LT1074/LT1076
Step-Down Switching
Regulator
FEATURES
s
s
s
s
s
s
s
s
5A Onboard 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
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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
s
s
s
s
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
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
TYPICAL APPLICATIO
Basic Positive Buck Converter
L1**
50
µ
H (LT1074)
100
µ
H (LT1076)
10V TO 40V
V
IN
LT1074
FB
R3
2.7k
C3
†
200
µ
F
C2
0.01
µ
F
V
SW
MBR745*
R1
2.8k
1%
R2
2.21k
1%
+
LT1074
100
EFFICIENCY (%)
GND
V
C
+
U
U
U
Buck Converter Efficiency
5V
5A
C1
500
µ
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
OUT
/2
90
80
V
OUT
= 12V, V
IN
= 20V
V
OUT
= 5V, V
IN
= 15V
70
60
50
0
1
2
3
4
5
6
L = 50µH TYPE 52 CORE
DIODE = MBR735
LT1074•TA01
OUTPUT LOAD CURRENT (A)
LT1074•TPC27
1
LT1074/LT1076
ABSOLUTE
AXI U RATI GS
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
PACKAGE/ORDER I FOR ATIO
FRONT VIEW
5
V
IN
V
SW
GND
V
C
FB/SENSE
ORDER PART
NUMBER
LT1076CQ
LT1076IQ
TAB IS
GND
4
3
2
1
Q PACKAGE
5-LEAD PLASTIC DD
LT1076:
θ
JC
= 4°C,
θ
JA
= 30°C/W
FRONT VIEW
7
6
5
4
3
2
1
R PACKAGE
7-LEAD PLASTIC DD
SHDN
V
C
FB/SENSE
GND
I
LIM
V
SW
V
IN
TAB IS
GND
LT1076CR
LT1076IR
LT1076HVCR
LT1076HVIR
TAB IS
GND
LT1076:
θ
JC
= 4°C,
θ
JA
= 30°C/W
FRONT VIEW
7
6
5
4
3
2
1
T7 PACKAGE
7-LEAD PLASTIC TO-220
SHDN
V
C
FB
GND
I
LIM
V
SW
V
IN
TAB IS
GND
LT1074CT7
LT1074HVCT7
LT1074IT7
LT1074HVIT7
LT1076CT7
LT1076HVCT7
LT1074:
θ
JC
= 2.5°C,
θ
JA
= 50°C/W
LT1076:
θ
JC
= 4°C,
θ
JA
= 50°C/W
*Assumes package is soldered to 0.5 IN
2
of 1 oz. copper over internal ground plane or over back side plane.
2
U
U
W
W W
U
W
(Note 1)
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
BOTTOM VIEW
V
C
1
2
3
V
IN
ORDER PART
NUMBER
CASE
IS GND
4
FB
V
SW
K PACKAGE
4-LEAD TO-3 METAL CAN
LT1074:
θ
JC
= 2.5°C,
θ
JA
= 35°C/W
LT1076:
θ
JC
= 4°C,
θ
JA
= 35°C/W
LT1074CK
LT1074HVCK
LT1074MK
LT1074HVMK
LT1076CK
LT1076HVCK
LT1076MK
LT1076HVMK
LT1074CT
LT1074HVCT
LT1074IT
LT1074HVIT
LT1076CT
LT1076HVCT
LT1076IT
LT1076HVIT
FRONT VIEW
5
4
3
2
1
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
LT1074:
θ
JC
= 2.5°C,
θ
JA
= 50°C/W
LT1076:
θ
JC
= 4°C,
θ
JA
= 50°C/W
LT1074/LT1076
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
Switch “On” Voltage (Note 2)
CONDITIONS
LT1074
I
SW
= 1A, T
j
≥
0°C
I
SW
= 1A, T
j
< 0°C
I
SW
= 5A, T
j
≥
0°C
I
SW
= 5A, T
j
< 0°C
I
SW
= 0.5A
I
SW
= 2A
V
IN
≤
25V, V
SW
= 0
V
IN
= V
MAX,
V
SW
= 0 (Note 8)
V
IN
= 25V, V
SW
= 0
V
IN
= V
MAX,
V
SW
= 0 (Note 8)
q
q
q
q
q
q
q
q
ELECTRICAL CHARACTERISTICS
MIN
TYP
MAX
1.85
2.1
2.3
2.5
1.2
1.7
UNITS
V
V
V
V
V
V
µA
µA
µA
µA
mA
mA
µA
V
V
A
A
A
A
A
A
%
kHz
kHz
kHz
kHz
%/V
V/V
µmho
µA
mA
µA
V
%
%
%/V
V
mV/°C
V
LT1076
