DESCRIPTIO Ultra-High Efficiency: Over Possible Current-Mode Oper

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DESCRIPTIO Ultra-High Efficiency: Over Possible Current-Mode Oper

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LTC1148 LTC1148-3.3/LTC1148-5 High Efficiency Synchronous Step-Down Switching Regulators
DESCRIPTIO
Ultra-High Efficiency: Over Possible 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 Wide Range: 3.5V* Short-Circuit Protection Very Dropout Operation: 100% Duty Cycle Synchronous Switching High Efficiency Adaptive Non-Overlap Gate Drives Output Externally Held High Shutdown Available 14-Pin Narrow Package
LTC1148 series family synchronous stepdown switching regulator controllers featuring automatic Burst Modeoperation maintain high efficiencies output currents. These devices drive external complementary power MOSFETs switching frequencies 250kHz 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* (20V maximum). Constant off-time architecture provides dropout regulation limited only RDS(ON) external MOSFET resistance inductor current sense resistor. LTC1148 series combines synchronous switching maximum efficiency high currents with automatic current operating mode, called Burst Mode operation which reduces switching losses. Standby power reduced only IOUT Load currents Burst Mode operation typically 300mA. operation input, LTC1149 LTC1159 data sheets Application Note
registered trademarks Linear Technology Corporation. Burst Mode trademark Linear Technology Corporation. LTC1148L LTC1148L-3.3 only.
APPLICATI
Notebook Palmtop Computers Portable Instruments Battery-Operated Digital Devices Cellular Telephones Power Distribution Systems Systems
TYPICAL APPLICATI
P-DRIVE LTC1148HV-5 SHUTDOWN SENSE SENSE N-DRIVE S-GND P-GND
(5.2V 18V)
P-CHANNEL Si9430DY 62µH
100µF
EFFICIENCY
NORMAL >1.5V SHUTDOWN
RSENSE** 0.05
VOUT 5V/2A
1000pF N-CHANNEL Si9410DY 1N5818
3300pF
470pF
COUT 390µF
LT1148 TA01
*COILTRONICS CTX62-2-MP **KRL SL-1-C1-0R050J
Figure High Efficiency Step-Down Converter
LTC1148-5 Efficiency
0.02 LOAD CURRENT
LTC1148 TA01
LTC1148 LTC1148-3.3/LTC1148-5 ABSOLUTE RATI
PACKAGE/ORDER ATIO
ORDER PART NUMBER
VIEW P-DRIVE SENSE N-DRIVE P-GND S-GND SHUTDOWN VFB* SENSE
Input Supply Voltage (Pin LTC1148 LTC1148L Series 0.3V LTC1148HV Series 0.3V Continuous Output Current (Pins 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
PACKAGE PACKAGE 14-LEAD PLASTIC 14-LEAD PLASTIC SOIC *FIXED OUTPUT VERSIONS
TJMAX 125°C, 70°C/ TJMAX 125°C, 110°C/
LTC1148CN LTC1148HVCN LTC1148CN-3.3 LTC1148HVCN-3.3 LTC1148CN-5 LTC1148HVCN-5 LTC1148CS LTC1148HVCS LTC1148LCS LTC1148CS-3.3 LTC1148HVCS-3.3 LTC1148LCS-3.3 LTC1148CS-5 LTC1148HVCS-5
Consult factory Industrial Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL VOUT PARAMETER Feedback Voltage (LTC1148, LTC1148L, LTC1148HV) Feedback Current (LTC1148, LTC1148L, LTC1148HV)
25°C, 10V, VSHUTDOWN unless otherwise noted.
