LT1074/LT1076
Step-Down Switching
Regulator
ponents, are included on the chip. The topology is a classic
FEATURES
positive  buck configuration but several design innova-
5A Onboard Switch (LT1074)
tions allow this device to be used as a positive-to-negative
Operates Up to 60V Input
converter, a negative boost converter, and as a flyback
100kHz Switching Frequency
converter. The switch output is specified to swing 40V
Greatly Improved Dynamic Behavior
below ground, allowing the LT1074 to drive a tapped-
Available in Low Cost 5 and 7-Lead Packages
inductor in the buck mode with output currents up to 10A.
Only 8.5mA Quiescent Current
The LT1074 uses a true analog multiplier in the feedback
Programmable Current Limit
loop. This makes the device respond nearly instanta-
Micropower Shutdown Mode
neously to input voltage fluctuations and makes loop gain
independent of input voltage. As a result, dynamic behav-
U
APPLICATIO S ior of the regulator is significantly improved over previous
designs.
Buck Converter with Output Voltage Range of 2.5V
On-chip pulse by pulse current limiting makes the LT1074
to 50V
nearly bust-proof for output overloads or shorts. The input
Tapped-Inductor Buck Converter with 10A Output
voltage range as a buck converter is 8V to 60V, but a self-
at 5V
boot feature allows input voltages as low as 5V in the
Positive-to-Negative Converter
inverting and boost configurations.
Negative Boost Converter
Multiple Output Buck Converter
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
DESCRIPTIO
which allows current limit to be adjusted down to zero. In
The LT®1074 is a 5A (LT1076 is rated at 2A) monolithic
addition, full micropower shutdown can be programmed.
bipolar switching regulator which requires only a few
See Application Note 44 for design details.
external parts for normal operation. The power switch, all
A fixed 5V output, 2A version is also available. See LT1076-5.
oscillator and control circuitry, and all current limit com-
, LTC and LT are registered trademarks of Linear Technology Corporation.
Buck Converter Efficiency
TYPICAL APPLICATIO
LT1074
Basic Positive Buck Converter
100
L1**
50µH (LT1074)
VOUT = 12V, V = 20V
IN
100µH (LT1076) 90
5V
VIN VSW
5A * USE MBR340 FOR LT1076
10V TO 40V
** COILTRONICS #50-2-52 (LT1074) 80
R1
LT1074
MBR745*
#100-1-52 (LT1076) VOUT = 5V, V = 15V
2.8k
IN
PULSE ENGINEERING, INC.
1%
70
FB
#PE-92114 (LT1074)
GND VC
#PE-92102 (LT1076)
R2
R3 L = 50µH TYPE 52 CORE
60
HURRICANE #HL-AK147QQ (LT1074)
2.21k
2.7k DIODE = MBR735
#HL-AG210LL (LT1076)
1%
+ +

C3 C2 C1
RIPPLE CURRENT RATING e" IOUT/2
50
200µF 0.01µF 500µF
0 1 2 3 4 5 6
25V
OUTPUT LOAD CURRENT (A)
LT1074" TA01 LT1074" TPC27
1074fd
1
EFFICIENCY (%)
U
U
LT1074/LT1076
ABSOLUTE AXI U RATI GS
(Note 1)
Input Voltage ILIM Pin Voltage (Forced) ............................................ 5.5V
LT1074/ LT1076 .................................................. 45V Maximum Operating Ambient Temperature Range
LT1074HV/LT1076HV ......................................... 64V Commercial .................................................0° C to 70° C
Switch Voltage with Respect to Input Voltage Industrial ................................................  40° C to 85° C
LT1074/ LT1076 .................................................. 64V Military (OBSOLETE) .....................  55° C to 125° C
LT1074HV/LT1076HV ......................................... 75V Maximum Operating Junction Temperature Range
Switch Voltage with Respect to Ground Pin (VSW Negative) Commercial ...............................................0° C to 125° C
LT1074/LT1076 (Note 7) ..................................... 35V Industrial ..............................................  40° C to 125° C
LT1074HV/LT1076HV (Note 7) ........................... 45V Military (OBSOLETE) ....................  55° C to 150° C
