DESCRIPTIO Very High Efficiency: Over Possible Wide Range: 3.5V*

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DESCRIPTIO Very High Efficiency: Over Possible Wide Range: 3.5V*

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LTC1147-3.3 LTC1147-5/LTC1147L High Efficiency Step-Down Switching Regulator Controllers
DESCRIPTIO
Very High Efficiency: Over Possible Wide Range: 3.5V* Current Mode Operation Excellent Line Load Transient Response High Efficiency Maintained Over Three Decades Output Current 160µA Standby Current Light Loads Logic Controlled Micropower Shutdown: 20µA Short-Circuit Protection Very Dropout Operation: 100% Duty Cycle High Efficiency Small Amount Board Space Output Externally Held High Shutdown Available 8-Pin Package
LTC1147 series step-down switching regulator controllers featuring automatic Burst Modeoperation maintain high efficiencies output currents. These devices drive external P-channel power MOSFET switching frequencies exceeding 400kHz using constant off-time current mode architecture providing constant ripple current inductor. operating current level user-programmable external current sense resistor. Wide input supply range allows operation from 3.5V* (16V maximum). Constant off-time architecture provides dropout regulation limited only RDS(ON) external MOSFET resistance inductor current sense resistor. LTC1147 series incorporate automatic power saving Burst Mode operation reduce switching losses when load currents drop below level required continuous operation. Standby power reduced only IOUT Load currents Burst Mode operation typically 300mA. applications where even higher efficiency required, refer LTC1148 data sheet Application Note
registered trademarks Linear Technology Corporation. Burst Mode trademark Linear Technology Corporation. *LTC1147L LTC1147L-3.3 only.
APPLICATI
Notebook Palmtop Computers Portable Instruments Battery-Operated Digital Devices Cellular Telephones Power Distribution Systems Systems
TYPICAL APPLICATI
P-DRIVE LTC1147-5 SHUTDOWN SENSE SENSE
(5.2V 14V)
P-CHANNEL Si9430DY 50µH
100µF RSENSE** 0.05
NORMAL >1.5V SHUTDOWN
VOUT 5V/2A
EFFICIENCY
1000pF
3300pF
MBRD330
470pF
COUT 390µF
LT1147
*COILTRONICS CTX50-2-MP **KRL SL-1-C1-0R050J
Figure High Efficiency Step-Down Converter
LTC1147-5 Efficiency
0.001 0.01
LT1147 TA01
LOAD CURRENT
LTC1147-3.3 LTC1147-5/LTC1147L ABSOLUTE RATI
PACKAGE/ORDER ATIO
VIEW SENSE P-DRIVE SHUTDOWN (VFB*) SENSE
Input Supply Voltage (Pin 0.3V Continuous Output Current (Pin 50mA Sense Voltages (Pins 0.3V Operating Ambient Temperature Range 70°C Extended Commercial Temperature Range 40°C 85°C Junction Temperature (Note 125°C Storage Temperature Range 65°C 150°C Lead Temperature (Soldering, sec). 300°C
ORDER PART NUMBER LTC1147CN8-3.3 LTC1147CN8-5 LTC1147CS8-3.3 LTC1147CS8-5 LTC1147LCS8 LTC1147LCS8-3.3 PART MARKING 11473 11475 1147L 1147L3
PACKAGE 8-LEAD PLASTIC PACKAGE 8-LEAD PLASTIC SOIC ADJUSTABLE OUTPUT VERSION
TJMAX 125°C, 110°C/ TJMAX 125°C, 150°C/
Consult factory Industrial Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL VOUT PARAMETER Feedback Voltage (LTC1147L) Feedback Current (LTC1147L) Regulated Output Voltage LTC1147-3.3, LTC1147L-3.3 LTC1147-5 Output Voltage Line Regulation Output Voltage Load Regulation LTC1147-3.3, LTC1147L-3.3 LTC1147-5 Burst Mode Output Ripple Input Supply Current (Note LTC1147 Series Normal Mode Sleep Mode Sleep Mode (LTC1147-5) Shutdown LTC1147L Series Normal Mode Sleep Mode Shutdown (LTC1147L-3.3) Current Sense Threshold Voltage (Note LTC1147-3.3, LTC1147L-3.3 LTC1147-5 LTC1147L
25°C, 10V, VSHUTDOWN unless otherwise noted.
