initially, assume that transistor off, inductor current zero, output voltage nominal value. this time, load current being supplied only will eventually fall below nominal. This deficiency will sensed control circuit will initiate on-cycle, driving into saturation. Current will start flow from through inductor rise rate V/L. voltage across inductor equal Vsat peak current
Vsat
When on-time completed, will turn magnetic field inductor will start collapse generating reverse voltage that forward biases supplying energy inductor current will decay rate voltage across equal Vout Vin. current instant
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Voltage Across Switch Vout Vsat Vout Vsat
Diode Voltage
Switch Current
Diode Current Iout Inductor Current IL(AVG) Capacitor Current
1/2(Ipk Iout) -Iout
Capacitor Ripple Voltage
Vout Vout Vout
Figure Step-Up Switching Regulator Waveforms IL(pk) Vout
Assuming that system operating discontinuous mode, current through inductor will reach zero after toff period completed. Then IL(pk) attained during must decay zero during toff ratio toff written.
Vsat toff Vsat toff
Note again, that volt-time product must equal that toff inductance value does affect this relationship. inductor current charges output filter capacitor through diode only during toff. output voltage remain constant, charge cycle delivered output filter capacitor must zero,
Ichg toff Idischg
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toff Vripple(p-p)
Iout
Figure shows step-up switching regulator waveforms. observing capacitor current making some substitutions above statement, formula peak inductor current obtained.
IL(pk) toff Iout (ton toff) IL(pk) Iout toff
peak inductor current also equal peak switch current, since series. With knowledge voltage across inductor during required peak current selected switch conduction time, minimum inductance value determined.
Lmin Vsat Ipk(switch) on(max)
ripple voltage calculated from known values on-time, off-time, peak inductor current, output current output capacitor value. Referring
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capacitor current waveforms Figure defined capacitor charging interval. Solving known terms yields:
Iout toff Iout toff Vout Vin(min) Vin(min) Vsat toff 6.75 6.75 3.42
cycle time network equal ton(max) toff.
ton(max) toff fmin
current during written:
Iout
ripple voltage
Vripple(p-p)
Iout
cycle
Next calculate toff from ratio ton/toff toff
toff 3.42
Iout (Ipk Iout)
15.5
Substituting yields:
(Ipk Iout) (Ipk Iout) toff (Ipk Iout)2 toff
Note that ratio ton/(ton toff) does exceed maximum 0.857. maximum on-time, ton(max), selecting value
(15.5
simplified formula that will give error less than voltage step-up greater than with ideal capacitor shown:
Vripple(p-p) Iout
peak switch current
Ipk(switch) Iout toff (3.42
This neglects small portion total area. area neglected equal
(toff Step-Up Switching Regulator Design Example
basic step-up regulator schematic shown Figure A78S40 again chosen order implement minimum component system. following conditions given: Vout Iout fmin Vin(min) 6.75 Vripple(p-p) 0.5% Vout mVp-p Determine ratio switch conduction versus diode conduction toff time.
minimum value inductance calculated since maximum on-time peak switch current known.
Lmin Vin(min) Vsat Ipk(switch) 6.75 15.5
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value current limit resistor, Rsc, determined using current level Ipk(switch) when
Vsat pk(switch) ton(max) Lmin 15.5 Iout ripple(p-p) 15.5
0.33 pk(switch) 0.33
ripple contribution gain comparator:
Vripple(p-p) Vref 1.25 33.6
0.55
Note that current limiting this basic step-up configuration will only protect switch transistor from overcurrent inductor saturation. output severely overloaded shorted, destroyed since they form direct path from Vout. Protection achieved current limiting replacing inductor with turns ratio transformer. approximate value ideal output filter capacitor
tantalum capacitor with 0.10 again chosen. ripple voltage capacitance value 28.7 44.2 ESR. This yields total ripple voltage
Eripple(p-p) Vref
33.6 28.7 44.2
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Comp
Test Line Regulation Load Regulation Output Ripple Efficiency, Internal Diode Efficiency, External Diode*
Iout Iout 6.75 Iout Iout Iout
Figure Step-Up Design Example
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1.25
1N5819
Vout V/50
Conditions
Results 0.21% 0.09% mVp-p 62.2% 74.2%
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Comp
Figure mA78S40 Ripple Reduction Technique
Comp 1.25 Reference Regulator
Vout
Vout 1.25
Figure MC34063 Ripple Reduction Technique
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1.25
Vout
nominal output voltage programmed divider.