Feedback Pin Voltage.....................................  2V, +10V Maximum Storage Temperature ...............  65° C to 150° C
Shutdown Pin Voltage (Not to Exceed VIN) .............. 40V Lead Temperature (Soldering, 10 sec)...................... 300° C
U W U
PACKAGE/ORDER I FOR ATIO
ORDER PART ORDER PART
FRONT VIEW BOTTOM VIEW
5 VIN VSW VC
NUMBER NUMBER
4 VSW
TAB IS
1
3 GND 2
GND LT1076CQ
LT1074CK
CASE
2 VC
IS GND
LT1076IQ 3 LT1074HVCK
4
1 FB/SENSE
LT1074MK
Q PACKAGE
VIN
FB
LT1074HVMK
5-LEAD PLASTIC DD
K PACKAGE
LT1076CK
4-LEAD TO-3 METAL CAN
LT1076: ¸ = 4° C, ¸ = 30° C/W
JC JA
LT1076HVCK
LT1074: ¸ = 2.5° C, ¸ = 35° C/W
JC JA
LT1076: ¸ = 4° C, ¸ = 35° C/W
JC JA LT1076MK
LT1076CR
FRONT VIEW
LT1076HVMK
OBSOLETE PACKAGE
7 SHDN LT1076IR
Consider the T5 Package for Alternate Source
6 VC
LT1076HVCR
TAB IS 5 FB/SENSE
4 GND
GND
LT1076HVIR
3 ILIM LT1074CT
FRONT VIEW
2 VSW
5 VIN LT1074HVCT
1 VIN
4 VSW
LT1074IT
R PACKAGE
TAB IS
7-LEAD PLASTIC DD 3 GND
LT1074HVIT
GND
2 VC
LT1076: ¸ = 4° C, ¸ = 30° C/W LT1076CT
JC JA
1 FB
LT1076HVCT
T PACKAGE
5-LEAD PLASTIC TO-220
LT1074CT7
LT1076IT
FRONT VIEW
LEADS ARE FORMED STANDARD FOR
LT1074HVCT7
LT1076HVIT
STRAIGHT LEADS, ORDER FLOW 06
SHDN
7
VC
6
LT1074IT7
FB
5
TAB IS LT1074: ¸ = 2.5° C, ¸ = 50° C/W
JC JA
GND
4 LT1074HVIT7
LT1076: ¸ = 4° C, ¸ = 50° C/W
JC JA
GND ILIM
3
LT1076CT7
VSW
2
VIN
1
LT1076HVCT7
T7 PACKAGE
7-LEAD PLASTIC TO-220
LT1074: ¸ = 2.5° C, ¸ = 50° C/W
JC JA
LT1076: ¸ = 4° C, ¸ = 50° C/W
JC JA
*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.
1074fd
2
WW
U
W
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 MIN TYP MAX UNITS
Switch  On Voltage (Note 2) LT1074 ISW = 1A, Tj e" 0° C 1.85 V
ISW = 1A, Tj < 0° C 2.1 V
ISW = 5A, Tj e" 0° C 2.3 V
ISW = 5A, Tj < 0° C 2.5 V
LT1076 ISW = 0.5A 1.2 V
ISW = 2A 1.7 V
Switch  Off Leakage LT1074 VIN d" 25V, VSW = 0 5 300 µ A
VIN = VMAX, VSW = 0 (Note 8) 10 500 µ A
LT1076 VIN = 25V, VSW = 0 150 µ A
VIN = VMAX, VSW = 0 (Note 8) 250 µ A
Supply Current (Note 3) VFB = 2.5V, VIN d" 40V 8.5 11 mA
40V < VIN < 60V 9 12 mA
VSHUT = 0.1V (Device Shutdown) (Note 9) 140 300 µ A
Minimum Supply Voltage Normal Mode 7.3 8 V
Startup Mode (Note 4) 3.5 4.8 V
Switch Current Limit (Note 5) LT1074 ILIM Open 5.5 6.5 8.5 A
RLIM = 10k (Note 6) 4.5 A
RLIM = 7k (Note 6) 3 A
LT1076 ILIM Open 2 2.6 3.2 A
RLIM = 10k (Note 6) 1.8 A
RLIM = 7k (Note 6) 1.2 A
Maximum Duty Cycle 85 90 %
Switching Frequency 90 100 110 kHz
Tj d" 125° C 85 120 kHz
Tj > 125° C 85 125 kHz
VFB = 0V through 2k&! (Note 5) 20 kHz
Switching Frequency Line Regulation 8V d" VIN d" VMAX (Note 8) 0.03 0.1 %/V
Error Amplifier Voltage Gain (Note 7) 1V d" VC d" 4V 2000 V/V
Error Amplifier Transconductance 3700 5000 8000 µ mho
Error Amplifier Source and Sink Current Source (VFB = 2V) 100 140 225 µ A
Sink (VFB = 2.5V) 0.7 1 1.6 mA
Feedback Pin Bias Current VFB = VREF 0.5 2 µ A
Reference Voltage VC = 2V 2.155 2.21 2.265 V
Reference Voltage Tolerance VREF (Nominal) = 2.21V Ä… 0.5 Ä… 1.5 %
All Conditions of Input Voltage, Output Ä… 1 Ä… 2.5 %
Voltage, Temperature and Load Current
Reference Voltage Line Regulation 8V d" VIN d" VMAX (Note 8) 0.005 0.02 %/V
VC Voltage at 0% Duty Cycle 1.5 V
Over Temperature  4 mV/° C
Multiplier Reference Voltage 24 V
Shutdown Pin Current VSH = 5V 5 10 20 µ A
VSH d" VTHRESHOLD (E" 2.5V) 50 µ A
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
1074fd
3
LT1074/LT1076
ELECTRICAL CHARACTERISTICS
Note 1: Absolute Maximum Ratings are those values beyond which the life Note 5: Switch frequency is internally scaled down when the feedback pin
of a device may be impaired. 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 1µ s.
Note 2: To calculate maximum switch  on voltage at currents between
low and high conditions, a linear interpolation may be used.
RLIM  1k RLIM  1k
Note 6: ILIM H" (LT1074), ILIM H" (LT1076).
