ANALOG DEVICES LT 1074 CT7, LT 1076 CT Datasheet

LT1074/LT1076

Step-Down Switching

Regulator

FEATURES

■

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

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APPLICATIO S

■

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

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DESCRIPTIO

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-

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.

The LT1074 uses a true analog multiplier in the feedback
loop. This makes the device respond nearly instanta­neously to input voltage fluctuations and makes loop gain
independent of input voltage. As a result, dynamic behav­ior 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 self­boot 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.

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

TYPICAL APPLICATIO

Basic Positive Buck Converter

50µH (LT1074)

100

µH (LT1076)

MBR745*

µF

10V TO 40V

LT1074

V

SW

FB

V

C

R3

2.7k

C2

0.01

V

IN

GND

+

†

C3
200µF

L1**

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R1

2.8k
1%

R2

2.21k
1%

Buck Converter Efficiency

100

5V

USE MBR340 FOR LT1076

*

5A

COILTRONICS #50-2-52 (LT1074)

**

PULSE ENGINEERING, INC.

HURRICANE #HL-AK147QQ (LT1074)

+

25V

†

C1

RIPPLE CURRENT RATING ≥ I

500

µF

#100-1-52 (LT1076)

#PE-92114 (LT1074)
#PE-92102 (LT1076)

#HL-AG210LL (LT1076)

/2

OUT

LT1074•TA01

90

80

EFFICIENCY (%)

70

60

50

0

1234

OUTPUT LOAD CURRENT (A)

LT1074

V = 12V, V = 20V

OUT IN

V = 5V, V = 15V

OUT IN

L = 50µH TYPE 52 CORE

DIODE = MBR735

5

LT1074•TPC27

sn1074 1074fds

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LT1074/LT1076

WW

W

ABSOLUTE AXI U RATI GS

U

(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 (V

Negative)

