ANALOG DEVICES LT 1074 CT Datasheet

LT1074/LT1076

Step-Down Switching

Regulator

EATU

F

■

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

■

Programmable Current Limit

■

Operates Up to 60V Input

■

Micropower Shutdown Mode

PPLICATI

A

■

Buck Converter with Output Voltage Range of 2.5V

RE

S

O

U
S

to 50V

■

Tapped-Inductor Buck Converter with 10A Output
at 5V

■

Positive-to-Negative Converter

■

Negative Boost Converter

■

Multiple Output Buck Converter

DUESCRIPTIO

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 com­ponents, 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 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 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.

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

10V TO 40V

U

O

A

PPLICATITYPICAL

Basic Positive Buck Converter

L1**

50 H (LT1074)

µ

100 H (LT1076)

µ

MBR745*

R1

2.8k
1%

R2

2.21k
1%

LT1074

V

SW

FB

V

C

R3

2.7k

C2

0.01 F

V

IN

GND

+

†

C3
200 F

µµ µ

5V
5A

+

C1
500 F

25V

*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 / 2

≥

OUT

LT1074 • TA01

Buck Converter Efficiency

100

90

80

EFFICIENCY (%)

70

60

50

0

LT1074

V = 12V, V = 20V

OUT IN

V = 5V, V = 15V

OUT IN

L = 50 H TYPE 52 CORE

µ

DIODE = MBR735

1234

OUTPUT LOAD CURRENT (A)

5

LT1074 • TPC27

1

6

LT1074/LT1076

5
4
3
2
1

V

GND
FB

V
V

C

FRONT VIEW

T PACKAGE, 5-LEAD T0-220

LEADS ARE FORMED STANDARD FOR
STRAIGHT LEADS, ORDER FLOW 06

IN

SW

LT1074: θJC = 2.5°C/W, θJA = 50°C/W
LT1076: θ

JC

= 4°C/W, θJA = 50°C/W

A

W

O

LUTEXI T

S

A

WUW

ARB

U
G

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

Negative)

SW

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

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

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

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

WU

/

PACKAGE

LT1076: θ

LT1076: θ

Y PACKAGE, 7-LEAD TO-220

LT1074: θ
LT1076: θ

* Assumes package is soldered to 0.5 IN2 of 1 oz. copper over internal ground

plane or over back side plane.

O

RDER I FOR ATIO

FRONT VIEW

5
4
3
2
1

Q PACKAGE

5-LEAD PLASTIC DD

= 4°C/W, θJA = 30°C/W*

JC

FRONT VIEW

7
6
5
4
3
2
1

R PACKAGE

7-LEAD PLASTIC DD

= 4°C/W, θJA = 30°C/W*

JC

FRONT VIEW

7

6

5

4

3

2

1

= 2.5°C/W, θ

JC

= 4°C/W, θ

JC

JA

= 50°C/W

JA

VIN
V

SW

GND
V

C

FB/SENSE

SHDN



V

C

FB/SENSE
GND
I

LIM

V

SW

V

IN

V

C

GND
V

SW

= 50°C/W









SHUTDOWN
FB
I

LIM

V

IN

ORDER PART

NUMBER

LT1076CQ

LT1076CR
LT1076HVCR

LT1074CY
LT1074HVCY
LT1074IY
LT1074HVIY
LT1076CY
LT1076HVCY

U

I

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

LIM

Maximum Operating Ambient Temperature Range

LT1074C/76C, LT1074HVC/76HVC ............ 0°C to 70°C

LT1074I/76I, LT1074HVI/76HVI ............. –40°C to 85°C

LT1074M/76M, LT1074HVM/76HVM ... –55°C to 125°C

Maximum Operating Junction Temperature Range

LT1074C/76C, LT1074HVC/76HVC .......... 0°C to 125°C

LT1074I/76I, LT1074HVI/76HVI ........... –40°C to 125°C

LT1074M/76M, LT1074HVM/76HVM ... –55°C to 150°C

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

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

ORDER PART

NUMBER

BOTTOM VIEW

V

C

1

2
3

4

FB

K PACKAGE, 4-LEAD TO-3 METAL CAN
LT1074: θJC = 2.5°C/W, θ

LT1076: θ

= 4°C/W, θJA = 35°C/W

JC

V

V

= 35°C/W

JA

IN

SW

CASE IS GND

LT1074CK
LT1074HVCK
LT1074MK
LT1074HVMK
LT1076CK
LT1076HVCK
LT1076MK
LT1076HVMK

LT1074CT
LT1074HVCT
LT1074IT
LT1074HVIT
LT1076CT
LT1076HVCT
LT1076IT

LECTRICAL C CHARA TERIST

E

PARAMETER CONDITIONS MIN TYP MAX UNITS

Switch “On” Voltage (Note 1) LT1074 I

2

ICS

SW

I

SW

I

SW

I

SW

LT1076 I

I

SW
SW

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

= 1A, Tj ≥ 0°C 1.85 V
= 1A, Tj < 0°C 2.1 V
= 5A, Tj ≥ 0°C 2.3 V
= 5A, Tj < 0°C 2.5 V