CONDITIONS
1.21
1.25
1.29
UNITS
Regulated Output Voltage LTC1148-3.3, LTC1148HV-3.3, LTC1148L-3.3 ILOAD 700mA LTC1148-5, LTC1148HV-5 ILOAD 700mA Output Voltage Line Regulation 12V, ILOAD 50mA Output Voltage Load Regulation LTC1148-3.3, LTC1148HV-3.3, LTC1148L-3.3 ILOAD LTC1148-5, LTC1148HV-5 ILOAD Output Ripple (Burst Mode) ILOAD
3.23 4.90
3.33 5.05
3.43 5.20
VOUT
mVP-P
Input Supply Current (Note LTC1148 Series Normal Mode Sleep Mode Sleep Mode (LTC1148-5) Shutdown LTC1148HV Series Normal Mode Sleep Mode Sleep Mode (LTC1148HV-5) Shutdown LTC1148L Series Normal Mode Sleep Mode Shutdown
(Note VSHUTDOWN 2.1V, VSHUTDOWN 2.1V, 3.5V 3.5V VSHUTDOWN 2.1V, 3.5V
LTC1148 LTC1148-3.3/LTC1148-5
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER Current Sense Threshold Voltage LTC1148, LTC1148HV, LTC1148L LTC1148-3.3, LTC1148HV-3.3 LTC1148L-3.3 LTC1148-5, LTC1148HV-5 tOFF Shutdown Threshold Shutdown Input Current Discharge Current Off-Time (Note Driver Output Transition Times
25°C, 10V, VSHUTDOWN unless otherwise noted.
CONDITIONS VSENSE VOUT/4 25mV (Forced) VSENSE VOUT/4 25mV (Forced) VSENSE VOUT 100mV (Forced) VSENSE VOUT 100mV (Forced) VSENSE VOUT 100mV (Forced) VSENSE VOUT 100mV (Forced) VSHUTDOWN VOUT Regulation, VSENSE VOUT VOUT 390pF, ILOAD 700mA 3000pF (Pins 14), UNITS
40°C 85°C (Note 10V, unless otherwise noted.
SYMBOL VOUT PARAMETER Feedback Voltage (LTC1148, LTC1148HV LTC1148L) Regulated Output Voltage LTC1148-3.3, LTC1148HV-3.3, LTC1148L-3.3 LTC1148-5, LTC1148HV-5 Input Supply Current (Note LTC1148 Series Normal Mode Sleep Mode Sleep Mode Shutdown LTC1148HV Series Normal Mode Sleep Mode Sleep Mode Shutdown LTC1148L Series Normal Mode Sleep Mode Shutdown Current Sense Threshold Voltage LTC1148, LTC1148HV, LTC1148L (Note CONDITIONS ILOAD 700mA ILOAD 700mA (Note VSHUTDOWN 2.1V, VSHUTDOWN 2.1V, 3.5V 3.5V VSHUTDOWN 2.1V, 3.5V VSENSE VOUT/4 25mV (Forced) VSENSE VOUT/4 25mV (Forced) 1.20 1.25 1.30 UNITS
3.17 4.85
3.33 5.05
0.55
LTC1148-3.3, LTC1148HV-3.3, LTC1148L-3.3 VSENSE VOUT 100mV (Forced) VSENSE VOUT 100mV (Forced) LTC1148-5, LTC1148HV-5 tOFF Shutdown Threshold Off-Time (Note 390pF, ILOAD 700mA VSENSE VOUT 100mV (Forced) VSENSE VOUT 100mV (Forced)
LTC1148 LTC1148-3.3/LTC1148-5
ELECTRICAL CHARACTERISTICS
denotes specifications which apply over full operating temperature range. Note calculated from ambient temperature power dissipation according following formulas: LTC1148CN, LTC1148CN-3.3, LTC1148CN-5: 70°C/W) LTC1148CS, LTC1148CS-3.3, LTC1148CS-5: 110°C/W) Note Dynamic supply current higher gate charge being delivered switching frequency. Applications Information. Note LTC1148 LTC1148HV versions tested with external feedback resistors resulting nominal output voltage LTC1148L version tested with external feedback resistors resulting nominal output voltage 2.5V. Note applications where RSENSE placed ground potential, offtime increases approximately 40%. Note LTC1148, LTC1148HV LTC1148L series tested quality assurance sampled -40°C 85°C. These specifications guaranteed design and/or correlation. Note LTC1148L LTC1148L-3.3 allow operation 3.5V.