SW

LT1074/LT1076 (Note 7) ..................................... 35V

LT1074HV/LT1076HV (Note 7) ........................... 45V

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

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

UUW

PACKAGE/ORDER I FOR ATIO

ORDER PART

LT1076CQ
LT1076IQ

LT1076CR
LT1076IR
LT1076HVCR
LT1076HVIR

LT1074CT7
LT1074HVCT7

C

LT1074IT7
LT1074HVIT7
LT1076CT7

SW
IN

LT1076HVCT7

NUMBER

TAB IS

GND

FRONT VIEW

TAB IS

GND

Q PACKAGE

5-LEAD PLASTIC DD

LT1076: θJC = 4°C, θJA = 30°C/W

FRONT VIEW

TAB IS

GND

R PACKAGE

7-LEAD PLASTIC DD

LT1076: θ

= 4°C, θJA = 30°C/W

JC

FRONT VIEW

T7 PACKAGE

7-LEAD PLASTIC TO-220

5

4

3

2

1

7
6
5
4
3
2
1

7
6
5
4
3
2
1

V

IN

V

SW

GND

V

C

FB/SENSE

SHDN
V

C

FB/SENSE
GND
I

LIM

V

SW

V

IN

SHDN
V
FB
GND
I

LIM

V
V

I

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

LIM

Maximum Operating Ambient Temperature Range

Commercial ................................................. 0°C to 70°C

Industrial ................................................ –40°C to 85°C

Military (OBSOLETE) ..................... –55°C to 125°C

Maximum Operating Junction Temperature Range

Commercial ............................................... 0°C to 125°C

Industrial .............................................. –40°C to 125°C

Military (OBSOLETE) .................... –55°C to 150°C

Maximum Storage Temperature ............... –65°C to 150°C

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

BOTTOM VIEW

V

C

1

2

3

4

V

IN

CASE
IS GND

ORDER PART

NUMBER

LT1074CK
LT1074HVCK
LT1074MK

FB

K PACKAGE

4-LEAD TO-3 METAL CAN

LT1074: θJC = 2.5°C, θJA = 35°C/W

LT1076: θ

OBSOLETE PACKAGE

Consider the T5 Package for Alternate Source

TAB IS

GND

LT1074: θ

LT1076: θ

= 4°C, θJA = 35°C/W

JC

FRONT VIEW

5

4

3

2

1

T PACKAGE

5-LEAD PLASTIC TO-220

LEADS ARE FORMED STANDARD FOR

STRAIGHT LEADS, ORDER FLOW 06

= 2.5°C, θJA = 50°C/W

JC

= 4°C, θJA = 50°C/W

JC

V

SW

LT1074HVMK
LT1076CK
LT1076HVCK
LT1076MK
LT1076HVMK

LT1074CT
LT1074HVCT

V

IN

V

SW

LT1074IT

GND

LT1074HVIT

V

C

LT1076CT

FB

LT1076HVCT
LT1076IT
LT1076HVIT

LT1074: θ

LT1076: θ

= 2.5°C, θJA = 50°C/W

JC

= 4°C, θJA = 50°C/W

JC

*Assumes package is soldered to 0.5 IN2 of 1 oz. copper over internal ground plane or over back side plane.
Consult LTC Marketing for parts specified with wider operating temperature ranges.

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sn1074 1074fds

LT1074/LT1076

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at T

The ● denotes the specifications which apply over the full operating

= 25°C. Tj = 25°C, VIN = 25V, unless otherwise noted.

A

PARAMETER CONDITIONS MIN TYP MAX UNITS

Switch “On” Voltage (Note 2) LT1074 I

LT1076 I

Switch “Off” Leakage LT1074 V

LT1076 V

Supply Current (Note 3) V

= 2.5V, V

FB

40V < V
V

SHUT

= 1A, Tj ≥ 0°C 1.85 V

SW

= 1A, Tj < 0°C 2.1 V

I

SW

I

= 5A, Tj ≥ 0°C 2.3 V

SW

= 5A, Tj < 0°C 2.5 V

I

SW

= 0.5A ● 1.2 V

SW

= 2A ● 1.7 V

I

SW

≤ 25V, V

IN

= V

V

IN

MAX, VSW

= 25V, V

IN

V

= V

IN

MAX, VSW

≤ 40V ● 8.5 11 mA

IN

< 60V ● 912 mA

IN

= 0 5 300 µA

SW

= 0 (Note 8) 10 500 µA

= 0 150 µA

SW

= 0 (Note 8) 250 µA

= 0.1V (Device Shutdown) (Note 9) ● 140 300 µA

Minimum Supply Voltage Normal Mode ● 7.3 8 V

Startup Mode (Note 4)

Switch Current Limit (Note 5) LT1074 I

LT1076 I

Open ● 5.5 6.5 8.5 A

LIM

= 10k (Note 6) 4.5 A

R

LIM

R

= 7k (Note 6) 3 A

LIM

Open ● 2 2.6 3.2 A

LIM

R

= 10k (Note 6) 1.8 A

LIM

R

= 7k (Note 6) 1.2 A

LIM

● 3.5 4.8 V

Maximum Duty Cycle ● 85 90 %

Switching Frequency 90 100 110 kHz

≤ 125°C ● 85 120 kHz

T

j

T

> 125°C ● 85 125 kHz

j

= 0V through 2kΩ (Note 5) 20 kHz

V

FB

Switching Frequency Line Regulation 8V ≤ V

IN

≤ V

(Note 8) ● 0.03 0.1 %/V

MAX

Error Amplifier Voltage Gain (Note 7) 1V ≤ VC ≤ 4V 2000 V/V
Error Amplifier Transconductance 3700 5000 8000 µmho

