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

U

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

U

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**

U

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

6

1

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.

2

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

3

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

4

BLOCK DIAGRAWDESCRIPTIO

LT1074/LT1076

U

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

5

LT1074/LT1076

UW

TYPICAL PERFOR A CE CHARACTERISTICS

VC Pin Characteristics VC Pin Characteristics Feedback Pin Characteristics

200

150

100

50

0

–50

CURRENT (mA)

–100

–150

–200

0

1234

V ADJUSTED FOR

FB

I = 0 AT V = 2V

CC

SLOPE ≈ 400kΩ

V ≤ 2V

FB

VOLTAGE (V)

5

789

6

LT1074•TPC01

2.0

1.5

1.0

0.5

0

–0.5

CURRENT (mA)

–1.0

–1.5

–2.0

1234

0

V ≥ 2.5V

FB

VOLTAGE (V)

5

789

6

LT1074•TPC02

–100

CURRENT (µA)

–200

–300

–400

–500

500

400

300

200

100

0

START OF
FREQUENCY SHIFTING

1

0

234

VOLTAGE (V)

5

78 109

6

LT1074•TPC03

Shutdown Pin Characteristics

40

30

20

V

= 50V

DETAILS OF THIS
AREA SHOWN IN
OTHER GRAPH

10

20 40

IN

WITH V

30

VOLTAGE (V)

IN

CURRENT (µA)

–10

–20

–30

–40

10

0

0

THIS POINT MOVES

50 60 70 80

LT1074•TPC04

Shutdown Pin Characteristics I

0

–5

–10

–15

–20

–25

CURRENT (µA)

–30

–35

–40

= 25°CT

j

CURRENT FLOWS OUT

OF SHUTDOWN PIN

SHUTDOWN
THRESHOLD

0

1.0 2.0

0.5

1.5

VOLTAGE (V)

2.5 3.0 3.5 4.0

LT1074•PC05

Supply Current

20

18

16

14

12

10

8

6

INPUT CURRENT (mA)

4

2

0

DEVICE NOT SWITCHING

V

C

0

20 30 60

10 40 50

INPUT VOLTAGE (V)

= 1V

LT1074•TPC11

–100

–150

–200

CURRENT (µA)

–250

–300

–350

–400

Pin Characteristics

LIM

100

50

0

–50

0

–1

–2

T = 25°C

j

12

VOLTAGE (V)

3

7

5

8

LT1074•TPC06

6

4

6

sn1074 1074fds

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Reference Voltage vs

Supply Current (Shutdown)

300

250

200

150

100

INPUT CURRENT (µA)

50

0

0

10 20 40

30 50 60

INPUT VOLTAGE (V)

Reference Shift with Ripple
Voltage

20

10

0

–10

–20

–30

–40

–50

–60

–70

CHANGE IN REFERENCE VOLTAGE (mV)

–80

0

SQUARE

WAVE

20 40 60 80

PEAK-TO-PEAK RIPPLE AT FB PIN (mV)

100 120

TRI WAVE

140

LT1074•TPC13

160 180 200

LT1074•TPC16

Temperature Switch “On” Voltage

2.25

2.24

2.23

2.22

2.21

VOLTAGE (V)

2.20

2.19

2.18

2.17
–25 0 25 50

–50

JUNCTION TEMPERATURE (°C)

75 100 125 150

Error Amplifier Phase and G

8k

7k

6k

(µmho)

5k

4k

3k

2k

TRANSCONDUCTANCE

1k

0

1k 100k 1M 10M

10k

FREQUENCY (Hz)

LT1074•TPC14

M

θ

G

M

LT1074•TPC17

200

150

100

50

0

–50

–100

–150

–200

PHASE (°)

LT1074/LT1076

3.0

2.5

2.0

1.5

“ON” VOLTAGE (V)

1.0

0.5
0

Switching Frequency vs
Temperature

120

115

110

105

100

95

FREQUENCY (kHz)

90

85

80

–50

–25 0 25 50

LT1074

LT1076

123

SWITCH CURRENT (A)

JUNCTION TEMPERATURE (°C)

T = 25°C

j

4 56

LT1074•TPC28

75

100

125

LT1074•TPC18

150

Feedback Pin Frequency Shift

160

140

120

100

80

60

40

SWITCHING FREQUENCY (kHz)

