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LT1371

500kHz High Efficiency

3A Switching Regulator

The LT

1371 is a monolithic high frequency current mode

switching regulator. It can be operated in all standard

switching configurations including boost, buck, flyback,

forward, inverting and “Cuk.” A 3A high efficiency switch

is included on the die, along with all oscillator, control and

protection circuitry.

The LT1371 typically consumes only 4mA quiescent

current and has higher efficiency than previous parts.

High frequency switching allows for very small inductors

to be used.

New design techniques increase flexibility and maintain

ease of use. Switching is easily synchronized to an exter-

nal logic level source. A logic low on the Shutdown pin

reduces supply current to 12µA. Unique error amplifier

circuitry can regulate positive or negative output voltage

while maintaining simple frequency compensation tech-

niques. Nonlinear error amplifier transconductance re-

duces output overshoot on start-up or overload recovery.

Oscillator frequency shifting protects external compo-

nents during overload conditions.

DESCRIPTION

■

Faster Switching with Increased Efficiency

■

Uses Small Inductors: 4.7µH

■

All Surface Mount Components

■

Low Minimum Supply Voltage: 2.7V

■

Quiescent Current: 4mA Typ

■

Current Limited Power Switch: 3A

■

Regulates Positive or Negative Outputs

■

Shutdown Supply Current: 12µA Typ

■

Easy External Synchronization

■

Boost Regulators

■

Laptop Computer Supplies

■

Multiple Output Flyback Supplies

■

Inverting Supplies

APPLICATIONS

TYPICAL APPLICATION

5V to 12V Boost Converter

OUTPUT CURRENT (A)

0.01

EFFICIENCY (%)

0.1 1

LT1371 • TA02

100

= 5V

12V Output Efficiency

LT1371

GND

LT1371 • TA01

S/S

L1*

4.7µH

C1**

22µF

25V

C4**

22µF

25V

× 2

C2

0.047µF

C3

0.0047µF



R3

R2

6.19k

R1

53.6k

OUT

†



12V

D1

MBRS330T3

OFF

*



COILCRAFT DO3316P-472 (4.7µH),

DO3316P-103 (10µH) OR 

SUMIDA CD104-100MC (10µH)

AVX TPSD226M025R0200



+ +

L1

4.7µH

10µH

OUT



0.7A

0.8A

†

MAX I

OUT

FEATURES

, LTC and LT are registered trademarks of Linear Technology Corporation.

Summary of content (16 pages)

