LT1076.PDF

LT1074/LT1076

Step-Down Switching

Regulator

FEATURE

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.

5A On-Board Switch (LT1074)

100kHz Switching Frequency

Greatly Improved Dynamic Behavior

Available in Low Cost 5 and 7-Lead Packages

Only 8.5mA Quiescent Current

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.

Programmable Current Limit

Operates Up to 60V Input

Micropower Shutdown Mode

PPLICATI

Buck Converter with Output Voltage Range of 2.5V

to 50V

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.

Tapped-Inductor Buck Converter with 10A Output

at 5V

Positive-to-Negative Converter

Negative Boost Converter

Multiple Output Buck Converter

The LT1074 is available in low cost TO-220 or TO-3

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.

ESCRIPTIO

The LT1074 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-

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

positive “buck” configuration but several design innova-

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

TYPICAL

PPLICATI

Buck Converter Efficiency

Basic Positive Buck Converter

L1**

50 H (LT1074)

100 H (LT1076)

LT1074

100

V = 12V, V = 20V

OUT

V IN

V SW

*USE MBR340 FOR LT1076

**COILTRONICS #50-2-52 (LT1074)

#100-1-52 (LT1076)

10V TO 40V

LT1074

MBR745*

2.8k

PULSE ENGINEERING, INC.

#PE-92114 (LT1074)

#PE-92102 (LT1076)

HURRICANE #HL-AK147QQ (LT1074)

#HL-AG210LL (LT1076)

V = 5V, V = 15V

OUT

GND

V C

2.21k

2.7k

0.01 F

RIPPLE CURRENT RATING I / 2

OUT

L = 50 H TYPE 52 CORE

DIODE = MBR735

200 F

500 F

25V

LT1074 • TA01

OUTPUT LOAD CURRENT (A)

LT1074 • TPC27

†

LT1074/LT1076

BSOLUTEI

I S

Input Voltage

LT1074/ LT1076 .................................................. 45V

LT1074HV/76HV.................................................. 64V

Switch Voltage with Respect to Input Voltage

LT1074/ 76 .......................................................... 64V

LT1074HV/76HV.................................................. 75V

Switch Voltage with Respect to Ground Pin (V SW Negative )

LT1074/76 (Note 6) ............................................. 35V

LT1074HV/76HV (Note 6) .................................... 45V

Feedback Pin Voltage ..................................... – 2V, +10V

Shutdown Pin Voltage (Not to Exceed V IN ) .............. 40V

I LIM Pin Voltage (Forced) ............................................ 5.5V

Maximum Operating Ambient Temperature Range

LT1074C/76C, LT1074HVC/76HVC ............ 0

C to 70

LT1074I/76I, LT1074HVI/76HVI ............. – 40

C to 85

LT1074M/76M, LT1074HVM/76HVM ... – 55

C to 125

Maximum Operating Junction Temperature Range

LT1074C/76C, LT1074HVC/76HVC .......... 0

C to 125

LT1074I/76I, LT1074HVI/76HVI ........... – 40

C to 125

LT1074M/76M, LT1074HVM/76HVM ... – 55

C to 150

Maximum Storage Temperature ................ – 65

C to 150

Lead Temperature (Soldering, 10 sec) ..................... 300

PACKAGE/ORDER I FOR ATIO

FRONT VIEW

ORDER PART

NUMBER

ORDER PART

NUMBER

V IN

V SW

GND

V C

FB/SENSE

BOTTOM VIEW

LT1076CQ

LT1074CK

LT1074HVCK

LT1074MK

LT1074HVMK

LT1076CK

LT1076HVCK

LT1076MK

LT1076HVMK

V C

V IN

Q PACKAGE

5-LEAD PLASTIC DD

CASE IS GND

LT1076:

q JC = 4°C/W,

q JA = 30°C/W*

FRONT VIEW

V SW

LT1076CR

LT1076HVCR

SHDN

V C

FB/SENSE

GND

I LIM

V SW

V IN

K PACKAGE, 4-LEAD TO-3 METAL CAN

q JA = 35°C/W

LT1076: q JC = 4°C/W, q JA = 35°C/W

q JC = 2.5°C/W,

R PACKAGE

7-LEAD PLASTIC DD

LT1076:

q JC = 4°C/W,

q JA = 30°C/W*

FRONT VIEW

LT1074CT

LT1074HVCT

LT1074IT

LT1074HVIT

LT1076CT

LT1076HVCT

LT1076IT

GND

FRONT VIEW

LT1074CY

LT1074HVCY

LT1074IY

LT1074HVIY

LT1076CY

LT1076HVCY

V C

SHUTDOWN

GND

I LIM

V IN

T PACKAGE, 5-LEAD T0-220

LEADS ARE FORMED STANDARD FOR

STRAIGHT LEADS, ORDER FLOW 06

V SW

Y PACKAGE, 7-LEAD TO-220

LT1074:

q JC = 2.5

C/W,

q JA = 50

C/W

LT1074:

q JC =

2.5°C/W,

q JA = 50°C/W

LT1076:

q JC = 4°C/W,

q JA = 50°C/W

LT1076:

q JC = 4

C/W,

q JA = 50

C/W

* Assumes package is soldered to 0.5 IN 2 of 1 oz. copper over internal ground

plane or over back side plane.

ELECTRICAL C

HARA TERISTICS

T j = 25

C, V IN = 25V, unless otherwise noted.

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

Switch “On” Voltage (Note 1)

LT1074 I SW = 1A, T j ³

1.85

I SW = 1A, T j < 0

2.1

I SW = 5A, T j ³

2.3

I SW = 5A, T j < 0

2.5

LT1076 I SW = 0.5A

1.2

I SW = 2A

1.7

LT1074:

LT1074/LT1076

ELECTRICAL C

HARA TERISTICS T j = 25 ° C, V IN = 25V, unless otherwise noted.

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

Switch “Off” Leakage

LT1074 V IN £ 25V, V SW = 0

300

m A

V IN = V MAX, V SW = 0 (Note 7)

500

LT1076 V IN = 25V, V SW = 0

150

m A

V IN = V MAX, V SW = 0 (Note 7)

250

m A

Supply Current (Note 2)

V FB = 2.5V, V IN £

40V

8.5

40V < V IN < 60V

V SHUT = 0.1V (Device Shutdown) (Note 8)

140

300

Minimum Supply Voltage

Normal Mode

7.3

Startup Mode (Note 3)

3.5

4.8

Switch Current Limit (Note 4)

LT1074 I LIM Open

5.5

6.5

8.5

R LIM = 10k (Note 5)

4.5

R LIM = 7k (Note 5)

LT1076 I LIM Open

2.6

3.2

R LIM = 10k (Note 5)

1.8

R LIM = 7k (Note 5)

1.2

Maximum Duty Cycle

Switching Frequency

100

110

kHz

T j £

125

120

kHz

T j > 125 ° C

125

kHz

V FB = 0V through 2k W (Note 4)

kHz

Switching Frequency Line Regulation

V IN £

V MAX (Note 7)

0.03

0.1

%/V

Error Amplifier Voltage Gain (Note 6)

1V £ V C £ 4V

2000

V/V

Error Amplifier Transconductance

3700

5000

8000

m mho

Error Amplifier Source and Sink Current

Source (V FB = 2V)

100

140

225

Sink (V FB = 2.5V)

0.7

1.6

Feedback Pin Bias Current

V FB = V REF

0.5

Reference Voltage

V C = 2V

2.155

2.21

2.265

Reference Voltage Tolerance

V REF (Nominal) = 2.21V

± 0.5

± 1.5

All Conditions of Input Voltage, Output

2.5

Voltage, Temperature and Load Current

Reference Voltage Line Regulation

8V £ V IN £ V MAX (Note 7)

0.005

0.02

%/V

V C Voltage at 0% Duty Cycle

1.5

Over Temperature

– 4

mV/

Multiplier Reference Voltage

Shutdown Pin Current

V SH = 5V

V SH

V THRESHOLD (

2.5V)

Shutdown Thresholds

Switch Duty Cycle = 0

2.2

2.45

2.7

Fully Shut Down

0.1

0.3

0.5

Thermal Resistance Junction to Case

LT1074

2.5

C/W

LT1076

4.0

° C/W

The

denotes the specifications which apply over the full operating temperature

Note 4: Switch frequency is internally scaled down when the feedback pin voltage

is less than 1.3V to avoid extremely short switch on times. During testing, V FB is

adjusted to give a minimum switch on time of 1

range.

Note 1: To calculate maximum switch “on” voltage at currents between low and

high conditions, a linear interpolation may be used.