20

–55°C

0

0

0.5 2.0 2.5

FEEDBACK PIN VOLTAGE (V)

150°C

25°C

1.0 1.5 3.0

LT1074•TPC19

Current Limit vs Temperature*

8

7

I PIN OPEN

LIM

6

5

4

3

R = 5kΩ

2

OUTPUT CURRENT LIMIT (A)

1

*MULTIPLY CURRENTS BY 0.4 FOR LT1076

0

–50 –25 0 25 50

LIM

JUNCTION TEMPERATURE (°C)

R = 10kΩ

LIM

75

100

125

LT1074•TPC22

150

sn1074 1074fds

7

LT1074/LT1076

∆∆V

VV

OUT

GND OUT

=

()()

221.

U

PI

U

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. Place the input capacitor very close to
the regulator and connect it with wide traces to avoid extra
inductance. Use radial lead capacitors.

dl

L

()

P

()

dt

STEP =

I

ESR

()

()

SW

RAMP =

I

T

()

()

SW

ON

C

LT1074•PD01

To ensure good load regulation, the ground pin must be
connected directly to the proper output node, so that no
high currents flow in this path. The output divider resistor
should also be connected to this low current connection
line as shown in Figure 2.

LT1074

FB

GND

R2

HIGH CURRENT
RETURN PATH

NEGATIVE OUTPUT NODE
WHERE LOAD REGULATION
WILL BE MEASURED

LT1074•PD02

Figure 1. Input Capacitor Ripple Figure 2. Proper Ground Pin Connection

LP = Total inductance in input bypass connections

and capacitor.

“Spike” height (dI/dt • LP) is approximately 2V per
inch of lead length for LT1074 and 0.8V per inch for
LT1076.

“Step” for ESR = 0.05Ω and I
“Ramp” for C = 200µF, T

ON

= 5A is 0.25V.

SW

= 5µs, and I

SW

= 5A,

is 0.12V.

Input current on the VIN Pin in shutdown mode is the sum
of actual supply current (≈140µA, with a maximum of
300µA), and switch leakage current. Consult factory for
special testing if shutdown mode input current is critical.

GROUND PIN

It might seem unusual to describe a ground pin, but in the
case of regulators, the ground pin must be connected
properly to ensure good load regulation. The internal
reference voltage is referenced to the ground pin; so any
error in ground pin voltage will be multiplied at the output;

FEEDBACK PIN

The feedback pin is the inverting input of an error amplifier
which controls the regulator output by adjusting duty
cycle. The noninverting input is internally connected to a
trimmed 2.21V reference. Input bias current is typically

0.5µA when the error amplifier is balanced (I

OUT

= 0). The
error amplifier has asymmetrical GM for large input sig­nals to reduce startup overshoot. This makes the amplifier
more sensitive to large ripple voltages at the feedback pin.
100mVp-p ripple at the feedback pin will create a 14mV
offset in the amplifier, equivalent to a 0.7% output voltage
shift. To avoid output errors, output ripple (P-P) should be
less than 4% of DC output voltage at the point where the
output divider is connected.

See the “Error Amplifier” section for more details.

Frequency Shifting at the Feedback Pin

The error amplifier feedback pin (FB) is used to downshift
the oscillator frequency when the regulator output voltage
is low. This is done to guarantee that output short-circuit

8

sn1074 1074fds

LT1074/LT1076

U

PI

U

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:

VV

+

t

ON

OUT D

=

Vf

•

IN

VD = Catch diode forward voltage ( ≈ 0.5V)
f = Switching frequency

At f = 100kHz, t

must drop to 0.2µs when V

ON

and the output is shorted (V

= 0V). In current limit,

OUT

IN

= 25V

the LT1074 can reduce tON to a minimum value of
≈0.6µs, much too long to control current correctly for
V

= 0. To correct this problem, switching frequency

OUT

is lowered from 100kHz to 20kHz as the FB pin drops
from 1.3V to 0.5V. This is accomplished by the circuitry

TO
OSCILLATOR

V

OUT

2.21V

Q1

R3
3k

R1

EXTERNAL
DIVIDER

FB

R2

2.21k

LT1074•PD03

+2V

+

ERROR

V

AMPLIFIER

C

–

Figure 3. Frequency Shifting

shown in Figure 3.

