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LT1074/LT1076

sn1074 1074fds

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

APPLICATIO S

FEATURES

DESCRIPTIO

TYPICAL APPLICATIO

Step-Down Switching

Regulator

The LT

1074 is a 5A (LT1076 is rated at 2A) monolithic

bipolar switching regulator which requires only a few

external parts for normal operation. The power switch, all

oscillator and control circuitry, and all current limit com-

■

Buck Converter with Output Voltage Range of 2.5V

to 50V

■

Tapped-Inductor Buck Converter with 10A Output

at 5V

■

Positive-to-Negative Converter

■

Negative Boost Converter

■

Multiple Output Buck Converter

■

5A Onboard Switch (LT1074)

■

Operates Up to 60V Input

■

100kHz Switching Frequency

■

Greatly Improved Dynamic Behavior

■

Available in Low Cost 5 and 7-Lead Packages

■

Only 8.5mA Quiescent Current

■

Programmable Current Limit

■

Micropower Shutdown Mode

ponents, are included on the chip. The topology is a classic

positive “buck” configuration but several design innova-

tions allow this device to be used as a positive-to-negative

converter, a negative boost converter, and as a flyback

converter. The switch output is specified to swing 40V

below ground, allowing the LT1074 to drive a tapped-

inductor in the buck mode with output currents up to 10A.

The LT1074 uses a true analog multiplier in the feedback

loop. This makes the device respond nearly instanta-

neously to input voltage fluctuations and makes loop gain

independent of input voltage. As a result, dynamic behav-

ior of the regulator is significantly improved over previous

designs.

On-chip pulse by pulse current limiting makes the LT1074

nearly bust-proof for output overloads or shorts. The input

voltage range as a buck converter is 8V to 60V, but a self-

boot feature allows input voltages as low as 5V in the

inverting and boost configurations.

The LT1074 is available in low cost TO-220 or DD packages

with frequency pre-set at 100kHz and current limit at 6.5A

(LT1076 = 2.6A). A 7-pin TO-220 package is also available

which allows current limit to be adjusted down to zero. In

addition, full micropower shutdown can be programmed.

See Application Note 44 for design details.

A fixed 5V output, 2A version is also available. See LT1076-5.

Buck Converter Efficiency

Basic Positive Buck Converter

GND

LT1074

500

µF

2.8k

2.21k

MBR745*

10V TO 40V

†

200µF

2.7k

0.01

µF

L1**

50µH (LT1074)

100

µH (LT1076)

USE MBR340 FOR LT1076

COILTRONICS #50-2-52 (LT1074)

#100-1-52 (LT1076)

PULSE ENGINEERING, INC.

#PE-92114 (LT1074)

#PE-92102 (LT1076)

HURRICANE #HL-AK147QQ (LT1074)

#HL-AG210LL (LT1076)

RIPPLE CURRENT RATING ≥ I

OUT

†

LT1074•TA01

25V

OUTPUT LOAD CURRENT (A)

EFFICIENCY (%)

1234

LT1074•TPC27

100

L = 50µH TYPE 52 CORE

DIODE = MBR735

V = 5V, V = 15V

OUT IN

V = 12V, V = 20V

OUT IN

LT1074

Summary of content (16 pages)