CONDITIONS ILOAD 700mA ILOAD 700mA 12V, ILOAD 50mA ILOAD ILOAD ILOAD (Note VSHUTDOWN 2.1V, 3.5V 3.5V VSHUTDOWN 2.1V, 3.5V VSENSE- VOUT 100mV (Forced) VSENSE- VOUT 100mV (Forced) VSENSE- VOUT 100mV (Forced) VSENSE- VOUT 100mV (Forced) VSENSE- VOUT/4 25mV (Forced) VSENSE- VOUT/4 25mV (Forced)
1.25
1.29 3.43 5.20
UNITS mVP-P
1.21
3.23 4.90
3.33 5.05
VOUT
LTC1147-3.3 LTC1147-5/LTC1147L
ELECTRICAL CHARACTERISTICS
SYMBOL tOFF PARAMETER Shutdown Threshold LTC1147-3.3, LTC1147-5, LTC1147L-3.3 Shutdown Input Current LTC1147-3.3, LTC1147-5, LTC1147L-3.3, Discharge Current Off-Time (Note Driver Output Transition Times
25°C, 10V, VSHUTDOWN unless otherwise noted.
CONDITIONS
UNITS
VSHUTDOWN VOUT Regulation, VSENSE VOUT VOUT 390pF, ILOAD 700mA 3000pF (Pins
40°C 85°C (Note 10V, unless otherwise noted.
SYMBOL VOUT PARAMETER Feedback Voltage (LTC1147L) Regulated Output Voltage LTC1147-3.3, LTC1147L-3.3 LTC1147-5 Input Supply Current (Note LTC1147 Series Normal Mode Sleep Mode Sleep Mode (LTC1147-5) Shutdown LTC1147L Series Normal Mode Sleep Mode Shutdown (LTC1147L-3.3) Current Sense Threshold Voltage (Note LTC1147-3.3 LTC1147-5 LTC1147L tOFF Shutdown Threshold LTC1147-3.3, LTC1147-5, LTC1147L-3.3, Off-Time (Note CONDITIONS ILOAD 700mA ILOAD 700mA (Note VSHUTDOWN 2.1V, 3.5V 3.5V VSHUTDOWN 2.1V, 3.5V <12V VSENSE VOUT 100mV (Forced) VSENSE VOUT 100mV (Forced) VSENSE VOUT 100mV (Forced) VSENSE VOUT 100mV (Forced) VSENSE- VOUT/4 25mV (Forced) VSENSE- VOUT/4 25mV (Forced) VSHUTDOWN 390pF, ILOAD 700mA
1.20 3.17 4.85
1.25 3.33 5.05
1.30
UNITS
0.55
denotes specifications which apply over full operating temperature range. Note calculated from ambient temperature power dissipation according following formulas: LTC1147CN8-3.3, LTC1147CN8-5: 110°C/W) LTC1147LCS, LTC1147CS8-3.3, LTC1147CS8-5: 150°C/W) Note Dynamic supply current higher gate charge being delivered switching frequency. Applications Information.
Note applications where RSENSE placed ground potential, offtime increases approximately 40%. Note LTC1147-3.3/LTC1147-5 tested quality assurance sampled 40°C 85°C. These specifications guaranteed design and/or correlation. Note LTC1147L LTC1147L-3.3 allow operation 3.5V Note LTC1147L tested with external feedback resistors resulting nominal output voltage 2.5V.
