LT1074/LT1076
1
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
U
FEATURES
DESCRIPTIO
U
TYPICAL APPLICATIO
U
Step-Down Switching
Regulator
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
s
Buck Converter with Output Voltage Range of 2.5V
to 50V
s
Tapped-Inductor Buck Converter with 10A Output
at 5V
s
Positive-to-Negative Converter
s
Negative Boost Converter
s
Multiple Output Buck Converter
s
5A Onboard Switch (LT1074)
s
100kHz Switching Frequency
s
Greatly Improved Dynamic Behavior
s
Available in Low Cost 5 and 7-Lead Packages
s
Only 8.5mA Quiescent Current
s
Programmable Current Limit
s
Operates Up to 60V Input
s
Micropower Shutdown Mode
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 tapped-
inductor 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 instanta-
neously to input voltage fluctuations and makes loop gain
independent of input voltage. As a result, dynamic behav-
ior of the regulator is significantly improved over previous
designs.
On-chip pulse by pulse current limiting makes the LT1074
nearly bust-proof for output overloads or shorts. The input
voltage range as a buck converter is 8V to 60V, but a self-
boot feature allows input voltages as low as 5V in the
inverting and boost configurations.
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.
Buck Converter Efficiency
Basic Positive Buck Converter
+
+
V
SW
V
IN
V
C
GND
FB
LT1074
5V
5A
C1
500
F
R1
2.8k
1%
R2
2.21k
1%
MBR745*
10V TO 40V
C3
200
F
R3
2.7k
C2
0.01
F
L1**
50
H (LT1074)
100
H (LT1076)
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
OUT
/2
*
**
LT1074TA01
25V
OUTPUT LOAD CURRENT (A)
0
EFFICIENCY (%)
60
70
80
90
1
2
3
4
LT1074TPC27
5
6
100
50
L = 50
H TYPE 52 CORE
DIODE = MBR735
V = 5V, V = 15V
OUT
IN
V = 12V, V = 20V
OUT
IN
LT1074
LT1074/LT1076
2
ABSOLUTE AXI U
RATI GS
W
W
W
U
PACKAGE/ORDER I FOR ATIO
U
U
W
(Note 1)
*Assumes package is soldered to 0.5 IN
2
of 1 oz. copper over internal ground plane or over back side plane.
LT1076CQ
LT1076IQ
LT1076CR
LT1076IR
LT1076HVCR
LT1076HVIR
LT1074CT7
LT1074HVCT7
LT1074IT7
LT1074HVIT7
LT1076CT7
LT1076HVCT7
LT1074CT
LT1074HVCT
LT1074IT
LT1074HVIT
LT1076CT
LT1076HVCT
LT1076IT
LT1076HVIT
LT1074CK
LT1074HVCK
LT1074MK
LT1074HVMK
LT1076CK
LT1076HVCK
LT1076MK
LT1076HVMK
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
SW
Negative)
LT1074/LT1076 (Note 7) ..................................... 35V
LT1074HV/LT1076HV (Note 7) ........................... 45V
Feedback Pin Voltage ..................................... 2V, +10V
Shutdown Pin Voltage (Not to Exceed V
IN
) .............. 40V
I
LIM
Pin Voltage (Forced) ............................................ 5.5V
Maximum Operating Ambient Temperature Range
Commercial ................................................. 0
C to 70
C
Industrial ................................................ 40
C to 85
C
Military ................................................. 55
C to 125
C
Maximum Operating Junction Temperature Range
Commercial ............................................... 0
C to 125
C
Industrial .............................................. 40
C to 125
C
Military ................................................. 55
C to 150
C
Maximum Storage Temperature ............... 65
C to 150
C