PAGE 1
LT1074/LT1076 Step-Down Switching Regulator ponents, are included on the chip. The topology is a classic positive “buck” configuration but several design innovations allow this device to be used as a positive-to-negative converter, a negative boost converter, and as a flyback converter. The switch output is specified to swing 40V below ground, allowing the LT1074 to drive a tappedinductor in the buck mode with output currents up to 10A.
PAGE 2
LT1074/LT1076 U W W W ABSOLUTE AXI U RATI GS (Note 1) Input Voltage LT1074/ LT1076 .................................................. 45V LT1074HV/LT1076HV ......................................... 64V Switch Voltage with Respect to Input Voltage LT1074/ LT1076 .................................................. 64V LT1074HV/LT1076HV ......................................... 75V Switch Voltage with Respect to Ground Pin (VSW Negative) LT1074/LT1076 (Note 7) .....................................
PAGE 3
LT1074/LT1076 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. Tj = 25°C, VIN = 25V, unless otherwise noted. PARAMETER CONDITIONS Switch “On” Voltage (Note 2) LT1074 ISW = 1A, Tj ≥ 0°C ISW = 1A, Tj < 0°C ISW = 5A, Tj ≥ 0°C ISW = 5A, Tj < 0°C LT1076 ISW = 0.
PAGE 4
LT1074/LT1076 ELECTRICAL CHARACTERISTICS Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: To calculate maximum switch “on” voltage at currents between low and high conditions, a linear interpolation may be used. Note 3: A feedback pin voltage (VFB) of 2.5V forces the VC pin to its low clamp level and the switch duty cycle to zero. This approximates the zero load condition where duty cycle approaches zero.
PAGE 5
LT1074/LT1076 U W BLOCK DIAGRA DESCRIPTIO A switch cycle in the LT1074 is initiated by the oscillator setting the R/S latch. The pulse that sets the latch also locks out the switch via gate G1. The effective width of this pulse is approximately 700ns, which sets the maximum switch duty cycle to approximately 93% at 100kHz switching frequency. The switch is turned off by comparator C1, which resets the latch.
PAGE 6
LT1074/LT1076 U W TYPICAL PERFOR A CE CHARACTERISTICS VC Pin Characteristics Feedback Pin Characteristics 2.0 500 150 1.5 400 VFB ADJUSTED FOR IC = 0 AT VC = 2V 0 SLOPE ≈ 400kΩ –100 VFB ≥ 2.5V 200 CURRENT (µA) 50 –50 300 1.0 CURRENT (mA) 100 CURRENT (mA) VC Pin Characteristics 200 0.5 0 –0.5 –400 –2.
PAGE 7
LT1074/LT1076 U W TYPICAL PERFOR A CE CHARACTERISTICS Reference Voltage vs Temperature Supply Current (Shutdown) 3.0 Tj = 25°C 2.24 250 2.5 150 100 “ON” VOLTAGE (V) 2.23 200 VOLTAGE (V) 2.22 2.21 2.20 LT1074 1.5 LT1076 1.0 2.18 2.
PAGE 8
LT1074/LT1076 U U PI DESCRIPTIO S VIN PIN The VIN pin is both the supply voltage for internal control circuitry and one end of the high current switch. It is important, especially at low input voltages, that this pin be bypassed with a low ESR, and low inductance capacitor to prevent transient steps or spikes from causing erratic operation. At full switch current of 5A, the switching transients at the regulator input can get very large as shown in Figure 1.
PAGE 9
