LT1270A/LT1270
8A and 10A High Efficiency
Switching Regulators
FEATURES
s
s
s
s
s
s
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s
DESCRIPTIO
Wide Input Voltage Range: 3.5V to 30V
Low Quiescent Current: 7mA
Internal 8A Switch (10A for LT1270A)
Very Few External Parts Required
Self-Protected Against Overloads
Shutdown Mode Draws Only 100µA Supply Current
Flyback-Regulated Mode Has Fully Floating Outputs
Comes in Standard 5-Pin Package
Can be Externally Synchronized
(See LT1072 Data Sheet)
APPLICATI
s
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S
The LT1270A/LT1270 are monolithic high power switch-
ing regulators. Identical to the popular LT1070, except for
switching frequency (60kHz) and higher switch current,
they can be operated in all standard switching configura-
tions including buck, boost, flyback, and inverting. A high
current, high efficiency switch is included on the die along
with all oscillator, control, and protection circuitry. Inte-
gration of all functions allows the LT1270A/LT1270 to be
built in a standard TO-220 power package. This makes it
extremely easy to use and provides “bust proof” operation
similar to that obtained with 3-pin linear regulators.
The LT1270A/LT1270 operate with supply voltages from
3.5V to 30V, and draw only 7mA quiescent current. By
utilizing current-mode switching techniques, they provide
excellent AC and DC load and line regulation.
The LT1270A/LT1270 use adaptive anti-sat switch drive to
allow very wide ranging load currents with no loss in
efficiency. An externally activated shutdown mode re-
duces total supply current to 100µA typical for standby
operation.
High Efficiency Buck Converter
PC Power Supply with Multiple Outputs
Battery Upconverter
Negative-to-Positive Converter
USER NOTE:
This data sheet is only intended to provide specifications, graphs and a general functional
description of the LT1270A/LT1270. Application circuits are included to show the capability of the
LT1270A/LT1270. A complete design manual (AN-19) should be obtained to assist in developing
new designs. AN-19 contains a comprehensive discussion of both the LT1070 and the external
components used with it, as well as complete formulas for calculating the values of these
components. AN-19 can also be used for the LT1270A/LT1270 by factoring in the higher switch
current rating and higher operating frequency.
A comprehensive CAD program called SwitcherCad is also available. Contact the local sales office
in your area or the factory direct.
TYPICAL APPLICATI
V
IN
(8V TO 25V)
1N5819
V
IN
V
SW
LT1270*
High Efficiency
†
Buck Converter
100
EFFICIENCY IS
≈
88% @ 2A, 86% @ 5A.
FOR
µPOWER
SHUTDOWN OPTION, CONSULT FACTORY.
*THESE COMPONENTS RATED CONSERVATIVELY TO
IMPROVE EFFICIENCY. MAXIMUM OUTPUT CURRENT
CAN BE 7A. FOR HIGH EFFICIENCY, THE INDUCTOR SHOULD
BE MOLYPERMALLOY OR FERRITE.
†
FB
GND
V
C
0.047µF
1k
OUTPUT POWER (W)
+
1µF
0.047µF
35µH, 8A*
5V*
5A
ANODE
9.1k
+
470µF
35V
MBR1035*
2N3904
360Ω
LT1431CZ REF
CATHODE
10k
1N4158
560Ω
1000µF
25V
LT1270 • TA01
U
Maximum Output Power*
BOOST
BUCK-BOOST,
V
O
= 12V
(INVERTING)
FLYBACK
†
80
60
40
20
BUCK-BOOST, V
O
= 5V
(INVERTING)
0
0
5
15
20
10
INPUT VOLTAGE (V)
25
30
LT1270 • TA02
UO
UO
*MULTIPLY BY 1.2 FOR LT1270A.
BUCK MODE OUTPUT POWER
≈
(7.5A)(V
OUT
)
†
TRANSFORMER TURNS RATIO MUST BE
OPTIMUM TO ACHIEVE FULL POWER
1
LT1270A/LT1270
ABSOLUTE
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
FRONT VIEW
5
4
3
2
1
T PACKAGE
5-LEAD TO-220
V
IN
V
SW
GND
FB
V
C
Supply Voltage ....................................................... 30V
Switch Output Voltage ............................................ 60V
Feedback Pin Voltage (Transient, 1ms) ................