Q1 is off when the output is regulating (VFB = 2.21V). As
the output is pulled down by an overload, VFB will eventu­ally reach 1.3V, turning on Q1. As the output continues to
drop, Q1 current increases proportionately and lowers the
frequency of the oscillator. Frequency shifting starts when
the output is ≈ 60% of normal value, and is down to its
minimum value of ≅ 20kHz when the output is ≅ 20% of
normal value. The rate at which frequency is shifted is
determined by both the internal 3k resistor R3 and the
external divider resistors. For this reason, R2 should not
be increased to more than 4kΩ, if the LT1074 will be
subjected to the simultaneous conditions of high input
voltage and output short-circuit.

SHUTDOWN PIN

The shutdown pin is used for undervoltage lockout, micro­power shutdown, soft-start, delayed start, or as a general
purpose on/off control of the regulator output. It controls
switching action by pulling the I

pin low, which forces

LIM

the switch to a continuous “off” state. Full micropower
shutdown is initiated when the shutdown pin drops below

0.3V.

The V/I characteristics of the shutdown pin are shown in
Figure 4. For voltages between 2.5V and ≈VIN, a current of
10µA flows

out

of the shutdown pin. This current in-

creases to ≈25µA as the shutdown pin moves through the

2.35V threshold. The current increases further to ≈30µA at
the 0.3V threshold, then drops to ≈15µA as the shutdown
voltage fall below 0.3V. The 10µA current source is in-
cluded to pull the shutdown pin to its high or default state
when left open. It also provides a convenient pull-up for
delayed start applications with a capacitor on the shut­down pin.

When activated, the typical collector current of Q1 in
Figure 5, is ≈2mA. A soft-start capacitor on the I

LIM

pin will

delay regulator shutdown in response to C1, by
≈(5V)(C

)/2mA. Soft-start after full micropower shut-

LIM

down is ensured by coupling C2 to Q1.

0

–5

–10

–15

–20

–25

CURRENT (µA)

–30

–35

–40

Figure 4. Shutdown Pin Characteristics

= 25°CT

j

CURRENT FLOWS OUT

OF SHUTDOWN PIN

SHUTDOWN
THRESHOLD

0

1.0 2.0

0.5

1.5

VOLTAGE (V)

2.5 3.0 3.5 4.0

LT1074•PC05

sn1074 1074fds

9

LT1074/LT1076

R

VVR

VV

R

R

SH

UTP SH

3

08 1

1

1

2

=

−

()()

− +

⎛
⎝

⎜

⎞
⎠

⎟

.

U

PI

SHUTDOWN

PIN

DESCRIPTIO S

10 Aµ

–

2.3V

+

–

0.3V

+

Figure 5. Shutdown Circuitry

U

V

IN

300 Aµ

I

LIM

Q1

TO TOTAL
REGULATOR
SHUTDOWN

PIN

6V

LT1074•PD07

EXTERNAL
C

LIM

C1

C2

Undervoltage Lockout

Undervoltage lockout point is set by R1 and R2 in Figure 6.
To avoid errors due to the 10µA shutdown pin current, R2
is usually set at 5k, and R1 is found from:

VV

−

TP SH

RR

12=

()

V

SH

VTP = Desired undervoltage lockout voltage

Hysteresis in undervoltage lockout may be accomplished
by connecting a resistor (R3) from the I

pin to the

LIM

shutdown pin as shown in Figure 7. D1 prevents the
shutdown divider from altering current limit.

V

R1

D1*

R3

R2

*1N4148

Figure 7. Adding Hysteresis

Trip Po V V

int .== +

TP

235 1

IN

SHUT

LT1074

I

LIM

OPTIONAL CURRENT
LIMIT RESISTOR

⎛

R

1

⎜

R

2

⎝

LT1074•PD09

⎞
⎟

⎠

If R3 is added, the lower trip point (VIN descending) will be
the same. The upper trip point (V

⎛

RRR

VV

=+

UTP SH

1

⎜
⎝

121

⎞

−

08

⎟

R

3

⎠

.