Switch “Off” Leakage
LT1074
LT1076
Supply Current (Note 3)
5
10
300
500
150
250
V
FB
= 2.5V, V
IN
≤
40V
40V < V
IN
< 60V
V
SHUT
= 0.1V (Device Shutdown) (Note 9)
Normal Mode
Startup Mode (Note 4)
LT1074
I
LIM
Open
R
LIM
= 10k (Note 6)
R
LIM
= 7k (Note 6)
I
LIM
Open
R
LIM
= 10k (Note 6)
R
LIM
= 7k (Note 6)
8.5
9
140
7.3
3.5
5.5
6.5
4.5
3
2.6
1.8
1.2
90
100
11
12
300
8
4.8
8.5
Minimum Supply Voltage
Switch Current Limit (Note 5)
LT1076
q
2
3.2
Maximum Duty Cycle
Switching Frequency
T
j
≤
125°C
T
j
> 125°C
V
FB
= 0V through 2kΩ (Note 5)
Switching Frequency Line Regulation
Error Amplifier Voltage Gain (Note 7)
Error Amplifier Transconductance
Error Amplifier Source and Sink Current
Feedback Pin Bias Current
Reference Voltage
Reference Voltage Tolerance
Source (V
FB
= 2V)
Sink (V
FB
= 2.5V)
V
FB
= V
REF
V
C
= 2V
V
REF
(Nominal) = 2.21V
All Conditions of Input Voltage, Output
Voltage, Temperature and Load Current
8V
≤
V
IN
≤
V
MAX
(Note 8)
Over Temperature
Multiplier Reference Voltage
Shutdown Pin Current
Shutdown Thresholds
Thermal Resistance Junction to Case
V
SH
= 5V
V
SH
≤
V
THRESHOLD
(≅2.5V)
Switch Duty Cycle = 0
Fully Shut Down
LT1074
LT1076
8V
≤
V
IN
≤
V
MAX
(Note 8)
1V
≤
V
C
≤
4V
q
q
q
q
85
90
85
85
110
120
125
0.1
8000
225
1.6
2
2.265
±1.5
±2.5
0.02
20
0.03
2000
3700
100
0.7
q
q
q
q
q
5000
140
1
0.5
2.21
±0.5
±1
0.005
1.5
–4
24
2.155
Reference Voltage Line Regulation
V
C
Voltage at 0% Duty Cycle
q
q
q
q
5
2.2
0.1
10
2.45
0.3
20
50
2.7
0.5
2.5
4.0
µA
µA
V
V
°C/W
°C/W
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
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 1µs.
R – 1k
R – 1k
(LT1074), I
LIM
≈
LIM
(LT1076).
Note 6:
I
LIM
≈
LIM
5.5k
2k
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.
BLOCK DIAGRA
INPUT SUPPLY
10
µ
A
0.3V
+
µ
-POWER
SHUTDOWN
–
6V
REGULATOR
AND BIAS
6V TO ALL
CIRCUITRY
CURRENT
LIMIT
COMP
C2
–
–
500
Ω
2.35V
+
SHUTDOWN*
2.21V
*AVAILABLE ON PACKAGES WITH PIN
COUNTS GREATER THAN 5.
4
W
LT1074
320
µ
A
CURRENT
LIMIT
SHUTDOWN
+
250
Ω
0.04
I
LIM
*
4.5V
10k
FREQ SHIFT
100kHz
OSCILLATOR
SYNC
3V(p-p)
V
IN
+
+
A1
ERROR
AMP
Z
ANALOG
X MULTIPLIER
XY
Z
Y
C1
–
PULSE WIDTH
COMPARATOR
400
Ω
15
Ω
R
R/S
S
Q
LATCH
R
G1
–
SWITCH
OUTPUT
(V
SW
)
LT1076
FB
V
C
24V (EQUIVALENT)
0.1
Ω
100
Ω
SWITCH
OUTPUT (V
SW
)
LT1074 • BD01
LT1074/LT1076
BLOCK DIAGRA
DESCRIPTIO
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 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 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
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
LIM
pin which is driven by an internal 320µA 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 150µA. A 10µA
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.
W
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