Note 3: A feedback pin voltage (VFB) of 2.5V forces the VC pin to its low
2k 5.5k
clamp level and the switch duty cycle to zero. This approximates the zero
Note 7: Switch to input voltage limitation must also be observed.
load condition where duty cycle approaches zero.
Note 8: VMAX = 40V for the LT1074/76 and 60V for the LT1074HV/76HV.
Note 4: Total voltage from VIN pin to ground pin must be e" 8V after start-
Note 9: Does not include switch leakage.
up for proper regulation.
BLOCK DIAGRA
INPUT SUPPLY
LT1074
320 µ A
10 µ A
0.3V
+
6V 500&!
6V TO ALL
µ-POWER
REGULATOR
SHUTDOWN CIRCUITRY
AND BIAS

CURRENT
LIMIT
0.04
COMP
+
CURRENT
2.35V
LIMIT
C2
+
250 &!
SHUTDOWN


ILIM*
SHUTDOWN*
4.5V 10k
FREQ SHIFT
R
100kHz
R/S G1
S Q
OSCILLATOR
LATCH
SYNC R
3V(p-p)
VIN
+
400 &! 15&!
Z
+ C1
ANALOG
A1
MULTIPLIER 
ERROR
X
XY PULSE WIDTH
AMP
Z
COMPARATOR
2.21V 
SWITCH
Y
OUTPUT
(VSW)
FB VC 24V (EQUIVALENT)
LT1076
0.1&!
*AVAILABLE ON PACKAGES WITH PIN
COUNTS GREATER THAN 5.
100&!
SWITCH
OUTPUT (VSW)
LT1074 " BD01
1074fd
4
W
LT1074/LT1076
BLOCK DIAGRA DESCRIPTIO
A switch cycle in the LT1074 is initiated by the oscillator voltages by feeding the FB signal into the oscillator and
setting the R/S latch. The pulse that sets the latch also creating a linear frequency downshift when the FB signal
locks out the switch via gate G1. The effective width of this drops below 1.3V. Current trip level is set by the voltage on
pulse is approximately 700ns, which sets the maximum the ILIM pin which is driven by an internal 320µ A current
switch duty cycle to approximately 93% at 100kHz switch- source. When this pin is left open, it self-clamps at about
ing frequency. The switch is turned off by comparator C1, 4.5V and sets current limit at 6.5A for the LT1074 and 2.6A
which resets the latch. C1 has a sawtooth waveform as one for the LT1076. In the 7-pin package an external resistor
input and the output of an analog multiplier as the other can be connected from the ILIM pin to ground to set a lower
input. The multiplier output is the product of an internal current limit. A capacitor in parallel with this resistor will
reference voltage, and the output of the error amplifier, A1, soft-start the current limit. A slight offset in C2 guarantees
divided by the regulator input voltage. In standard buck that when the ILIM pin is pulled to within 200mV of ground,
regulators, this means that the output voltage of A1 C2 output will stay high and force switch duty cycle to zero.
required to keep a constant regulated output is indepen-
The  Shutdown pin is used to force switch duty cycle to
dent of regulator input voltage. This greatly improves line
zero by pulling the ILIM pin low, or to completely shut down
transient response, and makes loop gain independent of
the regulator. Threshold for the former is approximately
input voltage. The error amplifier is a transconductance
2.35V, and for complete shutdown, approximately 0.3V.
type with a GM at null of approximately 5000µ mho. Slew
Total supply current in shutdown is about 150µ A. A 10µ A
current going positive is 140µ A, while negative slew
pull-up current forces the shutdown pin high when left
current is about 1.1mA. This asymmetry helps prevent
open. A capacitor can be used to generate delayed start-
overshoot on start-up. Overall loop frequency compensa-
up. A resistor divider will program  undervoltage lockout
tion is accomplished with a series RC network from VC to
if the divider voltage is set at 2.35V when the input is at the
ground.
desired trip point.
Switch current is continuously monitored by C2, which
The switch used in the LT1074 is a Darlington NPN (single
resets the R/S latch to turn the switch off if an overcurrent
NPN for LT1076) driven by a saturated PNP. Special
condition occurs. The time required for detection and
patented circuitry is used to drive the PNP on and off very
switch turn off is approximately 600ns. So minimum
quickly even from the saturation state. This particular
switch  on time in current limit is 600ns. Under dead
switch arrangement has no  isolation tubs connected to
shorted output conditions, switch duty cycle may have to
the switch output, which can therefore swing to 40V below
be as low as 2% to maintain control of output current. This
ground.
would require switch on time of 200ns at 100kHz switch-
ing frequency, so frequency is reduced at very low output
1074fd
5
U
W
LT1074/LT1076
TYPICAL PERFOR A CE CHARACTERISTICS
VC Pin Characteristics VC Pin Characteristics Feedback Pin Characteristics
2.0
200 500
400
1.5
150
300
V ADJUSTED FOR 1.0
100 FB
V e" 2.5V
FB
200
I = 0 AT V = 2V START OF
C C
FREQUENCY SHIFTING
0.5
50
100
0
0 0
 100
 0.5
 50
SLOPE H" 400k&!