PAGE 1
LT1371 500kHz High Efficiency 3A Switching Regulator U DESCRIPTION FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ The LT ®1371 is a monolithic high frequency current mode switching regulator. It can be operated in all standard switching configurations including boost, buck, flyback, forward, inverting and “Cuk.” A 3A high efficiency switch is included on the die, along with all oscillator, control and protection circuitry. Faster Switching with Increased Efficiency Uses Small Inductors: 4.
PAGE 2
LT1371 W W W AXI U U ABSOLUTE RATI GS Supply Voltage ....................................................... 30V Switch Voltage LT1371 ............................................................... 35V LT1371HV .......................................................... 42V S/S, SHDN, SYNC Pin Voltage ................................ 30V Feedback Pin Voltage (Transient, 10ms) .............. ±10V Feedback Pin Current ...........................................
PAGE 3
LT1371 ELECTRICAL CHARACTERISTICS VIN = 5V, VC = 0.6V, VFB = VREF, VSW, S/S, SHDN, SYNC and NFB pins open, unless otherwise noted. SYMBOL PARAMETER CONDITIONS VNFB Negative Feedback Reference Voltage Measured at Negative Feedback Pin Feedback Pin Open, VC = 0.8V – 2.540 ● – 2.570 INFB Negative Feedback Input Current VNFB = VNFR ● Negative Feedback Reference Voltage Line Regulation 2.7V ≤ VIN ≤ 25V, VC = 0.
PAGE 4
LT1371 U W TYPICAL PERFORMANCE CHARACTERISTICS Switch Saturation Voltage vs Switch Current Switch Current Limit vs Duty Cycle 25°C 0.8 0.7 0.6 0.5 –55°C 0.4 0.3 0.2 6 3.0 5 2.8 25°C AND 125°C 4 –55°C 3 2 1 2.4 2.2 2.0 0 2.0 18 1.8 14 1.4 12 1.2 10 1.0 SHUTDOWN DELAY 0.8 6 0.6 4 0.4 2 0.2 0 –50 –25 0 SHUTDOWN THRESHOLD (V) 1.6 MINIMUM SYNCHRONIZATION VOLTAGE (VP-P) 20 8 0 25 50 75 100 125 150 TEMPERATURE (°C) 3.0 400 2.5 2.0 1.5 1.0 0.
PAGE 5
LT1371 U W TYPICAL PERFORMANCE CHARACTERISTICS VC Pin Threshold and High Clamp Voltage vs Temperature Feedback Input Current vs Temperature 2.4 1.8 1.6 1.4 1.2 VC THRESHOLD 1.0 0.8 700 NEGATIVE FEEDBACK INPUT CURRENT (µA) FEEDBACK INPUT CURRENT (nA) VC HIGH CLAMP 2.0 VC PIN VOLTAGE (V) 0 800 2.2 VFB =VREF 600 500 400 300 200 100 0.6 0.
PAGE 6
LT1371 W BLOCK DIAGRAM VIN SHUTDOWN DELAY AND RESET SHDN SW LOW DROPOUT 2.3V REG ANTI-SAT S/S* SYNC SYNC LOGIC OSC DRIVER SWITCH 5:1 FREQUENCY SHIFT + 100k NFB NFBA – COMP 50k – FB + 1.245V REF GND SENSE + EA IA VC AV ≈ 6 *R AND T7 PACKAGES ONLY 0.04Ω – GND LT1371 • BD U OPERATION The LT1371 is a current mode switcher. This means that switch duty cycle is directly controlled by switch current rather than by output voltage.
PAGE 7
LT1371 U U W U APPLICATIO S I FOR ATIO Unique error amplifier circuitry allows the LT1371 to directly regulate negative output voltages. The negative feedback amplifier’s 100k source resistor is brought out for negative output voltage sensing. The NFB pin regulates at – 2.49V while the amplifier output internally drives the FB pin to 1.245V. This architecture, which uses the same main error amplifier, prevents duplicating functions and maintains ease of use.
PAGE 8
LT1371 U U W U APPLICATIO S I FOR ATIO Shutdown and Synchronization The 7-pin R and T7 package devices have a dual function S/S pin which is used for both shutdown and synchronization. The SW package device has both a Shutdown (SHDN) pin and a Synchronization (SYNC) pin which can be used separately or tied together. These pins are logic level compatible and can be pulled high, tied to VIN or left floating for normal operation.
PAGE 9