Vout 1.25
standard tolerance, resistor selected that divider current about
1.25
2500
Then
Vout 1.25 1.25
2200
this design example, output switch transistor driven into saturation with forced gain input voltage required base drive
Ipk(switch)
current required drive internal base-emitter resistor
22.1
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I170 VBE(switch)
driver collector current equal 22.1 26.2 Allow driver saturation drop across (0.5 Ipk). Then driver collector resistor equal
Rdriver Vsat(driver) VRSC I170 (22.1 4.1)
Energy stored inductor during conduction time Upon turn-off, energy transferred output filter capacitor load. Notice that this configuration output voltage derived only from inductor. This allows magnitude output value. less than, equal greater than that input following relationship:
Vout toff
voltage-inverting converter operates almost identically that step-up previously discussed. voltage across inductor during Vsat during toff voltage equal negative magnitude Vout Remember that volt-time product must equal that toff, ratio toff determined.
(Vin Vsat) (|Vout| toff |Vout| toff Vsat
VOLTAGE-INVERTING SWITCHING REGULATOR OPERATION basic voltage-inverting switching regulator shown Figure operating waveforms Figure
derivations formulas Ipk(switch), Lmin, same that step-up converter.
Voltage Across Switch
Vsat
(-Vout (Vin Vsat) Vout Diode Voltage Switch Current IC(AVG) Diode Current Iout Inductor Current
Capacitor Current Iout
1/2(Ipk Iout) -Iout
Capacitor Ripple Voltage
-Vout Vout -Vout
Figure Voltage-Inverting Switching Regulator Waveforms
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toff Vripple(p-p)
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Voltage-Inverting Switching Regulator Design Example
circuit diagram basic voltage-inverting regulator shown Figure A78S40 selected this design since reference both comparator inputs pinned out. following operating conditions given: Vout Iout fmin Vin(min) 13.5 Vripple(p-p) 0.4% Vout mVp-p Determine ratio switch conduction versus diode conductions toff time.
|Vout| Vsat toff 13.5 1.24
cycle time network equal ton(max) toff.
ton(max) toff fmin
Calculate toff from ratio ton/toff toff
toff 1.24
11.1
Note again that ratio ton/(ton toff) does exceed maximum 0.857.
0.12 Comp
U51A
Heatsink, IERC PSC2-3CB Test Line Regulation Load Regulation Output Ripple Short Circuit Current Efficiency Conditions Iout Iout 13.5 Iout Iout Results 0.01% 0.09% mVp-p 80.6%
Figure Voltage-Inverting Design Example http://onsemi.com
1.25
1N5822 66.5 Vout V/0.5
AN920/D
value must selected order ton(max).
(11.1
capacitors with 0.020 each chosen. ripple voltage capacitance value 22.4 ESR. This yields total ripple voltage
|Vout| Eripple(p-p) Vref 46.3
peak switch current
Ipk(switch) Iout toff (500 2.24 (1.24
22.4
minimum required inductance value
Lmin Vin(min) Vsat Ipk(switch)
nominal output voltage programmed divider. Note that with negative output voltage, inverting input comparator referenced ground. Therefore, voltage junction noninverting input must also ground potential when Vout regulation. magnitude Vout
|Vout| 1.25
13.5 11.1 2.24 66.5
divider current about desired this example.
1.25
current-limit resistor value selected determining level Ipk(switch) 16.5
pk(switch) Vsat Lmin 16.5 11.1 66.5
3,125
Then
|Vout| 1.25
2.62 0.33 pk(switch) 0.33 2.62 0.13 0.12
1.25
Output switch transistor driven into soft saturation with forced gain input voltage 13.5 order enhance turn-off switching time. required base drive
Ipk(switch)
approximate value ideal output filter capacitor
Iout Vripple(p-p) 11.1
2.24
value base-emitter turn-off resistor determined
pk(switch)
92.5
ripple contribution gain comparator
|Vout| Vripple(p-p) Vref 1.25
(35) 2.24
156.3
additional base current required
IRBE (Q2)
given level ripple, output filter capacitor becomes dominant factor choosing value capacitance. Therefore
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Then (Q2) equal Allow driver saturation drop across (0.12 2.24 Ipk). Then base driver resistor equal
Vin(min) Vsat(IC) VRSC VBE(Q2) I160
+Vin
+Vout
13.5 165.2
Step-Down Vout
+Vin
+Vout
STEP UP/DOWN SWITCHING REGULATOR OPERATION When designing board level sometimes becomes necessary generate constant output voltage that less than that battery. step-down circuit shown Figure will perform this function efficiently. However, battery discharges, terminal voltage will eventually fall below desired output, order utilize remaining battery energy step-up circuit shown Figure will required.