Lead Temperature (Soldering, 10 sec) ...................... 300
C
ORDER PART
NUMBER
ORDER PART
NUMBER
LT1076:
JC
= 4
C,
JA
= 30
C/W
LT1076:
JC
= 4
C,
JA
= 30
C/W
LT1074:
JC
= 2.5
C,
JA
= 50
C/W
LT1076:
JC
= 4
C,
JA
= 50
C/W
LT1074:
JC
= 2.5
C,
JA
= 35
C/W
LT1076:
JC
= 4
C,
JA
= 35
C/W
LT1074:
JC
= 2.5
C,
JA
= 50
C/W
LT1076:
JC
= 4
C,
JA
= 50
C/W
Q PACKAGE
5-LEAD PLASTIC DD
FRONT VIEW
V
IN
V
SW
GND
V
C
FB/SENSE
5
4
3
2
1
R PACKAGE
7-LEAD PLASTIC DD
FRONT VIEW
SHDN
V
C
FB/SENSE
GND
I
LIM
V
SW
V
IN
7
6
5
4
3
2
1
2
4
1
3
V
C
V
IN
V
SW
CASE
IS GND
FB
K PACKAGE
4-LEAD TO-3 METAL CAN
BOTTOM VIEW
T PACKAGE
5-LEAD PLASTIC TO-220
LEADS ARE FORMED STANDARD FOR
STRAIGHT LEADS, ORDER FLOW 06
V
IN
V
SW
GND
V
C
FB
FRONT VIEW
5
4
3
2
1
T7 PACKAGE
7-LEAD PLASTIC TO-220
SHDN
V
C
FB
GND
I
LIM
V
SW
V
IN
FRONT VIEW
7
6
5
4
3
2
1
TAB IS
GND
TAB IS
GND
TAB IS
GND
TAB IS
GND
LT1074/LT1076
3
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C. T
j
= 25
C, V
IN
= 25V, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Switch "On" Voltage (Note 2)
LT1074
I
SW
= 1A, T
j
0
C
1.85
V
I
SW
= 1A, T
j
< 0
C
2.1
V
I
SW
= 5A, T
j
0
C
2.3
V
I
SW
= 5A, T
j
< 0
C
2.5
V
LT1076
I
SW
= 0.5A
q
1.2
V
I
SW
= 2A
q
1.7
V
Switch "Off" Leakage
LT1074
V
IN
25V, V
SW
= 0
5
300
A
V
IN
= V
MAX,
V
SW
= 0 (Note 8)
10
500
A
LT1076
V
IN
= 25V, V
SW
= 0
150
A
V
IN
= V
MAX,
V
SW
= 0 (Note 8)
250
A
Supply Current (Note 3)
V
FB
= 2.5V, V
IN
40V
q
8.5
11
mA
40V < V
IN
< 60V
q
9
12
mA
V
SHUT
= 0.1V (Device Shutdown) (Note 9)
q
140
300
A
Minimum Supply Voltage
Normal Mode
q
7.3
8
V
Startup Mode (Note 4)
q
3.5
4.8
V
Switch Current Limit (Note 5)
LT1074
I
LIM
Open
q
5.5
6.5
8.5
A
R
LIM
= 10k (Note 6)
4.5
A
R
LIM
= 7k (Note 6)
3
A
LT1076
I
LIM
Open
q
2
2.6
3.2
A
R
LIM
= 10k (Note 6)
1.8
A
R
LIM
= 7k (Note 6)
1.2
A
Maximum Duty Cycle
q
85
90
%
Switching Frequency
90
100
110
kHz
T
j
125
C
q
85
120
kHz
T
j
> 125
C
q
85
125
kHz
V
FB
= 0V through 2k
(Note 5)
20
kHz
Switching Frequency Line Regulation
8V
V
IN
V
MAX
(Note 8)
q
0.03
0.1
%/V
Error Amplifier Voltage Gain (Note 7)
1V
V
C
4V
2000
V/V
Error Amplifier Transconductance
3700
5000
8000
mho
Error Amplifier Source and Sink Current
Source (V
FB
= 2V)
100
140
225
A
Sink (V
FB
= 2.5V)
0.7
1
1.6
mA
Feedback Pin Bias Current
V
FB
= V
REF
q
0.5
2
A
Reference Voltage
V
C
= 2V
q
2.155
2.21
2.265
V
Reference Voltage Tolerance
V
REF
(Nominal) = 2.21V
0.5
1.5
%
All Conditions of Input Voltage, Output
q
1
2.5
%
Voltage, Temperature and Load Current
Reference Voltage Line Regulation
8V
V
IN
V
MAX
(Note 8)
q
0.005
0.02
%/V
V
C
Voltage at 0% Duty Cycle
1.5
V
Over Temperature
q
4
mV/
C
Multiplier Reference Voltage
24
V
Shutdown Pin Current
V
SH
= 5V
q
5
10
20
A
V
SH
V
THRESHOLD
(
2.5V)
q
50
A
Shutdown Thresholds
Switch Duty Cycle = 0
q
2.2
2.45
2.7
V
Fully Shut Down
q
0.1
0.3
0.5
V
Thermal Resistance Junction to Case
LT1074
2.5
C/W
LT1076
4.0
C/W
ELECTRICAL CHARACTERISTICS
LT1074/LT1076
4
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
FB
) of 2.5V forces the V
C
pin to its low
clamp level and the switch duty cycle to zero. This approximates the zero
load condition where duty cycle approaches zero.
Note 4: Total voltage from V
IN
pin to ground pin must be
8V after start-
up for proper regulation.