 200
 1.0
 100
 300
V FB d" 2V
 1.5
 150
 400
 2.0
 200  500
0 1 2 3 4 5 6 7 8 9
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 10
VOLTAGE (V)
VOLTAGE (V) VOLTAGE (V)
LT1074" TPC02
LT1074" TPC01 LT1074" TPC03
Shutdown Pin Characteristics Shutdown Pin Characteristics ILIM Pin Characteristics
40 0 100
Tj = 25°C
50
30  5
CURRENT FLOWS OUT
0
OF SHUTDOWN PIN
20  10
T = 25°C
j
 50
VIN = 50V
10  15
SHUTDOWN  100
THIS POINT MOVES
THRESHOLD
0 WITH VIN  20
 150
 200
 10  25
 250
 20  30
DETAILS OF THIS  300
AREA SHOWN IN
 30  35
 350
OTHER GRAPH
 40  40
 400
0 10 20 30 40 50 60 70 80 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
 2  1 0 1 2 3 4 5 6 7 8
VOLTAGE (V) VOLTAGE (V)
VOLTAGE (V)
LT1074" TPC04 LT1074" PC05
LT1074" TPC06
Supply Current
20
18
16
14
DEVICE NOT SWITCHING
12
V = 1V
C
10
8
6
4
2
0
0 10 20 30 40 50 60
INPUT VOLTAGE (V)
LT1074" TPC11
1074fd
6
CURRENT (mA)
CURRENT (
µ
A)
CURRENT (mA)
CURRENT (
µ
A)
CURRENT (
µ
A)
CURRENT (
µ
A)
INPUT CURRENT (mA)
U
W
LT1074/LT1076
TYPICAL PERFOR A CE CHARACTERISTICS
Reference Voltage vs
Supply Current (Shutdown) Temperature Switch  On Voltage
300 2.25
3.0
T = 25°C
j
2.24
250
2.5
2.23
200
2.22
2.0 LT1074
150 2.21
1.5
2.20
100
2.19 LT1076
1.0
50
2.18
2.17
0
0.5
0 10 20 30 40 50 60  50  25 0 25 50 75 100 125 150
0 1 2 3 4 5 6
INPUT VOLTAGE (V) JUNCTION TEMPERATURE (°C)
SWITCH CURRENT (A)
LT1074" TPC13 LT1074" TPC14
LT1074" TPC28
Switching Frequency vs
Reference Shift with Ripple
Temperature
Voltage Error Amplifier Phase and GM
20 8k 200 120
10
7k 150 115
0
¸
6k 100 110
 10
TRI WAVE
5k 50 105
 20
SQUARE
 30 4k 0 100
WAVE
GM
 40
3k  50 95
 50
2k 90
 100
 60
1k 85
 150
 70
0 80
 80  200
0 20 40 60 80 100 120 140 160 180 200 1k 10k 100k 1M 10M  50  25 0 25 50 75 100 125 150
PEAK-TO-PEAK RIPPLE AT FB PIN (mV) FREQUENCY (Hz) JUNCTION TEMPERATURE (°C)
LT1074" TPC16 LT1074" TPC17 LT1074" TPC18
Feedback Pin Frequency Shift Current Limit vs Temperature*
160 8
140
7
I PIN OPEN
LIM
120 6
100
5
R = 10k&!
LIM
80
4
150°C
60
3
 55°C
R = 5k&!
LIM
40
2
25°C
20
1
*MULTIPLY CURRENTS BY 0.4 FOR LT1076
0 0
0 0.5 1.0 1.5 2.0 2.5 3.0
 50  25 0 25 50 75 100 125 150
FEEDBACK PIN VOLTAGE (V)
JUNCTION TEMPERATURE (°C)
LT1074" TPC19
LT1074" TPC22
1074fd
7
PHASE (
°
)
VOLTAGE (V)
 ON VOLTAGE (V)
INPUT CURRENT (
µ
A)
FREQUENCY (kHz)
TRANSCONDUCTANCE (
µ
mho)
CHANGE IN REFERENCE VOLTAGE (mV)
OUTPUT CURRENT LIMIT (A)
SWITCHING FREQUENCY (kHz)
U
W
LT1074/LT1076
PI DESCRIPTIO S
VIN PIN
" VGND VOUT
( )( )
The VIN pin is both the supply voltage for internal control " VOUT =
221
.
circuitry and one end of the high current switch. It is
important, especially at low input voltages, that this pin be
To ensure good load regulation, the ground pin must be
bypassed with a low ESR, and low inductance capacitor to
connected directly to the proper output node, so that no
prevent transient steps or spikes from causing erratic
high currents flow in this path. The output divider resistor
operation. At full switch current of 5A, the switching
should also be connected to this low current connection
transients at the regulator input can get very large as
line as shown in Figure 2.
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.
LT1074
FB
GND
dl
LP
( )
( )
dt R2
STEP =
ISW ESR
( )( )
RAMP =
HIGH CURRENT NEGATIVE OUTPUT NODE
ISW TON
( )( )
RETURN PATH WHERE LOAD REGULATION
C
WILL BE MEASURED
LT1074" PD01 LT1074" PD02
Figure 1. Input Capacitor Ripple Figure 2. Proper Ground Pin Connection
LP = Total inductance in input bypass connections FEEDBACK PIN
and capacitor.