General Applications
Step-Up Vout
Figure Basic Switching Regulator Configurations
+Vin
+Vout
combining circuits unique step-up/down configuration created (Figure which still employs simple inductor voltage transformation. Energy stored inductor during time that transistors "on" state. Upon turn-off, energy transferred output filter capacitor load forward biasing diodes Note that during this circuit identical basic step-up, during toff output voltage derived only from inductor with respect ground instead Vin. This allows output voltage value, thus less than, equal greater than that input. Current limit protection cannot employed basic step-up circuit. output severely overloaded shorted, destroyed since they form direct path from Vout. step-up/down configuration allows control circuit implement current limiting because series with Vout, step-down circuit.
Step-Up/Down Switching Regulator Design Example
Step-Up/Down Vout
Figure Combined Configuration
Determine ratio switch conduction versus diode conduction toff time.
Vout VFD1 VFD2 Vin(min) VsatQ1 VsatQ2 toff
cycle time network equal ton(max) toff.
ton(max) toff fmin cycle
complete step-up/down switching regulator design example shown Figure This regulator designed operate from standard battery pack with following conditions: 14.5 Vout fmin Iout Vripple(p-p) Vout mVp-p following design procedure provided that user select proper component values specific converter application.
Next calculate toff from ratio ton/toff ton(max) toff
toff ton(max) toff
toff
13.1
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maximum on-time selecting value
ton(max) (13.1 Lmin Vin(min) VsatQ1 VsatQ2 Ipk(switch)
13.1
standard capacitor. peak switch current
Ipk(switch) Iout toff (120 (1.9
inductor selected Lmin. value current limit resistor, Rsc, determined using current limit level Ipk(switch) when 14.5
pk(switch) VsatQ1 VsatQ2 ton(max) Lmin
minimum value inductance calculated since maximum on-time peak switch current known.
MPSU51A 0.22 14.5
14.5 13.1 1.41
1N5818 Vout V/120
1N5818
Comp 1.25 Reference Regulator MC34063
Test Line Regulation Load Regulation Output Ripple Short Circuit Current Efficiency
Conditions 14.5 Iout 12.6 Iout 12.6 Iout 12.6 14.5 Iout
Results 0.11% 0.015% mVp-p 1.54
Figure Step-Up/Down Switching Regulator Design Example
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0.33 pk(switch)
value base-emitter turn-off resistor determined
pk(switch)
0.33 1.41 0.23
standard 0.22 resistor. minimum value ideal output filter capacitor
Iout Vripple(p-p) 13.1
standard resistor selected. additional base current required
IRBE BEQ1
(20)
15.7
Ideally this would satisfy design goal, however, even solid tantalum capacitor this value will have typical (equivalent series resistance) which will contribute additional ripple. Also there ripple component gain comparator equal
Vripple(p-p) Vref 1.25
base drive resistor equal
Vin(min) Vsat(driver) VRSC VBEQ1 IRBE
0.15
ripple components phase, assumed conservative design. From above becomes apparent that dominant factor selection output filter capacitor. with 0.12 selected satisfy this design example following:
Vripple(p-p)
Iout
standard resistor used. circuit performance data shows excellent line load regulation. There some loss conversion efficiency over basic step-up step-down circuits added switch transistor diode "on" losses. However, this unique converter demonstrates that with simple inductor, step-up/down converter with current limiting constructed. DESIGN CONSIDERATIONS previously stated, design equations Lmin were based upon assumption that switching regulator operating onset continuous conduction with fixed input voltage, maximum output load current, minimum charge-current oscillator. Typically oscillator charge-current will greater than specific minimum microamps, thus will somewhat shorter actual operating frequency will greater than predicted. Also note that voltage drop developed across current-limit resistor accounted ton/toff Lmin design formulas. This voltage drop must considered when designing high current converters that operate with input voltage less than When checking initial switcher operation with oscilloscope, there will some concern circuit instability apparent random switching output. oscilloscope will difficult synchronize. This problem. normal operating characteristic this type switching regulator caused asynchronous operation comparator that oscillator. oscilloscope synchronized varying input voltage load current slightly from design nominals.
Vout VRef
Ipk(switch)
nominal output voltage programmed resistor divider.