TYPICAL PERFOR CHARACTERISTICS
Efficiency Input Voltage
EFFICIENCY
FIGURE CIRCUIT
ILOAD
VOUT (mV)
VOUT (mV)
ILOAD 100mA
INPUT VOLTAGE
LTC1148 TPC01
Supply Current
INCLUDING GATE CHARGE CURRENT SUPPLY CURRENT (µA)
NORMALIZED FREQUENCY
SUPPLY CURRENT (mA)
ACTIVE MODE
SLEEP MODE INPUT VOLTAGE
LTC1148 TPC04
Line Regulation
INPUT VOLTAGE
LTC1148 TPC02
Load Regulation
FIGURE CIRCUIT RSENSE 0.05
FIGURE CIRCUIT ILOAD
-100
LOAD CURRENT
LTC1148 TPC03
Supply Current Shutdown
INPUT VOLTAGE
LTC1148 TPC05
Operating Frequency (VIN VOUT)
VOUT (VIN VOUT) VOLTAGE 70°C 25°C
VSHUTDOWN
LTC1148 TPC06
LTC1148 LTC1148-3.3/LTC1148-5
TYPICAL PERFOR CHARACTERISTICS
Gate Charge Supply Current
OPERATING FREQUENCY (kHz)
LTC1148 TPC07
GATE CHARGE CURRENT (mA)
SENSE VOLTAGE (mV)
OFF-TIME (µs)
100nC
50nC
CTIO
P-Channel Drive (Pin High Current Drive P-Channel MOSFET. Voltage swing this from ground. (Pin Connection. connect power ground. (Pin Main Supply Pin. Must closely decoupled power ground (Pin External Capacitor from Ground Sets Operating Frequency. actual frequency also dependent upon input voltage. (Pin Internal Supply Voltage, Nominally 3.3V. decoupled signal ground. externally load this pin. (Pin Gain Amplifier Decoupling Point. current comparator threshold increases with voltage. Sense (Pin Connects internal resistive divider which sets output voltage LTC1148-3.3 LTC1148-5 versions. also input current comparator. Sense (Pin Input Current Comparator. built-in offset between pins conjunction with RSENSE sets current trip threshold. (Pin LTC1148 adjustable version, serves feedback from external resistive divider used output voltage. LTC1148-3.3 LTC1148-5 versions this used. Shutdown (Pin 10): When grounded, LTC1148 series operates normally. Pulling high holds both MOSFETs puts LTC1148 series micropower shutdown mode. Requires CMOS logic signal with 1µs, should left floating. S-Ground (Pin 11): Small Signal Ground. Must routed separately from other grounds terminal COUT. P-Ground (Pin 12): Driver Power Ground. Connects source N-channel MOSFET terminal CIN. (Pin 13): Connection. connect power ground. N-Channel Drive (Pin 14): High Current Drive Bottom N-Channel MOSFET. Voltage swing from ground VIN.
Off-Time VOUT
LTC1148-3.3 LTC1148-5 VSENSE VOUT
Current Sense Threshold Voltage
MAXIMUM THRESHOLD
MINIMUM THRESHOLD
OUTPUT VOLTAGE
LTC1148 TPC08
TEMPERATURE (°C)
LTC1148 TPC09
LTC1148 LTC1148-3.3/LTC1148-5
CTIO DIAGRA
SIGNAL GROUND
P-GND SLEEP
VTH2 VTH1
25mV 150mV
OFF-TIME CONTROL
SENSE
SHUTDOWN
TEST CIRCUIT
IRF9Z34 1N5818
P-DRIVE
N-DRIVE
50µH
LTC1148 P-GND
S-GND
SHUTDOWN SENSE (VFB) SENSE
100pF
1000pF
RSENSE 0.05
390pF
10nF
3300pF
connection shown LTC1148-3.3 LTC1148-5; changes create LTC1148.