LT1371 U U W U APPLICATIO S I FOR ATIO saturate abruptly and other core materials fall in between. The following formula assumes continuous mode operation but it errs only slightly on the high side for discontinuous mode, so it can be used for all conditions. ) ) V (V –V ) V IPEAK = (IOUT) OUT + IN OUT IN VIN 2(f)(L)(VOUT) VIN = Minimum Input Voltage f = 500kHz Switching Frequency 3.
PAGE 10
LT1371 U U W U APPLICATIO S I FOR ATIO Output Capacitor Ripple Current (RMS) DC IRIPPLE (RMS) = IOUT 1 – DC = IOUT VOUT – VIN VIN DC = Switch Duty Cycle Input Capacitors The input capacitor of a boost converter is less critical due to the fact that the input current waveform is triangular and does not contain large squarewave currents as is found in the output capacitor. Capacitors in the range of 10µF to 100µF, with an ESR of 0.2Ω or less, work well up to full 3A switch current.
PAGE 11
LT1371 U U W U APPLICATIO S I FOR ATIO maximum output load current and will also be increased if poor quality (high ESR) output capacitors are used. The addition of a 0.0047µF capacitor on the VC pin reduces switching frequency ripple to only a few millivolts. A low value for RC will also reduce VC pin ripple, but loop phase margin may be inadequate. FB Layout Considerations VC For maximum efficiency, LT1371 switch rise and fall times are made as short as possible.
PAGE 12
LT1371 U U W U APPLICATIO S I FOR ATIO VIN T7 Package Layout Considerations Electrical connection to the tab of a T7 package is required for proper device operation. If the tab is tied directly to the ground plane (Figure 4) no other considerations are necessary. If the tab is not connected directly to the ground plane, as in a vertically mounted application, a separate electrical connection from the tab to a “floating node” is required.
PAGE 13
LT1371 U TYPICAL APPLICATIONS N Positive-to-Negative Converter with Direct Feedback VIN 2.7V TO 13V + 2 D2 P6KE-15A D3 1N4148 1 • VIN OFF VSW S/S R2 6.19k 1% T1* C1 100µF ON 4 + • R2 2.49k 1% NFB + –VOUT† –5V C1 22µF ON 1N4148 VIN VSW FB OFF MBRS360T3 T1* 2, 3 7 + P6KE-20A • S/S 8, 9 •4 10 • LT1371 NFB C2 0.047µF R1 2k C3 0.0047µF VIN 2.7V TO 10V R3 2.49k 1% GND R1 68.
PAGE 14
LT1371 U TYPICAL APPLICATIONS N 20W CCFL Supply 47pF LAMP 1N4148 11 L1 5 8 1 4 3 2 + 0.47µF 22µF Q2 Q1 150Ω INTENSITY CONTROL L2 15µH MUR405 1N4148 1N4148 VIN 9V TO 15V 22k VSW VIN + 10k FB LT1371 140Ω 1µF 2.2µF GND VC L1=COILTRONICS CTX02-11128 L2=COILCRAFT DO3316P-153 Q1, Q2=ZETEX ZTX849, ZDT1048 OR ROHM 2SC5001 0.47 µF=WIMA 3X 0.15 µF TYPE MKP-20 COILTRONICS (407) 241-7876 + LT1371 • TA07 2.2µF Laser Power Supply 1800pF 10kV 0.
PAGE 15
LT1371 U PACKAGE DESCRIPTION Dimensions in inches (millimeters) unless otherwise noted. R Package 7-Lead Plastic DD Pak (LTC DWG # 05-08-1462) 0.256 (6.502) 0.060 (1.524) TYP 0.060 (1.524) 0.390 – 0.415 (9.906 – 10.541) 0.165 – 0.180 (4.191 – 4.572) 0.045 – 0.055 (1.143 – 1.397) 15° TYP 0.060 (1.524) 0.183 (4.648) 0.059 (1.499) TYP 0.330 – 0.370 (8.382 – 9.398) ( +0.008 0.004 –0.004 +0.203 0.102 –0.102 ) 0.095 – 0.115 (2.413 – 2.921) 0.075 (1.905) 0.300 (7.620) ( +0.012 0.143 –0.
PAGE 16
LT1371 U PACKAGE DESCRIPTION Dimensions in inches (millimeters) unless otherwise noted. T7 Package 7-Lead Plastic TO-220 (Standard) (LTC DWG # 05-08-1422) 0.165 – 0.180 (4.293 – 4.572) 0.147 – 0.155 (3.734 – 3.937) DIA 0.390 – 0.415 (9.906 – 10.541) 0.045 – 0.055 (1.143 – 1.397) 0.230 – 0.270 (5.842 – 6.858) 0.570 – 0.620 (14.478 – 15.748) 0.460 – 0.500 (11.684 – 12.700) 0.330 – 0.370 (8.382 – 9.398) 0.620 (15.75) TYP 0.700 – 0.728 (17.780 – 18.491) 0.152 – 0.202 0.260 – 0.320 (3.860 – 5.