LTC1147-3.3 LTC1147-5/LTC1147L
TYPICAL PERFOR CHARACTERISTICS
Efficiency Input Voltage
FIGURE CIRCUIT
EFFICIENCY
VOUT (mV)
VOUT (mV)
ILOAD
ILOAD 100mA
INPUT VOLTAGE
LTC1147 TPC01
Supply Current
SUPPLY CURRENT (mA)
INCLUDING GATE CHARGE CURRENT SUPPLY CURRENT (µA)
NORMALIZED FREQUENCY
ACTIVE MODE
SLEEP MODE INPUT VOLTAGE
LTC1147 TPC04
Gate Charge Supply Current
GATE CHARGE CURRENT (mA)
SENSE VOLTAGE (mV)
OFF-TIME (µs)
50nC
29nC OPERATING FREQUENCY (kHz)
LTC1147 TPC07
Line Regulation
LTC1147 TPC02
Load Regulation
FIGURE CIRCUIT RSENSE 0.05
FIGURE CIRCUIT ILOAD
-100
INPUT VOLTAGE
LOAD CURRENT
LTC1147 TPC03
Supply Current Shutdown
INPUT VOLTAGE VSHUTDOWN (NOT AVAILABLE LTC1147L)
Operating Frequency (VIN VOUT)
VOUT 70°C 25°C
(VIN VOUT) VOLTAGE
LTC1147 TPC05
LTC1148 TPC06
Off-Time VOUT
VSENSE VOUT
Current Sense Threshold Voltage
MAXIMUM THRESHOLD
LTC1147-3.3 LTC1147-5
MINIMUM THRESHOLD
OUTPUT VOLTAGE
LTC1147 TPC08
TEMPERATURE (°C)
LTC1147 TPC09
LTC1147-3.3 LTC1147-5/LTC1147L
CTIO
(VIN): Main Supply Pin. Must closely decoupled ground (CT): External capacitor from ground sets operating frequency. actual frequency also dependent upon input voltage. (ITH): Gain Amplifier Decoupling Point. current comparator threshold increases with voltage. (Sense Connects internal resistive divider which sets output voltage. also input current comparator. (Sense input current comparator. built-in offset between pins conjunction with RSENSE sets current trip threshold. (Shutdown/VFB): When grounded, fixed output versions LTC1147 family operate normally. Pulling high holds P-channel MOSFET puts LTC1147 micropower shutdown mode. Requires CMOS logic signal with 1µs. leave this floating. LTC1147L this serves feedback from external resistive divider used output voltage. (GND): independent ground lines must routed separately terminal COUT, cathode Schottky diode terminal CIN. (P-Drive): High current drive P-channel MOSFET. Voltage swing this from ground.
CTIO DIAGRA
SLEEP
25mV 150mV
OFF-TIME CONTROL
SENSE SHUTDOWN REFERENCE
LTC1147
VTH2
VTH1
Connection Shown LTC1147-3.3 LTC1147-5; Changes Create LTC1147L.
SENSE+ GROUND SENSE
P-DRIVE
1.25V 100k
LTC1147-3.3 LTC1147-5/LTC1147L
OPERATIO
LTC1147 series current mode, constant off-time architecture switch external P-channel power MOSFET. Operating frequency external capacitor (pin output voltage sensed internal voltage divider connected Sense- (pin voltage comparator gain block compare divided output voltage with reference voltage 1.25V. optimize efficiency, LTC1147 series automatically switch between modes operation, burst continuous. voltage comparator primary control element when device Burst Mode operation, while gain block controls output voltage continuous mode. During switch "on" cycle continuous mode, current comparator monitors voltage between pins connected across external shunt series with inductor. When voltage across shunt reaches threshold value, P-drive output switched VIN, turning P-channel MOSFET. timing capacitor connected allowed discharge rate determined off-time controller. discharge current made proportional output voltage (measured model inductor current, which decays rate which also proportional output voltage. When voltage timing capacitor discharged past VTH1, comparator trips, setting flip-flop. This causes P-drive output turning P-channel MOSFET back cycle then repeats. load current increases, output voltage decreases slightly. This causes output gain stage
APPLICATIO ATIO
LTC1147L Adjustable Applications
When output voltage other than 3.3V required, LTC1147L adjustable version used with external resistive divider from VOUT (See Figure regulated voltage determined
VOUT 1.25
(Refer Functional Diagram)
(pin increase current comparator threshold, thus tracking load current. sequence events Burst Mode operation very similar continuous operation with cycle interrupted voltage comparator. When output voltage above desired regulated value, P-channel MOSFET held comparator timing capacitor continues discharge below VTH1. When timing capacitor discharges past VTH2, voltage comparator trips, causing internal sleep line low. circuit enters sleep mode with power MOSFET turned off. sleep mode, majority circuitry turned off, dropping quiescent current from 1.6mA 160µA. load current being supplied from output capacitor. When output voltage dropped amount hysteresis comparator P-channel MOSFET again turned this process repeats. avoid operation current loop interfering with Burst Mode operation, built-in offset incorporated gain stage. This prevents current comparator threshold from increasing until output voltage dropped below minimum threshold. Using constant off-time architecture, operating frequency function input voltage. minimize frequency variation dropout approached, off-time controller increases discharge current drops below VOUT 1.5V. dropout P-channel MOSFET turned continuously (100% duty cycle), providing dropout operation with VOUT VIN.