P-DRIVE SENSE+ ADJUSTABLE VERSION N-DRIVE SENSE
1.25V
100k
REFERENCE
LTC1148
330µF
IRFZ34
440µF
VOUT
LTC1148 LTC1148-3.3/LTC1148-5
OPERATIO
LTC1148 series uses current mode, constant offtime architecture synchronously switch external pair complementary power MOSFETs. Operating frequency external capacitor timing capacitor output voltage sensed internal voltage divider connected Sense (LTC1148-3.3 LTC1148-5) external divider returned (LTC1148). voltage comparator gain block compare divided output voltage with reference voltage 1.25V. optimize efficiency, LTC1148 series automatically switches 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. While timing capacitor discharging, N-drive output goes VIN, turning N-channel MOSFET. When voltage timing capacitor discharged past VTH1, comparator trips, setting flip-flop. This causes N-drive output (turning Nchannel MOSFET) P-drive output also (turning P-channel MOSFET back on). cycle then repeats. load current increases, output voltage decreases slightly. This causes output gain stage
(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 Nchannel MOSFET turn off. circuit enters sleep mode with both power MOSFETs 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 process repeats. avoid operation current loop interfering with Burst Mode operation, built-in offset (VOS) incorporated gain stage. This prevents current comparator threshold from increasing until output voltage dropped below minimum threshold. prevent both external MOSFETs from ever being turned same time, feedback incorporated sense state driver output pins. Before N-drive output high, P-drive output must also high. Likewise, P-drive output prevented from going while N-drive output high. 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 extremely dropout operation.
LTC1148 LTC1148-3.3/LTC1148-5
APPLICATIO ATIO
basic LTC1148 series application circuit (fixed output versions) shown Figure External component selection driven load requirement, begins with selection RSENSE. Once RSENSE known, chosen. Next, power MOSFETs selected. Finally, COUT selected loop compensated. circuit shown Figure configured operation input voltage 20V. application requires higher input voltage, then LTC1149 LTC1159 should used. RSENSE Selection Output Current RSENSE chosen based required output current. LTC1148 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 LTC1148 series external component values yields:
RSENSE
RSENSE 100mV IMAX
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
LTC1148
Figure Selecting RSENSE
LTC1148 series automatically extends 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 LTC1148 series uses 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,
Assumes 2VOUT, Figure circuit. graph selecting versus frequency including effects input voltage given Figure operating frequency increased gate charge losses will higher, reducing efficiency (see Efficiency Considerations). complete expression operating frequency circuit Figure given
LTC1148 LTC1148-3.3/LTC1148-5
APPLICATIO ATIO
1000 VSENSE VOUT
CAPACITANCE (pF)
FREQUENCY (kHz)
LTC1148
Figure Timing Capacitor Value
tOFF
VOUT
where:
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, LTC1148 series 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 decrease past zero change polarity. consequence this that LTC1148 series enter Burst Mode operation efficiency will severely degraded currents.
Inductor Core Selection Once minimum value known, type inductor must selected. highest efficiency will obtained using ferrite, Kool 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 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. allow core saturate! Kool (from Magnetics, Inc.) 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 Beckman Industrial Corp. which increase height significantly. Power MOSFET Selection external power MOSFETs must selected with LTC1148 series: P-channel MOSFET main switch, N-channel MOSFET synchronous switch. main selection criteria power MOSFETs threshold voltage VGS(TH) resistance RDS(ON). minimum input voltage determines whether standard threshold logic-level threshold MOSFETs must used. standard threshold MOSFETs (VGS(TH) used. expected drop below logicKool registered trademark Magnetics, Inc.