LT1074/LT1076 U U PI DESCRIPTIO S current is well controlled even when switch duty cycle must be extremely low. Theoretical switch “on” time for a buck converter in continuous mode is: tON = VOUT + VD VIN • f VD = Catch diode forward voltage ( ≈ 0.5V) f = Switching frequency At f = 100kHz, tON must drop to 0.2µs when VIN = 25V and the output is shorted (VOUT = 0V). In current limit, the LT1074 can reduce tON to a minimum value of ≈0.6µs, much too long to control current correctly for VOUT = 0.
PAGE 10
LT1074/LT1076 U U PI DESCRIPTIO S Hysteresis in undervoltage lockout may be accomplished by connecting a resistor (R3) from the ILIM pin to the shutdown pin as shown in Figure 7. D1 prevents the shutdown divider from altering current limit. V IN 300 µ A 10 µ A SHUTDOWN PIN – ILIM PIN C1 2.3V VIN R1 + SHUT Q1 6V – EXTERNAL CLIM R3 I LIM R2 C2 0.3V LT1074 D1* OPTIONAL CURRENT LIMIT RESISTOR + LT1074•PD09 *1N4148 TO TOTAL REGULATOR SHUTDOWN Figure 7.
PAGE 11
LT1074/LT1076 U U PI DESCRIPTIO S ILIM PIN The ILIM pin is used to reduce current limit below the preset value of 6.5A. The equivalent circuit for this pin is shown in Figure 8. TO LIMIT CIRCUIT from forcing current back into the ILIM pin. To calculate a value for RFB, first calculate RLIM, the RFB: RFB = VIN 320 µ A *Change 0.44 to 0.16, and 0.5 to 0.18 for LT1076. D2 Example: ILIM = 4A, ISC = 1.5A, RLIM = (4)(2k) + 1k = 9k Q1 D1 R1 8K (ISC − 0.44 *)(RL ) (RL in kΩ) 0.5 *(RL − 1kΩ) − ISC 4.
PAGE 12
LT1074/LT1076 U U PI DESCRIPTIO S 5.8V Q4 90 µ A 90 µ A Q3 50 µ A Q2 Q1 X1.8 VC D1 FB 50 µ A 90 µ A D2 Q6 2.21V EXTERNAL FREQUENCY COMPENSATION RC 140 µ A CC 300 Ω ALL CURRENTS SHOWN ARE AT NULL CONDITION LT1074 • PD11 Figure 10.
PAGE 13
LT1074/LT1076 U TYPICAL APPLICATIO S Tapped-Inductor Buck Converter L2 5µH L1* VIN 20V† TO 35V VIN VSW 3 D2 35V 5W LT1074HV GND + VC C3 200µF 50V R1 2.8k D1** + FB R3 1k C2 0.2µF VOUT 5V, 10A† 1 D3 1N5819 R2 2.21k C1 4400µF (2 EA 2200µF, 16V) + C4 390µF 16V 0.01µF * PULSE ENGINEERING #PE±65282 ** MOTOROLA MBR2030CTL † IF INPUT VOLTAGE IS BELOW 20V, MAXIMUM OUTPUT CURRENT WILL BE REDUCED. SEE AN44 LT1074 •TA02 Positive-to-Negative Converter with 5V Output + + VIN 4.
PAGE 14
LT1074/LT1076 U PACKAGE DESCRIPTIO K Package 4-Lead TO-3 Metal Can (Reference LTC DWG # 05-08-1311) 0.760 – 0.775 (19.30 – 19.69) 0.320 – 0.350 (8.13 – 8.89) 0.060 – 0.135 (1.524 – 3.429) 0.420 – 0.480 (10.67 – 12.19) 0.038 – 0.043 (0.965 – 1.09) 1.177 – 1.197 (29.90 – 30.40) 0.655 – 0.675 (16.64 – 19.05) 0.470 TP P.C.D. 0.151 – 0.161 (3.84 – 4.09) DIA 2 PLC 0.167 – 0.177 (4.24 – 4.49) R 0.490 – 0.510 (12.45 – 12.
PAGE 15
LT1074/LT1076 U PACKAGE DESCRIPTIO R Package 7-Lead Plastic DD Pak (Reference 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) ( +0.203 0.102 –0.102 BOTTOM VIEW OF DD PAK HATCHED AREA IS SOLDER PLATED COPPER HEAT SINK ) 0.095 – 0.115 (2.413 – 2.921) 0.075 (1.905) 0.300 (7.620) +0.008 0.004 –0.
PAGE 16
LT1074/LT1076 U TYPICAL APPLICATIO Negative Boost Converter R1 12.7k 100pF VIN VFB R2 2.21k LT1074 C3 200µF 15V VC GND + VSW + L1 25µH C2 1nF R3 750Ω 0.01µF D1* C1 1000µF 25V VOUT –15V** VIN –5V TO –15V * MBR735 ** IOUT (MAX) = 1A TO 3A DEPENDING ON INPUT VOLTAGE. SEE AN44 + 100µF 5µH OPTIONAL OUTPUT FILTER LT1074 • TA04 U PACKAGE DESCRIPTIO T7 Package 7-Lead Plastic TO-220 (Standard) (Reference LTC DWG # 05-08-1422) 0.165 – 0.180 (4.191 – 4.572) 0.147 – 0.155 (3.734 – 3.937) DIA 0.