prevent stray pickup 100pF capacitor suggested across located close LTC1147L. Figure applications with VOUT below when RSENSE moved ground, current sense comparator inputs operate near ground. When current comparator operated less than common mode, off-time increases approximately 40%, requiring smaller timing capacitor
LTC1147-3.3 LTC1147-5/LTC1147L
APPLICATIO ATIO
basic LTC1147 application circuit shown Figure External component selection driven load requirement, begins with selection RSENSE. Once RSENSE known, chosen. Next, power MOSFET selected. Finally, COUT selected loop compensated. circuit shown Figure configured operation input voltage 16V. application requires higher input voltage, then synchronous switched LTC1149 should used. Consult factory lower minimum input voltage version. RSENSE Selection Output Current RSENSE chosen based required output current. LTC1147 series current comparator threshold range which extends from minimum 25mV/ RSENSE maximum 150mV/RSENSE. current comparator threshold sets peak inductor ripple current, yielding maximum output current IMAX equal peak value less half peak-to-peak ripple current. proper Burst Mode operation, IRIPPLE(P-P) must less than equal minimum current comparator threshold. Since efficiency generally increases with ripple current, maximum allowable ripple current assumed, i.e., IRIPPLE(P-P) 25mV/RSENSE (See Selection Operating Frequency). Solving RSENSE allowing margin variations LTC1147 series external component values yields:
RSENSE 100mV IMAX
RSENSE
graph selecting RSENSE versus maximum output current given Figure load current below which Burst Mode operation commences, IBURST peak short-circuit current ISC(PK), both track IMAX. Once RSENSE been chosen, IBURST ISC(PK) predicted from following:
IBURST 15mV RSENSE ISC(PK) 150mV RSENSE
0.20 0.15 0.10 0.05 MAXIMUM OUTPUT CURRENT
LTC1147
Figure Selecting RSENSE
LTC1147 series automatically extend tOFF during short circuit allow sufficient time inductor current decay between switch cycles. resulting ripple current causes average short-circuit current ISC(AVG) reduced approximately IMAX. Selection Operating Frequency LTC1147 series constant off-time architecture with tOFF determined external timing capacitor Each time P-channel MOSFET switch turns voltage reset approximately 3.3V. During off-time, discharged current which proportional VOUT. voltage analogous current inductor which likewise decays rate proportional VOUT. Thus inductor value must track timing capacitor value. value calculated from desired continuous mode operating frequency:
VOUT
Where drop across Schottky diode. graph selecting versus frequency including effects input voltage given Figure
LTC1147-3.3 LTC1147-5/LTC1147L
APPLICATIO ATIO
1000 VSENSE VOUT
CAPACITANCE (pF)
FREQUENCY (kHz)
LTC1147
Figure Timing Capacitor Value
operating frequency increased gate charge losses will reduce efficiency (see Efficiency Considerations). complete expression operating frequency given
where:
tOFF
VOUT
tOFF
VREG VOUT
VREG desired output voltage (i.e., 3.3V). VOUT measured output voltage. Thus VREG/VOUT regulation. Note that decreases, frequency decreases. When input output voltage differential drops below 1.5V, LTC1147 reduces tOFF increasing discharge current This prevents audible operation prior dropout. Once frequency been inductor must chosen provide more than 25mV/RSENSE peak-to-peak inductor ripple current. This results minimum required inductor value LMIN RSENSE VREG inductor value increased from minimum value, requirements output capacitor eased expense efficiency. small
inductor used, inductor current will become discontinuous before LTC1147 series enters Burst Mode operation. consequence this that LTC1147 series will delay entering Burst Mode operation efficiency will degraded currents. Inductor Core Selection Once minimum value known, type inductor must selected. Highest efficiency will obtained using ferrite, Kool (from Magnetics, Inc.) molypermalloy (MPP) cores. Lower cost powdered iron cores provide suitable performance efficiency Actual core loss independent core size fixed inductor value, very dependent inductance selected. inductance increases, core losses down. Unfortunately, increased inductance requires more turns wire therefore copper losses will increase. Ferrite designs have very core loss, design goals concentrate copper loss preventing saturation. Ferrite core material saturates "hard," which means that inductance collapses abruptly when peak design current exceeded. This results abrupt increase inductor ripple current consequent output voltage ripple which cause Burst Mode operation falsely triggered LTC1147. allow core saturate! Kool very good, loss core material toroids with "soft" saturation characteristic. Molypermalloy slightly more efficient high (>200kHz) switching frequencies quite more expensive. Toroids very space efficient, especially when several layers wire. Because they generally lack bobbin, mounting more difficult. However, designs surface mount available from Coiltronics, Sumida Beckman Industrial Corp. which increase height significantly. Power MOSFET Selection external P-channel power MOSFET must selected with LTC1147 series. main selection criteria power MOSFET threshold voltage VGS(TH) "on" resistance RDS(ON).