LTC1148 LTC1148-3.3/LTC1148-5
APPLICATIO ATIO
level threshold MOSFETs (VGS(TH) 2.5V) strongly recommended. LTC1148/LTC1148HV series supply voltage must always less than absolute maximum ratings MOSFETs. maximum output current IMAX determines RDS(ON) requirement MOSFETs. When LTC1148 series operating continuous mode, simplifying assumption made that MOSFETs always conducting average load current. duty cycles MOSFETs given
P-Ch Duty Cycle N-Ch Duty Cycle (VIN VOUT)
From duty cycles required RDS(ON) each MOSFET derived:
P-Ch RDS(ON) N-Ch RDS(ON) VOUT IMAX2 (VIN VOUT) IMAX2
where allowable power dissipations temperature dependencies 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. Schottky diode shown Figure only conducts during dead-time between conduction power MOSFETs. D1's sole purpose life prevent body diode N-channel MOSFET from turning storing charge during dead-time, which could cost much efficiency (although there other harmful effects omitted). Therefore, should selected forward voltage less than 0.7V when conducting IMAX.
COUT Selection continuous mode, source 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. Always consult manufacturer there question. additional 0.1µF ceramic capacitor also required high frequency decoupling. selection COUT driven required effective series resistance (ESR). COUT must less than twice value RSENSE proper operation LTC1148 series: COUT Required 2RSENSE 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.
LTC1148 LTC1148-3.3/LTC1148-5
APPLICATIO ATIO
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 mode (see Figure When COUT made small, output ripple frequencies will large enough trip voltage comparator. This causes Burst Mode operation activated when LTC1148 series would normally continuous operation. effect most pronounced with values RSENSE improved operating higher frequencies with lower values output remains regulation times.
1000 50µH RSENSE 0.02 25µH RSENSE 0.02
COUT (µF)
50µH RSENSE 0.05 (VIN VOUT) VOLTAGE
LTC1148
Figure Minimum Value COUT
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% 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, three main sources usually account most losses LTC1148 series circuits: LTC1148 bias current, MOSFET gate charge current, losses. supply current current which flows into less gate charge current.
LTC1148 LTC1148-3.3/LTC1148-5
APPLICATIO ATIO
LTC1148 supply current 160µA load, increases proportionally with load constant 1.6mA after LTC1148 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 MOSFETs. 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 QP). typical gate charge N-channel power MOSFET 25nC, P-channel about twice that value. This results IGATECHG 7.5mA 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 MOSFETs 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 P-channel N-channel MOSFETs. MOSFETs have approximately same RDS(ON), then resistance MOSFET simply summed with resistances RSENSE obtain losses. example, each RDS(ON) 0.1, 0.15, RSENSE 0.05, then total resistance 0.3. This results losses ranging from output current increases from 0.5A losses cause efficiency roll-off high output currents. Figure shows efficiency losses typical LTC1148 series regulator being apportioned.
EFFICIENCY/LOSS
GATE CHARGE LTC1148 0.01 0.03 OUTPUT CURRENT
LTC1148
Figure Efficiency Loss
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 dominate high load currents. Other losses including COUT dissipative losses, MOSFET switching losses, Schottky conduction losses during dead-time, inductor core losses, generally account less than total additional loss. Design Example design example, assume (nominal), VOUT IMAX 200kHz; RSENSE, immediately calculated: RSENSE 100mV/2 0.05 tOFF (1/200kHz) (5/12)] 2.92µs 2.92µs/(1.3 104) 220pF LMIN 0.05 220pF 28µH Assume that MOSFET dissipations limited 250mW. 50°C thermal resistance each MOSFET 50°C/ then junction temperatures will 63°C
LTC1148 LTC1148-3.3/LTC1148-5
APPLICATIO ATIO
0.007(63 0.27. required RDS(ON) each MOSFET calculated: P-Ch RDS(ON) N-Ch RDS(ON) 12(0.25) 0.12 5(2)2 (1.27) 12(0.25) 0.085 7(2)2 (1.27)
P-channel requirement Si9430DY, while N-channel requirement exceeded Si9410DY. Note that most stringent requirement N-channel MOSFET with VOUT (i.e., short circuit). During continuous short circuit, worst-case N-channel dissipation rises ISC(AVG)2 RDS(ON) With 0.05 sense resistor ISC(AVG) will result, increasing 0.085 N-channel dissipation 450mW temperature 73°C. will require current rating least temperature, COUT will require 0.05 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) fMIN (1/2.92µs)[1 (5V/7V)] 98kHz
5V(0.12)(2A)2(1.27) 435mW
This last step necessary assure that power dissipation junction temperature P-channel exceeded. LTC1148 Adjustable Applications When output voltage other than 3.3V required, LTC1148 adjustable version used with external resistive divider from VOUT (see Figure regulated voltage determined
VOUT 1.25
prevent stray pickup 100pF capacitor suggested across located close LTC1148. 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 Auxiliary Windings Suppressing Burst Mode Operation LTC1148 synchronous switch removes normal limitation that power must drawn from inductor primary winding order extract power from auxiliary windings. With synchronous switching, auxiliary outputs loaded without regard primary output load, providing that loop remains continuous mode operation. Burst Mode operation suppressed output currents with simple external network which cancels 25mV minimum current comparator threshold. This technique also useful eliminating audible noise from certain types inductors high current (IOUT applications when they lightly loaded. external offset series with Sense subtract from built-in 25mV offset. example this technique shown Figure resistors inserted series with leads from sense resistor.