Vout VRef 1.25
chosen then would both being standard resistor values. Transistor driven into saturation with forced gain approximately input voltage required base drive
Ipk(switch)
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High frequency circuit layout techniques imperative with switching regulators. minimize EMI, high current loops should kept short possible using heavy copper runs. current signal high current switch output grounds should return separate paths back input filter capacitor. output voltage divider should located close possible eliminate noise pick-up into feedback loop. circuit diagrams were purposely drawn manner depict this. circuits used molypermalloy power toroid cores magnetics where only inductance value given. number turns, wire core size information given since attempt made optimize their design. Inductor transformer design information obtained from magnetic core assembly companies listed switching regulator component source table. some circuit designs, mainly step-up voltage-inverting, ratio ton/(ton toff) greater than 0.857 required. This obtained addition ratio extender circuit shown Figure This circuit uses germanium components temperature sensitive. negative temperature coefficient timing capacitor will help reduce this sensitivity. Figure shows output switch time versus with without ratio extender circuit. Notice that without circuit, ratio ton/(ton toff) limited 0.857 only values greater than With circuit, ratio variable depending upon value chosen since toff nearly constant. Current limiting must used step-up voltage-inverting designs using ratio extender circuit. This will allow inductor time reset between cycles overcurrent during initial power switcher. When output filter capacitor reaches nominal voltage, voltage feedback loop will control regulation.
toff Scope 1N270 1.25 Reference Regulator 2N524 toff Ratio Extender Circuit
Comp
Figure Output Switch On-Off Time Test Circuit
ton-toff, Output Switch On-Off Time 1000 Comparator Noninverting Input Vref Comparator Inverting Input Ipk(sense) 25°C toff Without Ratio Extender Circuit With Ratio Extender Circuit Oscillator Timing Capacitor (nF) toff
Figure Output Switch On-Off Time versus Oscillator Timing Capacitor
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APPLICATIONS SECTION Listed below index converter circuits shown this application note. They categorized into three
INDEX CONVERTER CIRCUITS
Main Output Configuration Step-Down A78S40 MC34063 MC34063 A78S40 A78S40 A78S40 Step-Up A78S40 MC34063 MC34063 A78S40 A78S40 A78S40 A78S40 A78S40 Power with Minimum Components Medium Power High Voltage, Power High Voltage, Medium Power Photoflash Linear Pass from Main Output Buffered Linear Pass from Main Output PROM Programmer Buffered Switch Buffered Linear Pass from Main Output Dual Switcher, Step-Up Step-Down with Buffered Switch 28/50 36/225 190/5.0 334/45 9.0/100 Circuit 15/1000 28/250 6/30 12/500 5.0/250 -12/50 Power with Minimum Components Medium Power Buffered Switch Second Output Linear Pass from Main Output Buffered Switch Buffered Linear Pass from Main Output Negative Input Negative Output 5/50 12/750 5.0/5000 24/500 15/3000 -12/500 12/300 15/50 12/1000 Input Output V/mA Output V/mA Output V/mA Figure
major groups based upon main output configuration. Each these circuits constructed tested, performance table included.
Step-Up/Down MC34063 Medium Power Step-Up/Down 14.5 10/120
Voltage-Inverting MC34063 A78S40 A78S40 A78S40 A78S40 Power Medium Power with Buffered Switch High Voltage, High Power with Buffered Switch Watt Off-Line Flyback Switcher Tracking Regulator with Buffered Switch Buffered Linear Pass from Input -12/100 -15/500 -120/850 5.0/4000 -12/500 12/700 12/500 -12/700
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AN920/D
Comp 1.25 Reference Regulator
1N5819
1.74
Vout V/750
Test Line Regulation Load Regulation Output Ripple Short Circuit Current Efficiency
Conditions Iout Iout Iout Iout
Results 0.063% 0.17% mVp-p 89.5%
maximum power transfer watts possible from 8-pin dual-in-line package with Vout
Figure Step-Down
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AN920/D
D45VH4 MBR2540 0.036 2200
1.4T 23.1
22,000
1N5819
Comp 1.25 Reference Regulator
Vout1 Heatsink, IERC HP3-T0127-CB Heatsink, IERC UP-000-CB Test Line Regulation Load Regulation Output Ripple Short Circuit Current Line Regulation Load Regulation Output Ripple Short Circuit Current Efficiency Vout1 Vout1 Vout1 Vout1 Vout2 Vout2 Vout2 Vout2 Conditions Iout1 Iout2 Iout1 Iout2 Iout1 Iout2 Iout1 Iout2 Iout2 Iout1 Iout1 Iout2 Iout1 Iout2
Vout2 V/300
Results 0.09% 0.2% mVp-p 11.4 0.3% 0.05% mVp-p 11.25
second output easily derived winding secondary main output inductor phasing that energy delivered Vout2 during toff. second output power should exceed main output. potentiometer used divide down voltage across 0.036 resistor thus fine tune current limit.