The feedback pin is the inverting input of an error amplifier
 Spike height (dI/dt " LP) is approximately 2V per which controls the regulator output by adjusting duty
inch of lead length for LT1074 and 0.8V per inch for cycle. The noninverting input is internally connected to a
LT1076. trimmed 2.21V reference. Input bias current is typically
0.5µ A when the error amplifier is balanced (IOUT = 0). The
 Step for ESR = 0.05&! and ISW = 5A is 0.25V.
error amplifier has asymmetrical GM for large input sig-
 Ramp for C = 200µ F, TON = 5µ s, and ISW = 5A,
nals to reduce startup overshoot. This makes the amplifier
is 0.12V.
more sensitive to large ripple voltages at the feedback pin.
Input current on the VIN Pin in shutdown mode is the sum
100mVp-p ripple at the feedback pin will create a 14mV
of actual supply current (H" 140µ A, with a maximum of
offset in the amplifier, equivalent to a 0.7% output voltage
300µ A), and switch leakage current. Consult factory for
shift. To avoid output errors, output ripple (P-P) should be
special testing if shutdown mode input current is critical.
less than 4% of DC output voltage at the point where the
output divider is connected.
GROUND PIN
See the  Error Amplifier section for more details.
It might seem unusual to describe a ground pin, but in the
Frequency Shifting at the Feedback Pin
case of regulators, the ground pin must be connected
properly to ensure good load regulation. The internal
The error amplifier feedback pin (FB) is used to downshift
reference voltage is referenced to the ground pin; so any
the oscillator frequency when the regulator output voltage
error in ground pin voltage will be multiplied at the output;
is low. This is done to guarantee that output short-circuit
1074fd
8
U
U
LT1074/LT1076
PI DESCRIPTIO S
current is well controlled even when switch duty cycle SHUTDOWN PIN
must be extremely low. Theoretical switch  on time for a
The shutdown pin is used for undervoltage lockout, micro-
buck converter in continuous mode is:
power shutdown, soft-start, delayed start, or as a general
purpose on/off control of the regulator output. It controls
VOUT + VD
switching action by pulling the ILIM pin low, which forces
tON =
VIN " f
the switch to a continuous  off state. Full micropower
shutdown is initiated when the shutdown pin drops below
VD = Catch diode forward voltage ( H" 0.5V)
0.3V.
f = Switching frequency
The V/I characteristics of the shutdown pin are shown in
At f = 100kHz, tON must drop to 0.2µ s when VIN = 25V
Figure 4. For voltages between 2.5V and H" VIN, a current of
and the output is shorted (VOUT = 0V). In current limit,
10µ A flows out of the shutdown pin. This current in-
the LT1074 can reduce tON to a minimum value of
creases to H" 25µ A as the shutdown pin moves through the
H" 0.6µ s, much too long to control current correctly for
2.35V threshold. The current increases further to H" 30µ A at
VOUT = 0. To correct this problem, switching frequency
the 0.3V threshold, then drops to H" 15µ A as the shutdown
is lowered from 100kHz to 20kHz as the FB pin drops
voltage fall below 0.3V. The 10µ A current source is in-
from 1.3V to 0.5V. This is accomplished by the circuitry
cluded to pull the shutdown pin to its high or default state
when left open. It also provides a convenient pull-up for
TO
delayed start applications with a capacitor on the shut-
OSCILLATOR
down pin.
VOUT
+2V Q1
When activated, the typical collector current of Q1 in
R1
Figure 5, is H" 2mA. A soft-start capacitor on the ILIM pin will
+ 2.21V R3
3k
ERROR
delay regulator shutdown in response to C1, by
AMPLIFIER
EXTERNAL
VC
H" (5V)(CLIM)/2mA. Soft-start after full micropower shut-
 DIVIDER
FB
down is ensured by coupling C2 to Q1.
R2
2.21k
0
LT1074" PD03
Tj = 25°C
 5
CURRENT FLOWS OUT
Figure 3. Frequency Shifting
OF SHUTDOWN PIN
 10
 15
shown in Figure 3.
SHUTDOWN
THRESHOLD
 20
Q1 is off when the output is regulating (VFB = 2.21V). As
 25
the output is pulled down by an overload, VFB will eventu-
 30
ally reach 1.3V, turning on Q1. As the output continues to
drop, Q1 current increases proportionately and lowers the
 35
frequency of the oscillator. Frequency shifting starts when
 40
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
the output is H" 60% of normal value, and is down to its
VOLTAGE (V)
minimum value of E" 20kHz when the output is E" 20% of
LT1074" PC05
normal value. The rate at which frequency is shifted is
Figure 4. Shutdown Pin Characteristics
determined by both the internal 3k resistor R3 and the
external divider resistors. For this reason, R2 should not
be increased to more than 4k&!, if the LT1074 will be
subjected to the simultaneous conditions of high input
voltage and output short-circuit.