UTP

⎛

V

⎜
⎝

R

R

) will be:

⎞

1

⎟

3

⎠

If R1 and R2 are chosen, R3 is given by:

VSH = Threshold for lockout on the

shutdown pin = 2.45V

If quiescent supply current is critical, R2 may be increased
up to 15kΩ, but the denominator in the formula for R2
should replace VSH with VSH – (10µA)(R2).

R1

R2

5k

Figure 6. Undervoltage Lockout

10

SHUT

V

IN

LT1074

GND

LT1074•PD08

Example: An undervoltage lockout is required such that
the output will not start until VIN = 20V, but will continue
to operate until VIN drops to 15V. Let R2 = 2.32k.

15 2 35

1234

=

Rk

()

3

Rk

=

20 2 35 1

()

.

235 08 125

...

−

()()

.

− +

.

VV

−

=

235

.

V

39

⎛

12 5

⎜

232

⎝

=

⎞

.

⎟

.

⎠

12 5

.

.

k

sn1074 1074fds

LT1074/LT1076

U

PI

I

LIM

The I

U

DESCRIPTIO S

PIN

pin is used to reduce current limit below the

LIM

preset value of 6.5A. The equivalent circuit for this pin is
shown in Figure 8.

When I

TO LIMIT
CIRCUIT

R1
8K

is left open, the voltage at Q1 base clamps at 5V

LIM

V

Q1

I

LIM

Figure 8. I

IN

320 Aµ

D1

LIM

D2

4.3V

D3
6V

LT1047•PD12

Pin Circuit

through D2. Internal current limit is determined by the
current through Q1. If an external resistor is connected
between I

and ground, the voltage at Q1 base can be

LIM

reduced for lower current limit. The resistor will have a
voltage across it equal to (320µA)(R), limited to ≈5V when
clamped by D2. Resistance required for a given current
limit is:

R

= I

(2kΩ) + 1kΩ (LT1074)

LIM

= I

(5.5kΩ) + 1kΩ (LT1076)

LIM

R

LIM

LIM

As an example, a 3A current limit would require
3A(2k) + 1k = 7kΩ for the LT1074. The accuracy of these
formulas is ± 25% for 2A ≤ I
7A ≤ I
25% above the

≤ 1.8A (LT1076), so I

LIM

peak

switch current required.

≤ 5A (LT1074) and

LIM

should be set at least

LIM

Foldback current limiting can be easily implemented by
adding a resistor from the output to the I

pin as shown

LIM

in Figure 9. This allows full desired current limit (with or
without R

) when the output is regulating, but reduces

LIM

current limit under short-circuit conditions. A typical value
for RFB is 5kΩ, but this may be adjusted up or down to set
the amount of foldback. D2 prevents the output voltage

from forcing current back into the I
value for RFB, first calculate R

IR

−

044

.*

SC L

R

FB

()()

=

−Ω

Rk I

05 1

.*

LSC

()

LIM

−

pin. To calculate a

LIM

, the RFB:

Rink

()

Ω

L

*Change 0.44 to 0.16, and 0.5 to 0.18 for LT1076.

Example: I

R

FB

= 4A, ISC = 1.5A, R

LIM

−

15 044 9

..

()

=

kk

059 1 15

..

()

R

LIM

Figure 9. Foldback Current Limit

k

Ω

()

−

−

LT1074

I

LIM

R

FB

38

()

FB

D2

1N4148

= (4)(2k) + 1k = 9k

LIM

Ω

k

.

V

OUT

LT1074•PD13

Error Amplifier

The error amplifier in Figure 10 is a single stage design
with added inverters to allow the output to swing above
and below the common mode input voltage. One side of
the amplifier is tied to a trimmed internal reference voltage
of 2.21V. The other input is brought out as the FB (feed­back) pin. This amplifier has a GM (voltage “in” to current
“out”) transfer function of ≈5000µmho. Voltage gain is
determined by multiplying GM times the total equivalent
output loading, consisting of the output resistance of Q4
and Q6 in parallel with the series RC external frequency
compensation network. At DC, the external RC is ignored,
and with a parallel output impedance for Q4 and Q6 of
400kΩ, voltage gain is ≈2000. At frequencies above a few
hertz, voltage gain is determined by the external compen­sation, RC and CC.