Figure Step-Down with Buffered Switch Second Output
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AN920/D
18.2 0.22
Comp Vout2 V/50 Vout1 V/500
Test Line Regulation Load Regulation Output Ripple Short Circuit Current Line Regulation Load Regulation Output Ripple Short Circuit Current Efficiency Vout1 Vout1 Vout1 Vout1 Vout2 Vout2 Vout2 Vout2
Iout1 Iout2 Iout1 Iout2 Iout1 Iout2 Iout1 Iout2 Iout2 Iout1 Iout1 Iout2 Iout1 Iout2
Figure Step-Down with Linear Pass from Main Output
1.25
1N5819
Conditions
Results 0.63% 0.15% mVp-p 0.067% 0.2% mVp-p 88.2%
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AN920/D
0.022
D45VH10
Vout1 V/3.0 MBR1540 2200
Comp
TIP31
Test Line Regulation Load Regulation Output Ripple Short Circuit Current Line Regulation Load Regulation Output Ripple Short Circuit Current Efficiency Vout1 Vout1 Vout1 Vout1 Vout2 Vout2 Vout2 Vout2
Figure Step-Down with Buffered Switch Buffered Linear Pass from Main Output
1.25
Vout2 V/1.0 *All devices mounted heatsink, extra hole required MBR2540, IERC HP3-T0127-4CB
Conditions Iout1 Iout2 Iout1 Iout2 Iout1 Iout2 Iout1 Iout2 Iout2 Iout1 Iout2 Iout1 Iout2
Results 0.043% 0.07% mVp-p 12.6 0.008% 0.08% mVp-p 78.5%
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11.8
3.09
Vout V/500 1N5819
Comp
-2.5
Test Line Regulation Load Regulation Output Ripple Efficiency
this step-down circuit, output switch must connected series with negative input, causing internal 1.25 reference with respect -Vin. second reference -2.5 with respect ground generated Amp. Note that resistors must matched pairs good line regulation that provision made output short-circuit protection.
Figure Step-Down with Negative Input Negative Output
1.25
Conditions Iout Iout Iout Iout
Results 0.104% 0.042% mVp-p 85.5%
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AN920/D
7.75T 1.25 1N5819 0.22 Comp 1.25 Reference Regulator
Vout V/225
Test Line Regulation Load Regulation Output Ripple Efficiency
Conditions Iout Iout Iout Iout
Results 0.028% 0.042% mVp-p 90.4%
maximum power transfer watts possible with Vout high efficiency partially tapped inductor. point voltage differential 1.25 range somewhat limited when using this method.
Figure Step-Up
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AN920/D
1N4936
Vout V/5.0 Display
0.24 to12 Comp
1.25 Reference Regulator
Primary Turns Secondary Turns Core Ferroxcube 1408P-L00-3CB Bobbin Ferroxcube 1408PCB1 0.003 Spacer primary inductance
Test Line Regulation Load Regulation Output Ripple Short Circuit Current Efficiency
Conditions Iout Iout Iout Iout
Results 0.61% 0.37% mVp-p
This circuit designed power Semiconductor Solid Ceramic Displays from design calculations based step-up converter with input output rated level maximum step-up allowed oscillator ratio ton/(ton toff). current level chosen that transformer primary power level about greater than that required load. maximum determined flyback leakage inductance voltages present collector output switch during turn-off must exceed
Figure High-Voltage, Power Step-Up Solid Ceramic Display
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AN920/D
0.022
D45VH10
16.5
2200
1600 MR817 Comp 0.033 G.E. FT-118 Shutter TRAID PL-10
Charging Indicator Primary Turns Secondary Turns Core Ferroxcube 2616P-L00-3C8 Bobbin Ferroxcube 2616PCB1 0.018 Spacer Primary Inductance 16.5
With this step-up converter will charge capacitor from seconds. switching operation will cease until bleeds down charging time between flashes seconds. output current
Figure High-Voltage Step-Up with Buffered Switch Photoflash Applications
1.25
Heatsink, IERC PB1-36CB
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AN920/D
0.22
1N5819
Vout1 V/100
Comp
Test Line Regulation Load Regulation Output Ripple Line Regulation Load Regulation Output Ripple Short Circuit Current Efficiency Vout1 Vout1 Vout1 Vout2 Vout2 Vout2 Vout2
Iout1 Iout2 Iout1 Iout2 Iout1 Iout2 Iout1 Iout2 Iout2 Iout1 Iout1 Iout2 Iout1 Iout2
Figure Step-Up with Linear Pass from Main Output
1.25
Vout2 V/30 38.3
Conditions
Results 0.11% 0.11% mVp-p 0.0083% 0.0083% mVp-p 68.3%
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16.5