1074fd
9
CURRENT (
µ
A)
U
U
LT1074/LT1076
PI DESCRIPTIO S
VIN
Hysteresis in undervoltage lockout may be accomplished
by connecting a resistor (R3) from the ILIM pin to the
300 µ A
10 µ A
shutdown pin as shown in Figure 7. D1 prevents the
shutdown divider from altering current limit.
SHUTDOWN

PIN
ILIM
PIN
C1
VIN
R1
2.3V +
SHUT
LT1074
EXTERNAL D1*
Q1 R3
6V
CLIM
ILIM

R2
C2
OPTIONAL CURRENT
LIMIT RESISTOR
0.3V +
LT1074" PD09
*1N4148
TO TOTAL
REGULATOR
Figure 7. Adding Hysteresis
SHUTDOWN
LT1074" PD07
Figure 5. Shutdown Circuitry
ëÅ‚
R1öÅ‚
Trip Point = VTP = 235VìÅ‚ 1+
.
R2÷Å‚
íÅ‚ Å‚Å‚
Undervoltage Lockout
Undervoltage lockout point is set by R1 and R2 in Figure#$6.
If R3 is added, the lower trip point (VIN descending) will be
To avoid errors due to the 10µ A shutdown pin current, R2
the same. The upper trip point (VUTP) will be:
is usually set at 5k, and R1 is found from:
ëÅ‚
R1 R1öÅ‚ ëÅ‚ R1öÅ‚
VUTP = VSHìÅ‚ 1 + - 08VìÅ‚
.
VTP - VSH
()
R2 R3÷Å‚ R3÷Å‚
íÅ‚ Å‚Å‚ íÅ‚ Å‚Å‚
R1= R2
VSH
If R1 and R2 are chosen, R3 is given by:
VTP = Desired undervoltage lockout voltage
VSH - 08V R1
.
()( )
VSH = Threshold for lockout on the
R3 =
shutdown pin = 2.45V
ëÅ‚
R1öÅ‚
VUTP - VSHìÅ‚ 1
+
If quiescent supply current is critical, R2 may be increased
R2÷Å‚
íÅ‚ Å‚Å‚
up to 15k&! , but the denominator in the formula for R2
should replace VSH with VSH  (10µ A)(R2).
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.
R1
VIN
SHUT
LT1074
15V - 2.35V
R1= 2.34k = 12.5k
( )( 235V )
R2
.
GND
5k
235 - 0.8 125
. .
()( )
R3 = = 39k
.
LT1074" PD08
ëÅ‚
12.5öÅ‚
20 - 2.35ìÅ‚ 1
+
Figure 6. Undervoltage Lockout
232÷Å‚
.
íÅ‚ Å‚Å‚
1074fd
10
U
U
LT1074/LT1076
PI DESCRIPTIO S
ILIM PIN from forcing current back into the ILIM pin. To calculate a
value for RFB, first calculate RLIM, the RFB:
The ILIM pin is used to reduce current limit below the
preset value of 6.5A. The equivalent circuit for this pin is
I - 044 * RL
.
(SC )( ))
shown in Figure 8.
RFB = RL in k&!
(
05* RL - 1k - ISC
. &!
TO LIMIT
()
VIN
CIRCUIT
320 µ A
*Change 0.44 to 0.16, and 0.5 to 0.18 for LT1076.
D2
Q1 Example: ILIM = 4A, ISC = 1.5A, RLIM = (4)(2k) + 1k = 9k
D1
4.3V
R1
15 - 044 9k&!
. .
()( )(38k&! )
8K
D3
RFB = .
6V
059k - 1k - 15
..
( )
I LIM
LT1047" PD12 VOUT
Figure 8. ILIM Pin Circuit
LT1074
FB
I LIM
When ILIM is left open, the voltage at Q1 base clamps at 5V
through D2. Internal current limit is determined by the
RFB D2
current through Q1. If an external resistor is connected
RLIM
1N4148
between ILIM and ground, the voltage at Q1 base can be
reduced for lower current limit. The resistor will have a
LT1074" PD13
voltage across it equal to (320µ A)(R), limited to H" 5V when
Figure 9. Foldback Current Limit
clamped by D2. Resistance required for a given current
limit is:
Error Amplifier
RLIM = ILIM(2k&! ) + 1k&! (LT1074)
The error amplifier in Figure 10 is a single stage design
with added inverters to allow the output to swing above
RLIM = ILIM(5.5k&! ) + 1k&! (LT1076)
and below the common mode input voltage. One side of
As an example, a 3A current limit would require
the amplifier is tied to a trimmed internal reference voltage
3A(2k) + 1k = 7k&! for the LT1074. The accuracy of these
of 2.21V. The other input is brought out as the FB (feed-
formulas is Ä… 25% for 2A d" ILIM d" 5A (LT1074) and
back) pin. This amplifier has a GM (voltage  in to current
7A d" ILIM d" 1.8A (LT1076), so ILIM should be set at least
 out ) transfer function of H" 5000µ mho. Voltage gain is
25% above the peak switch current required.
determined by multiplying GM times the total equivalent
Foldback current limiting can be easily implemented by output loading, consisting of the output resistance of Q4
adding a resistor from the output to the ILIM pin as shown and Q6 in parallel with the series RC external frequency
in Figure 9. This allows full desired current limit (with or compensation network. At DC, the external RC is ignored,
without RLIM) when the output is regulating, but reduces and with a parallel output impedance for Q4 and Q6 of
current limit under short-circuit conditions. A typical value 400k&! , voltage gain is H" 2000. At frequencies above a few
for RFB is 5k&! , but this may be adjusted up or down to set hertz, voltage gain is determined by the external compen-
the amount of foldback. D2 prevents the output voltage sation, RC and CC.