sn1074 1074fds

11

LT1074/LT1076

PI

U

DESCRIPTIO S

U

90 Aµ

Q3

50 Aµ

Q2

Q1

X1.8

2.21V

140 Aµ

ALL CURRENTS SHOWN ARE AT NULL CONDITION

FB

Figure 10. Error Amplifier

300

50 Aµ

D2

5.8V

Q4

µ

90 A

D1

Ω

V

C

90 Aµ

Q6

EXTERNAL
FREQUENCY
COMPENSATION

R

C

C

C

LT1074 • PD11

G

2π

m

fC

••

at mid frequencies

C

A

=

V

A G R at high frequencies

=•

VmC

Phase shift from the FB pin to the VC pin is 90° at mid
frequencies where the external CC is controlling gain, then
drops back to 0° (actually 180° since FB is an inverting
input) when the reactance of CC is small compared to RC.
The low frequency “pole” where the reactance of CC is
equal to the output impedance of Q4 and Q6 (rO), is:

f

POLE

Although f

=

1

rk

≈Ω

400

rC

••

2

π

varies as much as 3:1 due to rO variations,

POLE

O

O

mid-frequency gain is dependent only on Gm, which is
specified much tighter on the data sheet. The higher
frequency “zero” is determined solely by RC and CC.

RC

••

2π

1

CC

f

ZERO

=

The error amplifier has asymmetrical peak output current.
Q3 and Q4 current mirrors are unity-gain, but the Q6
mirror has a gain of 1.8 at output null and a gain of 8 when
the FB pin is high (Q1 current = 0). This results in a
maximum positive output current of 140µA and a maxi-
mum negative (sink) output current of ≅1.1mA. The asym­metry is deliberate—it results in much less regulator
output overshoot during rapid start-up or following the
release of an output overload. Amplifier offset is kept low
by area scaling Q1 and Q2 at 1.8:1.

Amplifier swing is limited by the internal 5.8V supply for
positive outputs and by D1 and D2 when the output goes
low. Low clamp voltage is approximately one diode drop
(≈0.7V – 2mV/°C).

Note that both the FB pin and the VC pin have other internal
connections. Refer to the frequency shifting and synchro­nizing discussions.

12

sn1074 1074fds

U

TYPICAL APPLICATIO S

LT1074/LT1076

Tapped-Inductor Buck Converter

V

20V† TO 35V

IN

+

**

* = 1% FILM RESISTORS
D1 = MOTOROLA-MBR745
C1 = NICHICON-UPL1C221MRH6
C2 = NICHICON-UPL1A102MRH6
L1 = COILTRONICS-CTX25-5-52

L2

µ

H

5

C1

+

4400
(2 EA
2200
16V)

µ

F

µ

F,

+

V

IN

LT1074HV

GND

C3

µ

F

200
50V

PULSE ENGINEERING #PE±65282*
MOTOROLA MBR2030CTL

†

IF INPUT VOLTAGE IS BELOW 20V,
MAXIMUM OUTPUT CURRENT WILL BE REDUCED. SEE AN44

V

SW

FB

V

C

R3
1k

C2

µ

F

0.2

31

D2
35V
5W

D3
1N5819

L1*

D1**

0.01µF

R1

2.8k

R2

2.21k

Positive-to-Negative Converter with 5V Output

V

IN

4.5V to
40V

+

LT1074

GND

†

LOWER REVERSE VOLTAGE RATING MAY BE USED FOR LOWER INPUT VOLTAGES.
LOWER CURRENT RATING IS ALLOWED FOR LOWER OUTPUT CURRENT. SEE AN44.

††

LOWER CURRENT RATING MAY BE USED FOR LOWER OUTPUT CURRENT. SEE AN44.