Kool registered trademark Magnetics, Inc.
LTC1147-3.3 LTC1147-5/LTC1147L
APPLICATIO ATIO
minimum input voltage determines whether standard threshold logic-level threshold MOSFET must used. standard threshold MOSFET (VGS(TH) used. expected drop below logic-level threshold MOSFET (VGS(TH) 2.5V) strongly recommended. When logic-level MOSFET used, LTC1147 supply voltage must less than absolute maximum ratings MOSFET. maximum output current IMAX determines RDS(ON) requirement power MOSFET. When LTC1147 series operating continuous mode, simplifying assumption made that either MOSFET Schottky diode always conducting average load current. duty cycles MOSFET diode given P-Ch Duty Cycle Schottky Diode Duty Cycle (VIN VOUT
From duty cycle required RDS(ON) MOSFET derived:
P-Ch RDS(ON) VOUT IMAX2
where allowable power dissipation temperature dependency RDS(ON). will determined efficiency and/or thermal requirements (see Efficiency Considerations). generally given MOSFET form normalized RDS(ON) temperature curve, 0.007/°C used approximation voltage MOSFETs. Output Diode Selection (D1) Schottky diode shown Figure only conducts during off-time. important adequately specify diode peak current average power dissipation exceed diode ratings. most stressful condition output diode under short circuit (VOUT Under this condition diode must safely handle ISC(PK) close 100% duty cycle. Under normal load conditions average current conducted diode
(VIN VOUT (ILOAD)
Remember keep lead lengths short observe proper grounding (see Board Layout Checklist) avoid ringing increased dissipation. forward voltage drop allowable diode calculated from maximum short-circuit current
ISC(PK)
where allowable power dissipation will determined efficiency and/or thermal requirements (see Efficiency Considerations). COUT Selection continuous mode, source current P-channel MOSFET square wave duty cycle VOUT/VIN. prevent large voltage transients, input capacitor sized maximum current must used. maximum capacitor current given
Required IRMS IMAX
[VOUT(VIN VOUT)]1/2
This formula maximum 2VOUT, where IRMS IOUT/2. This simple worst case condition commonly used design because even significant deviations offer much relief. Note that capacitor manufacturer's ripple current ratings often based only 2000 hours life. This makes advisable further derate capacitor, choose capacitor rated higher temperature than required. Several capacitors also paralleled meet size height requirements design. Always consult manufacturer there question. additional 0.1µF ceramic decoupling capacitor also required (pin high frequency decoupling. selection COUT driven required effective series resistance (ESR). COUT must less than twice value RSENSE proper operation LTC1147 COUT Required 2RSENSE
LTC1147-3.3 LTC1147-5/LTC1147L
APPLICATIO ATIO
Optimum efficiency obtained making equal RSENSE. increased 2RSENSE, efficiency degrades less than greater than 2RSENSE, voltage ripple output capacitor will prematurely trigger Burst Mode operation, resulting disruption continuous mode efficiency which several percent. Manufacturers such Nichicon United Chemicon should considered high performance capacitors. OS-CON semiconductor dielectric capacitor available from Sanyo lowest ESR/size ratio aluminum electrolytic somewhat higher price. Once requirement COUT been met, current rating generally exceeds IRIPPLE(P-P) requirement. surface mount applications multiple capacitors have paralleled meet capacitance, ESR, current handling requirements application. Aluminum electrolytic tantalum capacitors both available surface mount configurations. case tantalum, critical that capacitors surge tested switching power supplies. excellent choice series surface mount tantalums, available case heights ranging from 4mm. example, 200µF/10V called application requiring height, 100µF/10V (P/N TPSD 107K010) could used. Consult manufacturer other specific recommendations. supply voltages, minimum capacitance COUT needed prevent abnormal frequency operating
1000 50µH RSENSE 0.02 25µH RSENSE 0.02
COUT (µF)
50µH RSENSE 0.05 (VIN VOUT) VOLTAGE
LTC1147
Figure Minimum Value COUT