1000pF SENSE (PIN RSENSE VOUT SENSE (PIN
COUT
LTC1148
Figure Suppression Burst Mode Operation
LTC1148 LTC1148-3.3/LTC1148-5
APPLICATIO ATIO
With addition current generated through causing offset
VOFFSET VOUT
VOFFSET 25mV, minimum threshold will cancelled Burst Mode operation prevented from occurring. Since VOFFSET constant, maximum load current also decreased same offset. Thus, back same IMAX, value sense resistor must lower:
RSENSE 75mV IMAX
prevent noise spikes from erroneously tripping current comparator, 1000pF capacitor needed across pins Output Crowbar added feature using N-channel MOSFET synchronous switch ability crowbar output with same MOSFET. Pulling timing capacitor above 1.5V when output voltage greater than desired regulated value will turn "on" N-channel MOSFET. fault condition which causes output voltage above maximum allowable value detected external circuitry. Turning N-channel MOSFET when this fault detected will cause large currents flow blow system fuse. N-channel MOSFET needs sized will safely handle this overcurrent condition. typical delay from pulling high drive going high 250ns. Note: Under shutdown conditions, N-channel held pulling high will cause N-channel MOSFET crowbar output. simple N-channel used interface between overvoltage detect circuitry LTC1148 shown Figure
FROM CROWBAR DETECT CIRCUIT (ACTIVE WHEN VGATE WHEN VGATE GROUND) VN2222LL LTC1148
LTC1148
Figure Output Crowbar Interface
Troubleshooting Hints Since efficiency critical LTC1148 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 should occur with waveform periodically falling ground shown Figure
3.3V
CONTINUOUS MODE OPERATION 3.3V
Burst ModeOPERATION
LTC1148
Figure Waveforms
observed falling ground high output currents, indicates poor decoupling improper grounding. Refer Board Layout Checklist.
LTC1148 LTC1148-3.3/LTC1148-5
APPLICATIO ATIO
Board Layout Checklist
When laying printed circuit board, following checklist should used ensure proper operation LTC1148 series. These items also illustrated graphically layout diagram Figure Check following your layout: signal power grounds segregated? LTC1148 signal ground must return plate COUT. power ground returns source N-channel MOSFET, anode Schottky diode, plate CIN, which should have short lead lengths possible. Does LTC1148 Sense connect point close RSENSE plate COUT? adjustable applications, resistive divider must connected between plate COUT signal ground.
BOLD LINES INDICATE HIGH CURRENT PATHS
N-CHANNEL 10nF 3300pF 1000pF SENSE RSENSE SHUTDOWN COUT
P-DRIVE
N-DRIVE
LTC1148 P-GND S-GND (VFB) SENSE
LTC1148
Figure LTC1148 Layout Diagram (See Board Layout Checklist)
Sense Sense leads routed together with minimum trace spacing? 1000pF capacitor between pins should close possible LTC1148. Does plate connect source P-channel MOSFET closely possible? This capacitor provides current P-channel MOSFET. decoupling capacitor connected closely between power ground This capacitor carries MOSFET driver peak currents. Shutdown actively pulled ground during normal operation? Shutdown high impedance must allowed float.