R1, R2, AND C4 ARE USED FOR LOOP FREQUENCY COMPENSATION WITH LOW INPUT VOLTAGE,

**

BUT R1 AND R2 MUST BE INCLUDED IN THE CALCULATION FOR OUTPUT VOLTAGE DIVIDER VALUES.
FOR HIGHER OUTPUT VOLTAGES, INCREASE R1, R2, AND R3 PROPORTIONATELY.
FOR INPUT VOLTAGE > 10V, R1, R2, AND C4 CAN BE ELIMINATED, AND COMPENSATION IS
DONE TOTALLY ON THE V PIN.
R3 = –2.37 (KΩ)

V
R1 = (R3) (1.86)
R2 = (R3) (3.65)

MAXIMUM OUTPUT CURRENT OF 1A IS DETERMINED BY MINIMUM INPUT

**

VOLTAGE OF 4.5V. HIGHER MINIMUM INPUT VOLTAGE WILL ALLOW MUCH HIGHER
OUTPUT CURRENTS. SEE AN44.

V

OUT

+

C1

µF

220
50V

L1

µH

25

††

5A

IN

V

SW

V

FB

V

C

C3

µF

0.1

C

R1**

5.1k

R2**
10k

†

D1
MBR745

C4**

0.01

R3*

2.74k

+

C2

µF

1000
10V

R4

1.82k*

µF

LT1074 • TA03

V

OUT

†

5V, 10A

C4

µ

F

390
16V

LT1074 •TA02

OPTIONAL FILTER

µH

5

–

+

–5V,1A***

200µF
10V

sn1074 1074fds

13

LT1074/LT1076

PACKAGE DESCRIPTIO

0.320 – 0.350
(8.13 – 8.89)

0.420 – 0.480

(10.67 – 12.19)

U

K Package

4-Lead TO-3 Metal Can

(Reference LTC DWG # 05-08-1311)

0.760 – 0.775

(19.30 – 19.69)

0.038 – 0.043
(0.965 – 1.09)

1.177 – 1.197

(29.90 – 30.40)

0.470 TP

0.060 – 0.135

(1.524 – 3.429)

0.655 – 0.675

(16.64 – 19.05)

P.C.D.

0.151 – 0.161
(3.84 – 4.09)

DIA 2 PLC

0.256

(6.502)

0.060

(1.524)

0.300

(7.620)

BOTTOM VIEW OF DD PAK

HATCHED AREA IS SOLDER PLATED

COPPER HEAT SINK

0.060

(1.524)

0.075

(1.905)

0.183

(4.648)

0.167 – 0.177
(4.24 – 4.49)

R

72°

18°

0.490 – 0.510

(12.45 – 12.95)

R

K4(TO-3) 1098

OBSOLETE PACKAGE

Q Package

5-Lead Plastic DD Pak

(Reference LTC DWG # 05-08-1461)

0.060

(1.524)

TYP

0.330 – 0.370

(8.382 – 9.398)

+0.012

0.143
–0.020

+0.305

3.632

()

–0.508

0.028 – 0.038

(0.711 – 0.965)

0.390 – 0.415

(9.906 – 10.541)

15° TYP

0.067
(1.70)

BSC

0.165 – 0.180

(4.191 – 4.572)

0.059

(1.499)

TYP

0.013 – 0.023

(0.330 – 0.584)

0.045 – 0.055

(1.143 – 1.397)

+0.008

0.004
–0.004

+0.203

0.102

()

–0.102

0.095 – 0.115

(2.413 – 2.921)

0.050 ± 0.012

(1.270 ± 0.305)

Q(DD5) 1098

14

sn1074 1074fds

PACKAGE DESCRIPTIO

LT1074/LT1076

U

R Package

7-Lead Plastic DD Pak

(Reference LTC DWG # 05-08-1462)

0.256

(6.502)

0.060

(1.524)

0.300

(7.620)

BOTTOM VIEW OF DD PAK

HATCHED AREA IS SOLDER PLATED

COPPER HEAT SINK

(1.524)

(1.905)

0.060

0.075

0.183

(4.648)

0.060

(1.524)

TYP

0.330 – 0.370

(8.382 – 9.398)

+0.012

0.143
–0.020

+0.305

3.632

()

–0.508

0.026 – 0.036

(0.660 – 0.914)

0.390 – 0.415

(9.906 – 10.541)

15° TYP

0.050
(1.27)

BSC

0.165 – 0.180

(4.191 – 4.572)

T Package

5-Lead Plastic TO-220 (Standard)

(Reference LTC DWG # 05-08-1421)

0.059

(1.499)

TYP

0.013 – 0.023

(0.330 – 0.584)