±15V
Operating Junction Temperature Range
LT1270AC/LT1270C (Oper.)................ 0°C to 125°C
LT1270AC/LT1270C (Short-Ckt) ......... 0°C to 140°C
Storage Temperature Range ................ – 65°c to 150°C
Lead Temperature (Soldering, 10 sec)................ 300°C
ORDER PART
NUMBER
LT1270CT
LT1270ACT
T
JMAX
= 100°C,
θ
JA
= 2°C/W,
θ
JA
= 50°C/ W
ELECTRICAL CHARACTERISTICS
SYMBOL
V
REF
I
B
gm
PARAMETER
Reference Voltage
Feedback Input Current
Error Amplifier
Transconductance
Error Amplifier Source of
Sink Current
Error Amplifier Clamp Voltage
Reference Voltage Line Regulation
A
V
I
Q
Error Amplifier Voltage Gain
Minimum Input Voltage
Supply Current
Control Pin Threshold
Normal/Flyback Threshold
on Feedback Pin
V
FB
V
FB
Flyback Reference Voltage
Change in Flyback Reference
Voltage
Flyback Reference Voltage
Line Regulation
Flyback Amplifier
Transconductance (gm)
Flyback Amplifier Source
and Sink Current
BV
V
SAT
Output Switch Breakdown
Voltage
Output Switch ON
Resistance (Note 1, 3)
Control Voltage to Switch
Current Transconductance
I
FB
= 50µA
CONDITIONS
V
IN
= 15V, V
C
= 0.5V, V
FB
= V
REF
, switch pin open, unless otherwise noted.
MIN
q
TYP
1.244
1.244
350
MAX
1.264
1.274
750
1100
6000
7000
350
400
2.30
0.52
0.03
UNITS
V
V
nA
nA
µmho
µmho
µA
µA
V
V
%/V
V/V
V
mA
V
V
V
V
V
V
%/V
µmho
µA
µA
V
Measured at Feedback Pin
V
C
= 0.8V
V
FB
= V
REF
∆l
C
=
±25µA
1.224
1.214
q
3000
2400
150
120
1.80
0.25
4400
200
V
C
= 1.5V
q
Hi Clamp, V
FB
= 1V
Lo Clamp, V
FB
= 1.5V
3V
≤
V
IN
≤
V
MAX
, V
C
= 0.8V
0.9V
≤
V
C
≤
1.4V
q
q
0.38
800
2.8
7
500
3.0
10
1.08
1.25
0.54
17.6
18.0
8.5
0.03
650
70
70
3V
≤
V
IN
≤
V
MAX
, V
C
= 0.6V
Duty Cycle = 0
q
0.80
0.60
0.40
15.0
14.0
4.5
0.90
0.45
16.3
6.8
0.01
q
0.05
≤
I
FB
≤
1mA
I
FB
= 50µA
3V
≤
V
IN
≤
V
MAX
∆l
C
=
±10µA
V
C
= 0.6V Source
I
FB
= 50µA Sink
3V
≤
V
IN
≤
V
MAX
I
SW
= 5mA
T
J
≤
100°C
T
J
≤
125°C
q
q
q
150
15
25
60
300
32
40
75
0.12
12
0.18
0.22
2
U
Ω
Ω
A/V
W
U
U
W W
W
LT1270A/LT1270
ELECTRICAL CHARACTERISTICS
SYMBOL
I
LIM
I
LIM
∆I
IN
∆I
SW
f
DC
MAX
PARAMETER
Switch Current Limit (LT1270)
(Note 3)
Switch Current Limit (LT1270A)
(Note 3)
Supply Current Increase
During Switch-ON Time
Switching Frequency
CONDITIONS
V
IN
= 15V, V
C
= 0.5V, V
FB
= V
REF
, switch pin open, unless otherwise noted.
MIN
q
q
q
q
TYP
MAX
16
14
16.0
14.0
UNITS
A
A
A
A
mA/A
kHz
kHz
%
µs
µA
mV
mV
Duty Cycle = 50%, T
J
≤
100°C
Duty Cycle = 80%, T
J
≤
100°C
Duty Cycle = 50%, T
J
≤
100°C
Duty Cycle = 80%, T
J
≤
100°C
8
6
10.0
7.5
25
50
50
80
60
92
1.5
100
40
70
70
95
400
250
300
q
Maximum Switch Duty Cycle
Flyback Sense Delay Time
Shutdown Mode Supply Current
Shutdown Mode Threshold Voltage
3V
≤
V
IN
≤
V
MAX
, V
C
= 0.05V
3V
≤
V
IN
≤
V
MAX
q
100
50
150
The
q
denotes specifications which apply over the full operating
temperature range.
Note 1:
Measured with V
C
in Hi Clamp, V
FB
= 0.8V.
Note 2:
For duty cycles (DC) between 50% and 80%, minimum guaranteed
switch current is given by I
LIM
= 6.67 (1.7 – DC) for the LT1270 and I
LIM
=
8.33 (1.7 – DC) for the LT1270A.
Note 3:
Minimum current limit is reduced by 0.5A at 125°C. 100°C test
limits are guaranteed by correlation to 125°C tests.
TYPICAL PERFOR A CE CHARACTERISTICS
Switch Current Limit vs Duty Cycle
2.4
1.6
T
J
= 25°C
SWITCH SATURATION VOLTAGE (V)
SWITCH CURRENT (A)
1.8
25°C
–55°C
1.2
125°C
6
0
0
20
60
40
DUTY CYCLE (%)
U W
Switch Saturation Voltage
1.2
0.8
0.4
0
80
100
0
2
6
8
4
SWITCH CURRENT (A)
10
LT1270 • TA04
LT1270 • TA03
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 circuits as described herein will not infringe on existing patent rights.
3
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