P-CHANNEL
VOUT
OUTPUT DIVIDER REQUIRED WITH ADJUSTABLE VERSION ONLY
LTC1148 LTC1148-3.3/LTC1148-5
TYPICAL APPLICATIO
5.2V
390pF
SENSE
SENSE
1000pF RSENSE**
*COILTRONICS CTX100-4 Kool CORE **KRL SP-1/2-A1-0R100
Figure 5V/1A High Efficiency Regulator with Extended Input Voltage Range
Si9405DY
MBRS140T3
Si9410DY
300pF 3300pF
P-DRIVE
N-DRIVE
1000pF RSENSE** VOUT 3.3V/1A SHUTDOWN COUT 220µF OS-CON 50µH
LTC1148HV-3.3 P-GND S-GND
SHUTDOWN SENSE SENSE
*COILTRONICS CTX50-4 Kool CORE **IRC LR2010-01-R100-G
Figure High Efficiency 3.3V/1A Converter with Extended Input Voltage Range
Si9430DY MBRS140T3
100µF
Si9410DY
3300pF
P-DRIVE
N-DRIVE
SHUTDOWN COUT 220µF 100µH
LTC1148HV-5 P-GND S-GND
SHUTDOWN
VOUT 5V/1A
LTC1148
100µF
LTC1148
LTC1148 LTC1148-3.3/LTC1148-5
TYPICAL APPLICATIO
IRF9Z34 1N5818
390pF
10nF 3300pF
SENSE
SENSE
0.01µF RSENSE** 0.033
*COILTRONICS CTX50-2-MP **KRL SL-1-C1-0R033J
VOUT 1.25V VALUES SHOWN VOUT
Figure High Efficiency Adjustable Regulator
3.5V
MMSF3P02HD MBRS140T3
100µF
MMSF5N02HD
270pF 3300pF
P-DRIVE
N-DRIVE
1000pF RSENSE** 0.05 VOUT 3.3V/2A SHUTDOWN COUT 220µF 25µH
LTC1148L-3.3 P-GND S-GND
SHUTDOWN SENSE SENSE
*COILTRONICS CTX25-5 Kool CORE **IRC LR2512-01-R050-G
LTC1148
Figure Input Voltage, 3.3V/2A Dropout, High Efficiency Regulator
330µF
IRFZ34
P-DRIVE
N-DRIVE
SHUTDOWN 50µH
LTC1148 P-GND S-GND
SHUTDOWN
100pF
COUT 220µF OS-CON
VOUT 5V/3A
LTC1148
LTC1148 LTC1148-3.3/LTC1148-5
TYPICAL APPLICATIO
VIN: ACTIVE SHUTDOWN TP0610L Si9430DY 1N5818
560pF
10nF 6800pF
SENSE
SENSE
1000pF RSENSE** 0.05
*COILTRONICS CTX50-2-MP **KRL SL-1-C1-0R050J
Figure Input Voltage 5V/1A Regulator
Si9433DY MBRS140T3
100µF
Si9410DY
0.1µF 270pF 3300pF
P-DRIVE
N-DRIVE
1000pF RSENSE** 0.02 SHUTDOWN COUT 220µF VOUT 3.3V/4.5A 10µH
LTC1148-3.3 P-GND S-GND
SHUTDOWN SENSE SENSE
*COILTRONICS CTX10-5P **KRL SP-1-C1-0R020
LTC1148
Figure High Efficiency 3.3V/4.5A Converter
220µF
Si9410DY
P-DRIVE
N-DRIVE
200pF VOUT -5V/1.4A 50µH
LTC1148 P-GND S-GND
SHUTDOWN
COUT 220µF OS-CON
LTC1148
LTC1148 LTC1148-3.3/LTC1148-5
TYPICAL APPLICATIO
P-DRIVE
LTC1148
0.1µF RSENSE** 0.082 SHUTDOWN
390pF 1N4148 VOUT
SHUTDOWN SENSE
3300pF
SENSE
*LOW REQUIRED **KRL NP-1A-C1-0R082J
LTC1148
Figure Input Voltage 5V/1A Regulator
MMSF3P02HD
MBRS140T3
MMSF5N02HD
390pF 10nF 3300pF
P-DRIVE
N-DRIVE
1000pF RSENSE** 0.05 SHUTDOWN 50µH
LTC1148
SHUTDOWN SENSE SENSE
100pF
*COILTRONICS CTX50-2-MP **KRL SL-1-C1-0R050R
Figure Logic Selectable 5V/1A 3.3V/1A 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.