mode (see Figure When COUT made small, output ripple frequencies will large enough trip voltage comparator. This causes Burst Mode operation activated when LTC1147 series would normally continuous operation. effect most pronounced with values RSENSE improved operating higher frequencies with lower values output remains regulation times. Checking Transient Response regulator loop response checked looking load transient response. Switching regulators take several cycles respond step (resistive) load current. When load step occurs, VOUT shifts amount equal ILOAD ESR, where effective series resistance COUT. ILOAD also begins charge discharge COUT until regulator loop adapts current change returns VOUT steady state value. During this recovery time VOUT monitored overshoot ringing which would indicate stability problem. external components shown Figure circuit will prove adequate compensation most applications. second, more severe transient caused switching loads with large (>1µF) supply bypass capacitors. discharged bypass capacitors effectively parallel with COUT, causing rapid drop VOUT. regulator deliver enough current prevent this problem load switch resistance driven quickly. only solution limit rise time switch drive that load rise time limited approximately CLOAD. Thus 10µF capacitor would require 250µs rise time, limiting charging current about 200mA. Efficiency Considerations percent efficiency switching regulator equal output power divided input power times 100%. often useful analyze individual losses determine what limiting efficiency which change would produce most improvement. Percent efficiency expressed %Efficiency 100%
LTC1147-3.3 LTC1147-5/LTC1147L
APPLICATIO ATIO
where etc., individual losses percentage input power. (For high efficiency circuits only small errors incurred expressing losses percentage output power). Although dissipative elements circuit produce losses, four main sources usually account most losses LTC1147 circuits: LTC1147 bias current, MOSFET gate charge current, losses, voltage drop Schottky diode. supply current current which flows into (pin less gate charge current. LTC1147 series supply current 160µA load, increases proportionally with load constant 1.6mA after LTC1147 series entered continuous mode. Because bias current drawn from VIN, resulting loss increases with input voltage. bias losses generally less than load currents over 30mA. However, very load currents bias current accounts nearly loss. MOSFET gate charge current results from switching gate capacitance power MOSFET. Each time MOSFET gate switched from high again, packet charge moves from ground. resulting dQ/dt current which typically much larger than supply current. continuous mode, IGATECHG f(QP). typical gate charge 0.135 P-channel power MOSFET 40nC. This results IGATECHG 100kHz continuous operation typical mid-current loss with 10V. Note that gate charge loss increases directly with both input voltage operating frequency. This principal reason highest efficiency circuits operate moderate frequencies. Furthermore, argues against using larger MOSFET than necessary control losses, since overkill cost efficiency well money! losses easily predicted from resistances MOSFET, inductor, current shunt. continuous mode average output current flows through RSENSE, "chopped" between
EFFICIENCY/LOSS
P-channel Schottky diode. MOSFET RDS(ON) multiplied P-channel duty cycle summed with resistances RSENSE obtain losses. example, RDS(ON) 0.1, 0.15, RSENSE 0.05, then total resistance 2VOUT. This results losses ranging from output current increases from 0.5A losses cause efficiency roll high output currents. Schottky diode major source power loss high currents gets worse high input voltages. diode loss calculated multiplying forward voltage drop times Schottky diode duty cycle multiplied load current. example, assuming duty cycle with Schottky diode forward voltage drop 0.4V, loss increases from 0.5% load current increases from 0.5A Figure shows efficiency losses typical LTC1147 series regulator being apportioned. gate charge loss responsible majority efficiency lost mid-current region. Burst Mode operation employed currents, gate charge loss alone would cause efficiency drop unacceptable levels. With Burst Mode operation, supply current represents lone (and unavoidable) loss component which continues become higher percentage output current reduced. expected, losses Schottky diode loss dominate high load currents.