= 0.5A ● 1.2 V
= 2A ● 1.7 V

LT1074/LT1076

LECTRICAL C CHARA TERIST

E

PARAMETER CONDITIONS MIN TYP MAX UNITS

Switch “Off” Leakage LT1074 V

LT1076 V

Supply Current (Note 2) V

Minimum Supply Voltage Normal Mode ● 7.3 8 V

Switch Current Limit (Note 4) LT1074 I

Maximum Duty Cycle ● 85 90 %
Switching Frequency 90 100 110 kHz

Switching Frequency Line Regulation 8V ≤ V
Error Amplifier Voltage Gain (Note 6) 1V ≤ VC ≤ 4V 2000 V/V
Error Amplifier Transconductance 3700 5000 8000 µmho
Error Amplifier Source and Sink Current Source (V

Feedback Pin Bias Current V
Reference Voltage VC = 2V ● 2.155 2.21 2.265 V
Reference Voltage Tolerance V

Reference Voltage Line Regulation 8V ≤ VIN ≤ V
VC Voltage at 0% Duty Cycle 1.5 V

Multiplier Reference Voltage 24 V
Shutdown Pin Current V

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

Thermal Resistance Junction to Case LT1074 2.5 °C/W

FB

40V < VIN < 60V ● 912 mA
V

SHUT

Startup Mode (Note 3) ● 3.5 4.8 V

LT1076 I

Tj ≤ 125°C ● 85 120 kHz
Tj > 125°C ● 85 125 kHz
V

FB

Sink (V

FB

REF

All Conditions of Input Voltage, Output ● ±1 ± 2.5 %
Voltage, Temperature and Load Current

Over Temperature ● – 4 mV/°C

SH

V

SH

Fully Shut Down ● 0.1 0.3 0.5 V

LT1076 4.0 °C/W

ICS

V

V

= 2.5V, V

= 0.1V (Device Shutdown) (Note 8) ● 140 300 µA

LIM

R
R

LIM

R
R

= 0V through 2kΩ (Note 4) 20 kHz

≤ V

IN

FB

= 2.5V) 0.7 1 1.6 mA

FB

= V

REF

(Nominal) = 2.21V ±0.5 ± 1.5 %

= 5V ● 51020 µA

≤ V

THRESHOLD

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

≤ 25V, V

IN

= V

IN

= 25V, V

IN

= V

IN

≤ 40V ● 8.5 11 mA

IN

Open ● 5.5 6.5 8.5 A

= 10k (Note 5) 4.5 A

LIM

= 7k (Note 5) 3 A

LIM

Open ● 2 2.6 3.2 A

= 10k (Note 5) 1.8 A

LIM

= 7k (Note 5) 1.2 A

LIM

(Note 7) ● 0.03 0.1 %/V

MAX

= 2V) 100 140 225 µA

(Note 7) ● 0.005 0.02 %/V

MAX

= 0 5 300 µA

SW

= 0 (Note 7) 10 500 µA

MAX, VSW

= 0 150 µA

SW

= 0 (Note 7) 250 µA

MAX, VSW

● 0.5 2 µA

(≅2.5V) ● 50 µA

The ● denotes the specifications which apply over the full operating temperature
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 (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.

Note 3: Total voltage from VIN pin to ground pin must be ≥ 8V after startup for
proper regulation.

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

R

– 1k

Note 5: I

Note 6: Switch to input voltage limitation must also be observed.
Note 7: V
Note 8: Does not include switch leakage.

LIM

≈ (LT1074), I

LIM

2k

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

MAX

R

– 1k

LIM

≈ (LT1076).

LIM

5.5k

3

LT1074/LT1076

BLOCK

IDAGRA

W

INPUT SUPPLY

10 Aµ

SHUTDOWN*

2.21V

0.3V
+

-POWER

µ

SHUTDOWN
–

2.35V
+

–

+

ERROR

–

FB V

CURRENT

LIMIT

SHUTDOWN

A1

AMP

6V

REGULATOR

AND BIAS

I *

LIM

MULTIPLIER

X

24V (EQUIVALENT)

C

FREQ SHIFT

SYNC

V

IN

Z

ANALOG

XY

Z
Y

320 Aµ

6V TO ALL
CIRCUITRY

100kHz

OSCILLATOR

3V(p-p)

+

C1

–

4.5V

10k

PULSE WIDTH

COMPARATOR

S

R

R/S

LATCH

R

CURRENT

LIMIT
COMP

Q

LT1076

LT1074

500

Ω

15

Ω

SWITCH 

OUTPUT

(V )

SW

0.04

+

C2

–

G1

250

Ω

400

Ω

4

*AVAILABLE ON PACKAGES WITH PIN
COUNTS GREATER THAN 5.

0.1

100

Ω

SWITCH

OUTPUT (V )

SW

Ω

LT1074 • BD01

LT1074/LT1076

W

BLOCK

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

IDAGRA

ESCRIPTIO

D

U

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

5