Note 2: A feedback pin voltage (V FB ) of 2.5V forces the V C pin to its low clamp

level and the switch duty cycle to zero. This approximates the zero load condition

where duty cycle approaches zero.

Note 3: Total voltage from V IN pin to ground pin must be

Note 5: I LIM

R LIM – 1k

(LT1074)

I LIM »

(LT1076).

R LIM – 1k

5.5k

8V after startup for

Note 6: Switch to input voltage limitation must also be observed.

Note 7: V MAX = 40V for the LT1074/76 and 60V for the LT1074HV/76HV.

Note 8: Does not include switch leakage.

proper regulation.

LT1074/LT1076

BLOCK

DAGRA

INPUT SUPPLY

LT1074

10 A

0.3V

320 A

REGULATOR

AND BIAS

500

-POWER

SHUTDOWN

6V TO ALL

CIRCUITRY

–

CURRENT

LIMIT

COMP

0.04

CURRENT

LIMIT

SHUTDOWN

2.35V

250

–

SHUTDOWN*

I *

4.5V

10k

FREQ SHIFT

100kHz

OSCILLATOR

R/S

LATCH

SYNC

3V(p-p)

V IN

400

ANALOG

MULTIPLIER

ERROR

AMP

–

PULSE WIDTH

COMPARATOR

2.21V

–

SWITCH

OUTPUT

(V )

24V (EQUIVALENT)

LT1076

0.1

*AVAILABLE ON PACKAGES WITH PIN

COUNTS GREATER THAN 5.

100

SWITCH

OUTPUT (V )

LT1074 • BD01

LIM

LT1074/LT1076

ESCRIPTIO

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 switch-

ing frequency. The switch is turned off by comparator C1,

which resets the latch. C1 has a sawtooth waveform as one

input and the output of an analog multiplier as the other

input. The multiplier output is the product of an internal

reference voltage, and the output of the error amplifier, A1,

divided by the regulator input voltage. In standard buck

regulators, this means that the output voltage of A1

required to keep a constant regulated output is indepen-

dent of regulator input voltage. This greatly improves line

transient response, and makes loop gain independent of

input voltage. The error amplifier is a transconductance

type with a G M at null of approximately 5000

DAGRA

A current

source. When this pin is left open, it self-clamps at about

4.5V and sets current limit at 6.5A for the LT1074 and 2.6A

for the LT1076. In the 7-pin package an external resistor

can be connected from the I LIM pin to ground to set a lower

current limit. A capacitor in parallel with this resistor will

soft start the current limit. A slight offset in C2 guarantees

that when the I LIM pin is pulled to within 200mV of ground,

C2 output will stay high and force switch duty cycle to zero.

mho. Slew

pull-up current forces the shutdown pin high when left

open. A capacitor can be used to generate delayed start-

up. A resistor divider will program “undervoltage lockout”

if the divider voltage is set at 2.35V when the input is at the

desired trip point.

A, while negative slew

current is about 1.1mA. This asymmetry helps prevent

overshoot on start-up. Overall loop frequency compensa-

tion is accomplished with a series RC network from V C to

ground.

Switch current is continuously monitored by C2, which

resets the R/S latch to turn the switch off if an overcurrent

condition occurs. The time required for detection and

switch turn off is approximately 600ns. So minimum

switch “on” time in current limit is 600ns. Under dead

shorted output conditions, switch duty cycle may have to

be as low as 2% to maintain control of output current. This

would require switch on time of 200ns at 100kHz switch-

ing frequency, so frequency is reduced at very low output

The switch used in the LT1074 is a Darlington NPN (single

NPN for LT1076) driven by a saturated PNP. Special

patented circuitry is used to drive the PNP on and off very

quickly even from the saturation state. This particular

switch arrangement has no “isolation tubs” connected to

the switch output, which can therefore swing to 40V below

ground.

BLOCK

voltages by feeding the FB signal into the oscillator and

creating a linear frequency downshift when the FB signal

drops below 1.3V. Current trip level is set by the voltage on

the I LIM pin which is driven by an internal 320

The “Shutdown” pin is used to force switch duty cycle to

zero by pulling the I LIM pin low, or to completely shut down

the regulator. Threshold for the former is approximately

2.35V, and for complete shutdown, approximately 0.3V.

Total supply current in shutdown is about 150

A. A 10

current going positive is 140

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