GATE CHARGE LTC1147 SCHOTTKY DIODE 0.01 0.03 OUTPUT CURRENT
LTC1147
Figure Efficiency Loss
LTC1147-3.3 LTC1147-5/LTC1147L
APPLICATIO ATIO
Other losses including COUT dissipative losses, MOSFET switching losses, inductor core losses, generally account less than total additional loss. Design Example design example, assume (nominal), VOUT 3.3V, IMAX 130kHz; RSENSE, immediately calculated: RSENSE 100mV/1A tOFF (1/130kHz) (3.3/5)] 2.61µs 2.61µs/(1.3 104) 220pF 220pF 3.3V 33µH Assume that MOSFET dissipation limited 250mW. 50°C thermal resistance MOSFET 50°C/ then junction temperatures will 63°C 0.007(63 0.27. required RDS(ON) MOSFET calculated:
P-Ch RDS(ON) 5(0.25) 3.3(1)2 (1.27)
P-channel requirement Si9430DY. Note that most stringent requirement Schottky diode with VOUT (i.e., short circuit). During continuous short circuit, worst-case Schottky diode dissipation rises
ISC(AVG)
3.3V
With sense resistor ISC(AVG) will result, increasing 0.4V Schottky diode dissipation 0.4W. will require current rating least 0.5A temperature, COUT will require optimum efficiency. allow drop minimum value. lower input voltages operating frequency will decrease P-channel will conducting most time, causing power dissipation increase. VIN(MIN) 4.5V, frequency will decrease P-channel will conducting most time causing power dissipation increase. VIN(MIN) 4.5V:
CONTINUOUS MODE OPERATION
fMIN 102kHz 2.61µs
3.3(0.125)(1A)2(1.27) 116mW
This last step necessary assure that power dissipation junction temperature P-channel exceeded. Troubleshooting Hints Since efficiency critical LTC1147 series applications, very important verify that circuit functioning correctly both continuous Burst Mode operation. waveform monitor voltage timing capacitor continuous mode (ILOAD IBURST) voltage should sawtooth with 0.9VP-P swing. This voltage should never below shown Figure When load currents (ILOAD IBURST) Burst Mode operation occurs. voltage falls ground periods time shown Figure During this time LTC1147 series sleep mode with quiescent current reduced 160µA. inductor current should also monitored. Look verify that peak-to-peak ripple current continuous mode operation approximately same Burst Mode operation.
3.3V
Burst Mode OPERATION
LTC1147
Figure Waveforms
observed falling ground high output currents, indicates poor decoupling improper grounding. Refer Board Layout Checklist.
LTC1147-3.3 LTC1147-5/LTC1147L
APPLICATIO ATIO
Board Layout Checklist When laying printed circuit board, following checklist should used ensure proper operation LTC1147 series. These items also illustrated graphically layout diagram Figure Check following your layout: signal power grounds segregated? LTC1147 ground (pin must return separately power signal grounds. power ground returns source anode Schottky diode, plate CIN, which should have lead lengths short possible. signal ground connects plate COUT. Does LTC1147 Sense (pin connect point close RSENSE plate COUT?
BOLD LINES INDICATE HIGH CURRENT PATHS
LTC1147-3.3 LTC1147-5 (LTC1147L) SENSE P-DRIVE GROUND SHUTDOWN
390pF
3300pF
SHUTDOWN (VFB)
SENSE
1000pF
Figure LTC1147 Layout Diagram (See Board Layout Checklist)
Sense Sense leads routed together with minimum trace spacing? 1000pF capacitor between pins should close possible LTC1147. Does plate connect source P-channel MOSFET closely possible? This capacitor provides current P-channel MOSFET. input decoupling capacitor (0.1µF/1µF) connected closely between (pin ground (pin This capacitor carries MOSFET driver peak currents. fixed output versions, Shutdown (pin actively pulled ground during normal operation? Shutdown high impedance must allowed float.