0.22 13.7
Comp
2N5089
16.5 0.058
WRITE Input
Switch Position NOTE:
WRITE 2.25 2.25
values volts. Vref 1.245
Used conjunction with transistors, A78S40 generate required voltage needed program erase EEPROMs from single supply. step-up converter provides selectable regulated voltage Pins This voltage used generate second reference point power linear regulator consisting internal TIP29 transistor. When WRITE input less than 2N5089 transistor OFF, allowing voltage rise exponentially with approximate time constant required some EEPROMs. linear regulator amplifies voltage four, generating required output voltage byte-erase write cycle. When WRITE input greater than 2.25 2N5089 turns clamping point internal reference level 1.245 output will approximately (1.245 Vsat 2N5089). A78AS40 reference only source current, therefore reference bias used. output short-circuit protected supply current over input range
Figure Step-Up with Buffered Linear Pass from Main Output Programming EEPROMs
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1.25
57.6
TIP29
Output 10.2
Voltage Pins 23.52 23.52 28.07 28.07 5.24 1.28 6.25 1.28 20.95 5.12 25.01 5.12
Contributed Steve Hageman Calex Mfg. Inc.
AN920/D
6800 0.022
1200 Comp
2N5824 2200
Vout1 V/1.0
TIP29 Vout2 V/50
Heatsink, IERC LAT0127B5CB Heatsink, IERC PB1-36CB
Test Line Regulation Load Regulation Output Ripple Line Regulation Load Regulation Output Ripple Short Circuit Current Line Regulation Load Regulation Output Ripple Short Circuit Current Efficiency NOTE: Vout1 Vout1 Vout1 Vout2 Vout2 Vout2 Vout2 Vout3 Vout3 Vout3 Vout3
Conditions
Iout1 0.25 Iout2 Iout3
outputs nominal load current unless otherwise noted.
Figure Step-Up with Buffered Switch Buffered Linear Pass from Main Output http://onsemi.com
1.25
1N5818
MC79L12
Vout3 V/50
1N5818
Results 0.06% 0.083% mVp-p 0.013% 0.021% mVp-p 0.008% 0.12% mVp-p 71.8%
AN920/D
0.24 Comp 1N5819
Vout1 V/250
1N5819
Test Line Regulation Load Regulation Output Ripple Short Circuit Current Line Regulation Load Regulation Output Ripple Efficiency Vout1 Vout1 Vout1 Vout1 Vout2 Vout2 Vout2
This circuit shows method using A78S40 construct independent converters. Output uses typical step-up circuit configuration while Output makes connected with positive feedback create free running step-down converter. slew rate limits maximum switching frequency rated load less than kHz.
Figure Dual Switcher, Step-Up Step-Down with Buffered Switch
1.25
MPSU51A
2200
Vout2 V/250
Conditions Iout1 Iout2 Iout1 Iout2 Iout1 Iout2 Iout1 Iout2 Iout2 Iout1 Iout1 Iout2 Iout1 Iout2
Results 0.054% 0.036% mVp-p 0.04% 0.18% mVp-p 81.8%
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AN920/D
0.24
Comp 1.25 Reference Regulator
1300 1N5819
|Vout| 1.25
1000
Vout V/100
Test Line Regulation Load Regulation Output Ripple Short Circuit Current Efficiency
Conditions Iout Iout Iout Iout
Results 0.008% 0.042% mVp-p
above circuit shows method using MC34063 construct power voltage-inverting converter. Note that integrated circuit ground, connected directly negative output, thus allowing internally connected comparator reference function properly output voltage control. With this configuration, Vout must exceed conversion efficiency modest since output switch connected Darlington on-voltage large portion minimum operating input voltage. improvement realized with addition external saturated switch when connected similar manner that shown Figure
Figure Power Voltage-Inverting
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AN920/D
0.022
2N6438
4700 1050 1200 Comp MR822 Vout -120 V/850
1000
Test Line Regulation Load Regulation Output Ripple Short Circuit Current Efficiency
This high power voltage-inverting circuit makes center tapped inductor step-up magnitude output. Without tap, output switch transistor would need breakdown greater than start toff; maximum rating this device calculations done typical voltage-inverting converter with input output -120 inductor value will center tapped value used. 1000 capacitor used filter spikes generated high switching current flowing through wiring inductance.