Si9430DY
100µF
N-DRIVE
50µH 220µF* OS-CON 50µH 1N5818
VOUT 5V/1A
Si9410DY
100pF
COUT 220µF OS-CON
5.2V
100µF
VN2222LL VOUT 3.3V VOUT
COUT 220µF OS-CON
LTC1148
VOUT 3.3V/2A 5V/2A
LTC1148 LTC1148-3.3/LTC1148-5
TYPICAL APPLICATIO
1N4148 2N3906 2N2222 MUR110 N-CH IRFZ44 1N5818 470nF
EFFICIENCY
VN2222LL
820pF 3300pF
P-DRIVE
N-DRIVE
33µH
LTC1148HV-5 P-GND S-GND
SHUTDOWN SENSE SENSE
SHUTDOWN COUT 2200µF
1000pF *COILTRONICS CTX33-10-KM **DALE LVR-3-0.01
RSENSE** 0.01 VOUT 5V/8A
LTC1148
Figure N-Channel 5V/8A High Efficiency Regulator (Burst Mode Operation Suppressed)
PACKAGE DESCRIPTIO
0.300 0.325 (7.620 8.255)
Dimensions inches (millimeters) unless otherwise noted.
0.770* (19.558)
0.130 0.005 (3.302 0.127) 0.015 (0.380)
Package 14-Lead Plastic
0.009 0.015 (0.229 0.381) +0.025 0.325 -0.015 +0.635 8.255 -0.381 0.075 0.015 (1.905 0.381) 0.100 0.010 (2.540 0.254)
0.125 (3.175)
0.010 0.020 (0.254 0.508) 0.008 0.010 (0.203 0.254)
0.053 0.069 (1.346 1.752)
Package 14-Lead Plastic SOIC
0.016 0.050 0.406 1.270
0.014 0.019 (0.355 0.483)
*THESE DIMENSIONS INCLUDE MOLD FLASH PROTRUSIONS. MOLD FLASH PROTRUSIONS SHALL EXCEED 0.006 INCH (0.15mm).
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, 95035-7487
(408) 432-1900 FAX: (408) 434-0507 TELEX: 499-3977
2700µF
N-CH IRFZ44
LOAD CURRENT
LTC1148
Figure N-Channel 5V/8A Efficiency
additional high efficiency application circuits, Application Note
0.045 0.065 (1.143 1.651)
0.065 (1.651) 0.018 0.003 (0.457 0.076)
0.255 0.015* (6.477 0.381)
*THESE DIMENSIONS INCLUDE MOLD FLASH PROTRUSIONS. MOLD FLASH PROTURSIONS SHALL EXCEED 0.010 INCH (0.254mm).
0.337 0.344* (8.560 8.738) 0.004 0.010 (0.101 0.254)
0.050 (1.270)
0.228 0.244 (5.791 6.197)
0.150 0.157* (3.810 3.988)
LT/GP 1294 PRINTED
LINEAR TECHNOLOGY CORPORATION 1993

DESCRIPTIO Ultra-High Efficiency: Over Possible Current-Mode Oper

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