0.045 – 0.055

(1.143 – 1.397)

+0.008

0.004
–0.004

+0.203

0.102

()

–0.102

0.095 – 0.115

(2.413 – 2.921)

0.050 ± 0.012

(1.270 ± 0.305)

R (DD7) 1098

0.390 – 0.415

(9.906 – 10.541)

0.460 – 0.500

(11.684 – 12.700)

0.067

BSC

(1.70)

0.147 – 0.155

(3.734 – 3.937)

DIA

0.230 – 0.270

(5.842 – 6.858)

0.570 – 0.620

(14.478 – 15.748)

0.330 – 0.370

(8.382 – 9.398)

SEATING PLANE

0.152 – 0.202

0.260 – 0.320
(6.60 – 8.13)

0.028 – 0.038

(0.711 – 0.965)

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen­tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

(3.861 – 5.131)

0.165 – 0.180

(4.191 – 4.572)

0.700 – 0.728

(17.78 – 18.491)

0.135 – 0.165

(3.429 – 4.191)

0.620

(15.75)

TYP

* MEASURED AT THE SEATING PLANE

0.045 – 0.055

(1.143 – 1.397)

0.095 – 0.115

(2.413 – 2.921)

0.155 – 0.195*
(3.937 – 4.953)

0.013 – 0.023

(0.330 – 0.584)

T5 (TO-220) 0399

sn1074 1074fds

15

LT1074/LT1076

TYPICAL APPLICATIO

U

Negative Boost Converter

200µF

V

–5V TO –15V

PACKAGE DESCRIPTIO

0.390 – 0.415

(9.906 – 10.541)

LT1074

GND

C3

+

15V

IN

MBR735

*

**

I

(MAX) = 1A TO 3A DEPENDING

OUT

ON INPUT VOLTAGE. SEE AN44

U

7-Lead Plastic TO-220 (Standard)

(Reference LTC DWG # 05-08-1422)

(3.734 – 3.937)

V

IN

V

FB

V

V

C

0.01µF

0.147 – 0.155

DIA

SW

R3
750Ω

T7 Package

C2
1nF

100pF

L1

D1*

25

µ

H

+

µ

H

5

OPTIONAL OUTPUT FILTER

0.165 – 0.180

(4.191 – 4.572)

R1

12.7k

R2

2.21k

100µF

+

C1

µ

F

1000
25V

V

OUT

–15V**

LT1074 • TA04

0.045 – 0.055

(1.143 – 1.397)

0.230 – 0.270

(5.842 – 6.858)

0.460 – 0.500

(11.684 – 12.700)

0.050

BSC

(1.27)

0.330 – 0.370

(8.382 – 9.398)

0.026 – 0.036

(0.660 – 0.914)

0.570 – 0.620

(14.478 – 15.748)

0.260 – 0.320

(6.604 – 8.128)

SEATING PLANE

0.152 – 0.202

(3.860 – 5.130)

0.700 – 0.728

(17.780 – 18.491)

0.135 – 0.165

(3.429 – 4.191)

0.620

(15.75)

TYP

*MEASURED AT THE SEATING PLANE

0.095 – 0.115

(2.413 – 2.921)

0.155 – 0.195*

(3.937 – 4.953)

0.013 – 0.023

(0.330 – 0.584)

T7 (TO-220) 0399

RELATED PARTS

PART NUMBER DESCRIPTION COMMENTS

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LT1374/LT1374HV 4.5A, 500kHz Step-Down Switching Regulators VIN Up to 25V (32V for HV), I

LT1370 6A, 500kHz High Efficiency Switching Regulator 6A/42V Internal Switch, 7-Lead DD/TO-220

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LT1765 3A, 1.25MHz, Step-Down Regulator VIN = 3V to 25V, VµF =1.2V, TSSOP-16E, SO8 Package

Linear Technology Corporation

16

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone]

●

www.linear.com

Up to 1.25A, SO-8

OUT

Up to 4.25A, SO-8/DD

OUT

Up to 0.5A, SO-8

OUT

Up to 50A, Current Mode

OUT

LT/CPI 0202 1.5K REV D • PRINTED IN USA

© LINEAR TECHNOLOGY CORPORATION 1994

sn1074 1074fds