Error Amplifier Source and Sink Current Source (V

Sink (V

Feedback Pin Bias Current V

FB

= 2V) 100 140 225 µA

FB

= 2.5V) 0.7 1 1.6 mA

FB

= V

REF

● 0.5 2 µA

Reference Voltage VC = 2V ● 2.155 2.21 2.265 V

Reference Voltage Tolerance V

(Nominal) = 2.21V ±0.5 ±1.5 %

REF

All Conditions of Input Voltage, Output

● ± 1 ± 2.5 %

Voltage, Temperature and Load Current

Reference Voltage Line Regulation 8V ≤ VIN ≤ V

(Note 8) ● 0.005 0.02 %/V

MAX

VC Voltage at 0% Duty Cycle 1.5 V

Over Temperature

● – 4 mV/°C

Multiplier Reference Voltage 24 V
Shutdown Pin Current V

= 5V ● 51020 µA

SH

V

SH

≤ V

THRESHOLD

(≅2.5V) ● 50 µA

Shutdown Thresholds Switch Duty Cycle = 0 ● 2.2 2.45 2.7 V

Fully Shut Down

● 0.1 0.3 0.6 V

Thermal Resistance Junction to Case LT1074 2.5 °C/W

LT1076 4.0 °C/W

sn1074 1074fds

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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 (V

) of 2.5V forces the VC pin to its low

FB

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

Note 4: Total voltage from V

pin to ground pin must be ≥ 8V after start-

IN

up for proper regulation.

W

BLOCK DIAGRA

INPUT SUPPLY

10 Aµ

SHUTDOWN*

0.3V

2.35V

+

-POWER

µ

SHUTDOWN
–

+

–

CURRENT

LIMIT

SHUTDOWN

6V

REGULATOR

AND BIAS

I *

LIM

320 Aµ

6V TO ALL
CIRCUITRY

Note 5: 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

Note 6: I

is adjusted to give a minimum switch on time of 1µs.

FB

R

LIM

2k

– 1k

LIM

≈ (LT1074), I

R

– 1k

LIM

≈ (LT1076).

LIM

5.5k

Note 7: Switch to input voltage limitation must also be observed.
Note 8: V

= 40V for the LT1074/76 and 60V for the LT1074HV/76HV.

MAX

Note 9: Does not include switch leakage.

LT1074

500

Ω

CURRENT

4.5V

10k

LIMIT
COMP

+

C2

–

250

Ω

0.04

+

A1

ERROR

2.21V

*AVAILABLE ON PACKAGES WITH PIN
COUNTS GREATER THAN 5.

AMP

–

FB V

X

C

FREQ SHIFT

100kHz

OSCILLATOR

SYNC

V

IN

Z

ANALOG

MULTIPLIER

XY

Z
Y

24V (EQUIVALENT)

3V(p-p)

+

–

C1

PULSE WIDTH
COMPARATOR

S

R

R/S

LATCH

R

Q

LT1076

100

Ω

SWITCH

OUTPUT (V )

G1

Ω

15

400

Ω

SWITCH
OUTPUT

(V )

SW

0.1

Ω

SW

LT1074 • BD01

sn1074 1074fds

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BLOCK DIAGRAWDESCRIPTIO

LT1074/LT1076

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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. 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 indepen­dent 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 GM at null of approximately 5000µmho. Slew
current going positive is 140µA, while negative slew
current is about 1.1mA. This asymmetry helps prevent
overshoot on start-up. Overall loop frequency compensa­tion is accomplished with a series RC network from VC 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 switch­ing frequency, so frequency is reduced at very low output

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

The “Shutdown” pin is used to force switch duty cycle to
zero by pulling the I
the regulator. Threshold for the former is approximately

2.35V, and for complete shutdown, approximately 0.3V.
Total supply current in shutdown is about 150µA. A 10µA
pull-up current forces the shutdown pin high when left
open. A capacitor can be used to generate delayed start­up. 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.

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.

pin which is driven by an internal 320µA current

LIM

pin to ground to set a lower

LIM

pin is pulled to within 200mV of ground,

LIM

pin low, or to completely shut down

LIM

sn1074 1074fds

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