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
FB
is adjusted to give a minimum switch on time of 1
s.
Note 6: I
LIM
(LT1074)
,
I
LIM
(LT1076).
Note 7: Switch to input voltage limitation must also be observed.
Note 8: V
MAX
= 40V for the LT1074/76 and 60V for the LT1074HV/76HV.
Note 9: Does not include switch leakage.
R
LIM
1k
2k
R
LIM
1k
5.5k
ELECTRICAL CHARACTERISTICS
6V
REGULATOR
AND BIAS
+
-POWER
SHUTDOWN
10 A
320 A
+
2.35V
0.3V
CURRENT
LIMIT
SHUTDOWN
4.5V
10k
S
+
CURRENT
LIMIT
COMP
C2
15
400
+
C1
R/S
LATCH
R
R
Q
X
Y
Z
+
A1
ERROR
AMP
FB
V
100kHz
OSCILLATOR
ANALOG
MULTIPLIER
XY
Z
FREQ SHIFT
SYNC
G1
PULSE WIDTH
COMPARATOR
24V (EQUIVALENT)
2.21V
SWITCH
OUTPUT
(V )
0.04
250
500
INPUT SUPPLY
SHUTDOWN*
I *
6V TO ALL
CIRCUITRY
LIM
SW
V
IN
C
LT1074 BD01
100
SWITCH
OUTPUT (V )
0.1
SW
3V(p-p)
LT1076
LT1074
*AVAILABLE ON PACKAGES WITH PIN
COUNTS GREATER THAN 5.
BLOCK DIAGRA
W
LT1074/LT1076
5
A switch cycle in the LT1074 is initiated by the oscillator
setting the R/S latch. The pulse that sets the latch also
locks out the switch via gate G1. The effective width of this
pulse is approximately 700ns, which sets the maximum
switch duty cycle to approximately 93% at 100kHz switch-
ing frequency. The switch is turned off by comparator C1,
which resets the latch. C1 has a sawtooth waveform as one
input and the output of an analog multiplier as the other
input. The multiplier output is the product of an internal
reference voltage, and the output of the error amplifier, A1,
divided by the regulator input voltage. In standard buck
regulators, this means that the output voltage of A1
required to keep a constant regulated output is indepen-
dent of regulator input voltage. This greatly improves line
transient response, and makes loop gain independent of
input voltage. The error amplifier is a transconductance
type with a G
M
at null of approximately 5000
mho. Slew
current going positive is 140
A, while negative slew
current is about 1.1mA. This asymmetry helps prevent
overshoot on start-up. Overall loop frequency compensa-
tion is accomplished with a series RC network from V
C
to
ground.
Switch current is continuously monitored by C2, which
resets the R/S latch to turn the switch off if an overcurrent
condition occurs. The time required for detection and
switch turn off is approximately 600ns. So minimum
switch "on" time in current limit is 600ns. Under dead
shorted output conditions, switch duty cycle may have to
be as low as 2% to maintain control of output current. This
would require switch on time of 200ns at 100kHz switch-
ing frequency, so frequency is reduced at very low output
voltages by feeding the FB signal into the oscillator and
creating a linear frequency downshift when the FB signal
drops below 1.3V. Current trip level is set by the voltage on
the I
LIM
pin which is driven by an internal 320
A current
source. When this pin is left open, it self-clamps at about
4.5V and sets current limit at 6.5A for the LT1074 and 2.6A
for the LT1076. In the 7-pin package an external resistor
can be connected from the I
LIM
pin to ground to set a lower
current limit. A capacitor in parallel with this resistor will
soft-start the current limit. A slight offset in C2 guarantees
that when the I
LIM
pin is pulled to within 200mV of ground,
C2 output will stay high and force switch duty cycle to zero.
The "Shutdown" pin is used to force switch duty cycle to
zero by pulling the I
LIM
pin low, or to completely shut down
the regulator. Threshold for the former is approximately
2.35V, and for complete shutdown, approximately 0.3V.
Total supply current in shutdown is about 150
A. A 10
A
pull-up current forces the shutdown pin high when left
open. A capacitor can be used to generate delayed start-
up. A resistor divider will program "undervoltage lockout"
if the divider voltage is set at 2.35V when the input is at the
desired trip point.
The switch used in the LT1074 is a Darlington NPN (single
NPN for LT1076) driven by a saturated PNP. Special
patented circuitry is used to drive the PNP on and off very
quickly even from the saturation state. This particular
switch arrangement has no "isolation tubs" connected to
the switch output, which can therefore swing to 40V below
ground.
BLOCK DIAGRA
W
DESCRIPTIO
U
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