P-CH RSENSE
COUT
VOUT
100pF
OUTPUT DIVIDER REQUIRED WITH ADJUSTABLE VERSION ONLY
LTC1147
additional High Efficiency application circuits Application Note
LTC1147-3.3 LTC1147-5/LTC1147L
TYPICAL APPLICATIO
3.5V
120pF
3300pF
0.01µF
RSENSE** 0.068
*SUMIDA CDR74B-100LC **IRC LRC-LR2010-01-R068-F
Figure 3.3V Dropout High Efficiency Regulator
Si9400DY 0.1µF
MBRS130LT3
33µF
LTC1147L SENSE P-DRIVE GROUND SHUTDOWN (VFB) SENSE
220µH
3300pF
0.01µF
RSENSE** 0.22
6.8k
COUT 100µF
IOUT
300pF
100pF
1.2k
VOUT
*SUMIDA CDR74-221 **IRC LRC-LR2010-01-R068-F
LTC1147
Figure Precision Constant Current Source
Figure IOUT VOUT Figure
Si9433DY MBRS130LT3
47µF
0.1µF 10µH LTC1147L-3.3 P-DRIVE GROUND SHUTDOWN SENSE SHUTDOWN COUT 100µF
SENSE
VOUT 3.3V/1.25A
LTC1147
12.6V
VOUT
LTC1147
LTC1147-3.3 LTC1147-5/LTC1147L
TYPICAL APPLICATIO
3.5V
120pF
3300pF
0.01µF
RSENSE** 0.05
*COILTRONICS CTX10-4 **IRC LR2512-01-0R050-G
Figure 2.5V, Regulator
Si9430DY
MBRS130LT3
0.1µF 20µH LTC1147-3.3 SENSE P-DRIVE GROUND SHUTDOWN SENSE SHUTDOWN COUT 220µF
220pF
3300pF
0.01µF
RSENSE** 0.05
*COILTRONICS CTX20-4 **KRL SP-1/2-A1-OR050
Figure 3.3V, Output High Efficiency Regulator
Information furnished Linear Technology Corporation believed accurate reliable. However, responsibility assumed use. Linear Technology Corporation makes representation that interconnection circuits described herein will infringe existing patent rights.
Si9433DY MBRD330
15µF
0.1µF 10µH LTC1147L P-DRIVE GROUND SHUTDOWN (VFB) SENSE 49.9k I00pF 49.9k COUT 220µF
SENSE
2.5V/2A
LTC1147
22µF
LTC1147
VOUT 3.3V/2A
LTC1147-3.3 LTC1147-5/LTC1147L
PACKAGE DESCRIPTIO
Dimensions inches (millimeters) unless otherwise noted. Package 8-Lead Plastic
0.400 (10.160)
0.250 0.010 (6.350 0.254)
0.300 0.320 (7.620 8.128)
0.045 0.065 (1.143 1.651)
0.130 0.005 (3.302 0.127)
0.009 0.015 (0.229 0.381)
0.065 (1.651) 0.125 (3.175) 0.020 (0.508)
+0.025 0.325 -0.015 8.255 +0.635 -0.381
0.045 0.015 (1.143 0.381) 0.100 0.010 (2.540 0.254)
0.018 0.003 (0.457 0.076)
0393
Package 8-Lead Plastic SOIC
0.189 0.197 (4.801 5.004)
0.228 0.244 (5.791 6.197)
0.150 0.157 (3.810 3.988)
0.010 0.020 (0.254 0.508) 0.008 0.010 (0.203 0.254) 0.016 0.050 0.406 1.270
0.053 0.069 (1.346 1.752) 0.004 0.010 (0.101 0.254)
0.014 0.019 (0.355 0.483)
0.050 (1.270)
0393
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, 95035-7487
(408) 432-1900 FAX: (408) 434-0507 TELEX: 499-3977
LT/GP 0195 PRINTED
LINEAR TECHNOLOGY CORPORATION 1993

DESCRIPTIO Very High Efficiency: Over Possible Wide Range: 3.5V*

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