Figure High Power Voltage-Inverting with Buffered Switch
economical watt off-line flyback switcher shown Figure this circuit A78S40 connected operate fixed frequency pulse width modulator. oscillator sawtooth waveform connected noninverting input comparator preset voltage derived from reference connected inverting input. preset voltage reduces maximum percent on-time output switch from nominal 85.7% about 45%. maximum must less than when equal turns ratio primary clamp winding
1.25
Center Tapped *Heatsink IERC Nested Pair HP1-T03-CB HP3-T03-CB Conditions Iout Iout Iout Iout Results 0.042% 0.029% mVp-p 81.8%
used. Output regulation isolation achieved TL431 output reference comparator, 4N35 optocoupler. output reaches nominal level, TL431 will start conduct current through 4N35. This turn will cause optoreceiver transistor turn raising voltage which will cause reduction percent on-time output switch. peak drain current output output loading increased, MPS6515 will activate Ipk(sense) shorten cycle-by-cycle basis.
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output shorted, Ipk(sense) circuit will cause charge beyond upper oscillator trip point oscillator frequency will decrease. This action will result lower average power dissipation output switching transistor. Each output series inductor second shunt filter capacitor forming filter. This used reduce level high frequency ripple spikes. Care must taken with layout grounds filter network. Each input output filter capacitor must have separate ground returns transformer shown circuit diagram. complete printed circuit board with component layout shown Figure A78S40 used previously shown circuit designs fixed frequency pulse width modulator, however consideration must given proper selection feedback loop elements order insure circuit stability.
MBR1635 Vout1 V/4.5
1N4303 Fusible Resistor
4N35
2200
1000
TL431 Return
1N965A 1000 MUR110
±20% Comp
Vout2 V/0.8
1000
4N50
1000
Return
MPSA55
1N4937
Primary: Pins Turns AWG, Bifilar Wound Pins Turns AWG, Bifilar Wound Secondary Turns (two strands) Bifilar Wound Secondary Turns Bifilar Wound
Figure Watt Off-Line Flyback Switcher with Primary Power Limiting
1.25
Vout3 V/0.8
0.24 0.0047 UL/CSA
MPS6515
1000
*Heatsink Thermalloy 6072B-MT
Core Bobbin: Coilcraft PT3995 Gap: 0.030 Spacer each primary inductance Primary primary leakage inductance must less than Coilcraft Z7156: Remove layer final inductance Coilcraft Z7157:
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Test Line Regulation Load Regulation Output Ripple Short Circuit Current Line Regulation Load Regulation Output Ripple Short Circuit Current Efficiency NOTE: Vout1 Vout1 Vout1 Vout1 Vout2 Vout3 Vout2 Vout3 Vout2 Vout3 Vout2 Vout3 Vac, Iout1 Vac, Vac, Iout2 Iout3 0.25 Vac, Conditions Results 0.01% 0.03% mVp-p 19.2 0.04% 1.6% mVp-p 10.8 75.7%
outputs nominal load current unless otherwise noted.
Figure (continued) Watt Off-Line Flyback Switcher with Primary Power Limiting
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Component Layout Bottom View
Printed Circuit Board Negative Bottom View
Figure Watt Off-Line
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AN920/D
0.15
MPSU51A
1N5822 Vout1 V/500 36.1
Vout2 Voltage
Comp *Heatsink IERC PSC2-3
MPSU01A
Tracking
Test Line Regulation Load Regulation Output Ripple Line Regulation Load Regulation Output Ripple Efficiency Vout1 Vout1 Vout1 Vout2 Vout2 Vout2
This tracking regulator provides output from single input. negative output generated voltage-inverting converter while positive linear pass regulator taken from input. outputs monitored corrective fashion that voltage center divider zero VIO. connected unity gain inverter when |Vout1| |Vout2|.