1074fd
11
U
U
LT1074/LT1076
PI DESCRIPTIO S
5.8V
Q4
90 µ A
µ
90 A
Q3
50 µ A
VC
D1
EXTERNAL
FREQUENCY
Q1 Q2 FB
COMPENSATION
50 µ A
90 µ A
X1.8
D2
RC
Q6
2.21V
140 µ A
CC
300&!
LT1074 " PD11
ALL CURRENTS SHOWN ARE AT NULL CONDITION
Figure 10. Error Amplifier
The error amplifier has asymmetrical peak output current.
Gm
Q3 and Q4 current mirrors are unity-gain, but the Q6
AV = at mid frequencies
2Ä„ " f " CC 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
AV = Gm " RC at high frequencies
maximum positive output current of 140µ A and a maxi-
Phase shift from the FB pin to the VC pin is 90° at mid mum negative (sink) output current of E" 1.1mA. The asym-
frequencies where the external CC is controlling gain, then metry is deliberate it results in much less regulator
drops back to 0° (actually 180° since FB is an inverting output overshoot during rapid start-up or following the
input) when the reactance of CC is small compared to RC. release of an output overload. Amplifier offset is kept low
The low frequency  pole where the reactance of CC is by area scaling Q1 and Q2 at 1.8:1.
equal to the output impedance of Q4 and Q6 (rO), is:
Amplifier swing is limited by the internal 5.8V supply for
positive outputs and by D1 and D2 when the output goes
1
low. Low clamp voltage is approximately one diode drop
fPOLE = rO H" 400k
&!
2Ä„ " rO " C
(H" 0.7V  2mV/° C).
Although fPOLE varies as much as 3:1 due to rO variations, Note that both the FB pin and the VC pin have other internal
mid-frequency gain is dependent only on Gm, which is connections. Refer to the frequency shifting and synchro-
specified much tighter on the data sheet. The higher nizing discussions.
frequency  zero is determined solely by RC and CC.
1
fZERO =
2Ä„ " RC " CC
1074fd
12
U
U
LT1074/LT1076
TYPICAL APPLICATIO S
Tapped-Inductor Buck Converter
L2
L1*
5µH
VIN VOUT
VIN VSW
20V TO 35V 5V, 10A
3 1
D2
R1
35V D1**
LT1074HV
2.8k
5W
C1
FB + 4400µF
GND VC
(2 EA
2200µF,
+ C4
D3
R2
R3
16V)
390µF
1N5819
2.21k
1k
16V
+
C3 C2
0.01µF
200µF 0.2µF
50V
* PULSE ENGINEERING #PEÄ… 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
+
C1
4.5V to
220µF
40V
50V
+
L1
25µH
5A
R3*
R1**
VIN VSW 2.74k
5.1k
+
C2
LT1074
1000µF
R2**
10V
10k
OPTIONAL FILTER
GND VC VFB
D1 5µH 
200µF
MBR745 R4
10V
C3 C4**
1.82k*
+
0.1µF 0.01µF
 5V,1A***

* = 1% FILM RESISTORS LOWER REVERSE VOLTAGE RATING MAY BE USED FOR LOWER INPUT VOLTAGES.
D1 = MOTOROLA-MBR745 LOWER CURRENT RATING IS ALLOWED FOR LOWER OUTPUT CURRENT. SEE AN44.
C1 = NICHICON-UPL1C221MRH6

LOWER CURRENT RATING MAY BE USED FOR LOWER OUTPUT CURRENT. SEE AN44.
C2 = NICHICON-UPL1A102MRH6
L1 = COILTRONICS-CTX25-5-52 ** 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.
C
R3 = VOUT  2.37 (K&!)
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.
LT1074 " TA03
1074fd
13
U
LT1074/LT1076
PACKAGE DESCRIPTIO
K Package
4-Lead TO-3 Metal Can
(Reference LTC DWG # 05-08-1311)
0.760  0.775
0.320  0.350
(19.30  19.69)
(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
72°
(12.45  12.95)
18°
R
K4(TO-3) 1098
OBSOLETE PACKAGE
Q Package
5-Lead Plastic DD Pak
(Reference LTC DWG # 05-08-1461)
0.060
0.390  0.415
(1.524)
(9.906  10.541)
0.060 TYP
0.165  0.180
0.256
(1.524)
(6.502) (4.191  4.572) 0.045  0.055
(1.143  1.397)
15° TYP
+0.008
0.004
0.060  0.004
0.183 0.059
0.330  0.370
(1.524)
(4.648) (1.499) +0.203
(8.382  9.398) 0.102
(  0.102
)
TYP
0.095  0.115
(2.413  2.921)
0.075
(1.905)
0.067 0.050 Ä… 0.012
0.300
+0.012 0.013  0.023
(1.70)
0.143 (1.270 Ä… 0.305)
(7.620)
 0.020
(0.330  0.584)
BSC
0.028  0.038
+0.305
BOTTOM VIEW OF DD PAK
3.632
Q(DD5) 1098
(0.711  0.965)
(  0.508
)
HATCHED AREA IS SOLDER PLATED
COPPER HEAT SINK
1074fd
14
U
LT1074/LT1076
PACKAGE DESCRIPTIO
R Package
7-Lead Plastic DD Pak
(Reference LTC DWG # 05-08-1462)
0.060
0.390  0.415
(1.524)
(9.906  10.541)
TYP
0.060 0.165  0.180
0.256
(1.524)
(4.191  4.572) 0.045  0.055
(6.502)
(1.143  1.397)
15° TYP
+0.008
0.004
 0.004
0.060
0.183 0.059
0.330  0.370
(1.524)
(4.648) (1.499) +0.203
(8.382  9.398) 0.102
(  0.102
)
TYP
0.095  0.115
(2.413  2.921)
0.075
(1.905)
0.050 0.050 Ä… 0.012
0.300
0.013  0.023
+0.012
(1.27)
0.143 (1.270 Ä… 0.305)
(7.620)  0.020
(0.330  0.584)
BSC
0.026  0.036
+0.305
BOTTOM VIEW OF DD PAK 3.632 (0.660  0.914)
(  0.508
)
R (DD7) 1098
HATCHED AREA IS SOLDER PLATED
COPPER HEAT SINK
T Package
5-Lead Plastic TO-220 (Standard)
(Reference LTC DWG # 05-08-1421)
0.165  0.180
0.147  0.155
0.390  0.415
(4.191  4.572)
(3.734  3.937)
0.045  0.055
(9.906  10.541)
DIA
(1.143  1.397)
0.230  0.270
(5.842  6.858)
0.570  0.620
0.620
0.460  0.500 (14.478  15.748)
(15.75)
(11.684  12.700)
TYP
0.330  0.370
0.700  0.728
(8.382  9.398)
(17.78  18.491)
0.095  0.115
SEATING PLANE
(2.413  2.921)
0.152  0.202
0.155  0.195*
(3.861  5.131)
0.260  0.320
(3.937  4.953)
(6.60  8.13)
0.013  0.023
(0.330  0.584)
0.067
BSC 0.135  0.165
0.028  0.038
(1.70)
(3.429  4.191)
(0.711  0.965) * MEASURED AT THE SEATING PLANE
T5 (TO-220) 0399
1074fd
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 represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 15
U
LT1074/LT1076
TYPICAL APPLICATIO
Negative Boost Converter
R1
100pF
12.7k
VIN VFB
R2
LT1074
2.21k
GND VC VSW
C3
+ + C1
200µF
1000µF
L1
C2
15V
D1* 25V
25µH
1nF
0.01µF
R3
750&!
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
PACKAGE DESCRIPTIO
T7 Package
7-Lead Plastic TO-220 (Standard)
(Reference LTC DWG # 05-08-1422)
0.165  0.180
0.147  0.155
0.390  0.415 (4.191  4.572)
(3.734  3.937)
0.045  0.055
(9.906  10.541)
DIA
(1.143  1.397)
0.230  0.270
(5.842  6.858)
0.570  0.620
0.620
0.460  0.500 (14.478  15.748)
(15.75)
(11.684  12.700)
TYP
0.330  0.370
0.700  0.728
(8.382  9.398)
(17.780  18.491)
0.095  0.115
SEATING PLANE
(2.413  2.921)
0.152  0.202
0.155  0.195*
(3.860  5.130)
0.260  0.320
(3.937  4.953)
(6.604  8.128)
0.013  0.023
0.050 0.026  0.036
BSC
(0.330  0.584)
(1.27) (0.660  0.914)
0.135  0.165
(3.429  4.191) *MEASURED AT THE SEATING PLANE
T7 (TO-220) 0399
RELATED PARTS
PART NUMBER DESCRIPTION COMMENTS
LT1375/LT1376 1.5A, 500kHz Step-Down Switching Regulators VIN Up to 25V, IOUT Up to 1.25A, SO-8
LT1374/LT1374HV 4.5A, 500kHz Step-Down Switching Regulators VIN Up to 25V (32V for HV), IOUT Up to 4.25A, SO-8/DD
LT1370 6A, 500kHz High Efficiency Switching Regulator 6A/42V Internal Switch, 7-Lead DD/TO-220
LT1676 Wide Input Range, High Efficiency Step-Down Regulator VIN from 7.4V to 60V, IOUT Up to 0.5A, SO-8
LT1339 High Power Synchronous DC/DC Controller VIN Up to 60V, IOUT Up to 50A, Current Mode
LT1765 3A, 1.25MHz, Step-Down Regulator VIN = 3V to 25V, Vµ F =1.2V, TSSOP-16E, SO8 Package
1074fd
LT/CPI 0202 1.5K REV D " PRINTED IN USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
16
[phone] FAX: [phone] www.linear.com © LINEAR TECHNOLOGY CORPORATION 1994
U
U