Figure Tracking Regulator, Voltage-Inverting with Buffered Switch Buffered Linear Pass from Input
1.25
Vout2 V/500
Conditions 14.5 Iout1 Iout2 Iout1 Iout2 Iout1 Iout2 14.5 Iout1 Iout2 Iout2 Iout1 Iout1 Iout2 Iout1 Iout2
0.042% 0.008% mVp-p 0.042% 0.021% mVp-p 77.2%
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Results
AN920/D
SUMMARY goal this application note convey theory operation MC34063 A78S40, show derivation basic first order design equations. circuits were chosen explore variety cost effective practical solutions designing switching converters. Another major objective show ease simplicity designing switching converters remove mystical "black magic" fears. Semiconductor maintains Linear Discrete products applications staff that dedicated assisting customers with design problems questions. BIBLIOGRAPHY Steve Hageman, DC/dc Converter Powers EEPROMs, EDN, January 1983. Design Manual SMPS Power Transformers, Copyright 1982 Pulse Engineering. HeatSink/Dissipator Products Thermal Management Guide, Copyright 1980 International Electronic Research Corporation. Linear Interface Integrated Circuits Data Manual, Copyright 1988 Motorola Inc. Linear Ferrite Materials Components Sixth Edition, Ferroxcube Corporation. Linear/Switchmode Voltage Regulator Hanbook Theory Practice, Copyright 1989 Motorola Inc. Molypermalloy Powder Cores, Catalog MPP-303U, Copyright 1981 Magnetics Inc. Motorola Power Data Book, Copyright 1989 Motorola Inc. Motorola Rectifiers Zener Diodes Data Book, Copyright 1988 Motorola Inc. Structured Design Switching Power Magnetics, AN-P100, Coilcraft Inc. Switching Linear Power Supply, Power Converter Design, Abraham Pressman, Copyright 1977 Hayden Book Company Inc. mA78S40 Switching Voltage Regulator, Application Note 370, Copyright 1982 Fairchild Camera Instruments Corporation. Switchmode Transformer Ferrite E-Core Packages, DS-P200, Coilcraft Inc. Lange, Tapped Inductor Improves Efficiency Switching Regulator, August 1979. Voltage Regulator Handbook, Copyright 1978 Fairchild Camera Instruments Corporation. SWITCHING REGULATOR COMPONENT SOURCES
Capacitor
Erie Technical Products P.O. Erie, 16512 (814) 452-5611 Mallory Capacitor P.O. 1284 Indianapolis, 46206 (317) 856-3731 United Chemi-Con 9801 Higgins Road Rosemont, 60018 (312) 696-2000
Heatsinks
IERC Magnolia Blvd. Burbank, (213) 849-2481 Thermalloy, Inc. 2021 Valley View Lane Dallas, Texas 75234 (214) 243-4321
Magnetic Assemblies
Coilcraft, Inc. 1102 Silver Lake Cary, 60013 (312) 639-2361 Pulse Engineering P.O. 12235 Diego, 92112 (714) 279-5900
Magnetic Cores
Ferroxcube 5083 Kings Highway Saugerties, 12477 (914) 246-2811 Magnetics, Inc. P.O. Butler, 16001 (412) 282-8282 Corporation America 4709 Golf Rd., Suite Skokie, 60076 (312) 679-8200
Semiconductor does endorse warrant suppliers referenced.
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Semiconductor registered trademarks Semiconductor Components Industries, (SCILLC). SCILLC reserves right make changes without further notice products herein. SCILLC makes warranty, representation guarantee regarding suitability products particular purpose, does SCILLC assume liability arising application product circuit, specifically disclaims liability, including without limitation special, consequential incidental damages. "Typical" parameters which provided SCILLC data sheets and/or specifications vary different applications actual performance vary over time. operating parameters, including "Typicals" must validated each customer application customer's technical experts. SCILLC does convey license under patent rights rights others. SCILLC products designed, intended, authorized components systems intended surgical implant into body, other applications intended support sustain life, other application which failure SCILLC product could create situation where personal injury death occur. Should Buyer purchase SCILLC products such unintended unauthorized application, Buyer shall indemnify hold SCILLC officers, employees, subsidiaries, affiliates, distributors harmless against claims, costs, damages, expenses, reasonable attorney fees arising directly indirectly, claim personal injury death associated with such unintended unauthorized use, even such claim alleges that SCILLC negligent regarding design manufacture part. SCILLC Equal Opportunity/Affirmative Action Employer. This literature subject applicable copyright laws resale manner.
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