1
LT1270A/LT1270
D
U
ESCRIPTIO
S
FEATURE
8A and 10A High Efficiency
Switching Regulators
s
Wide Input Voltage Range: 3.5V to 30V
s
Low Quiescent Current: 7mA
s
Internal 8A Switch (10A for LT1270A)
s
Very Few External Parts Required
s
Self-Protected Against Overloads
s
Shutdown Mode Draws Only 100
A Supply Current
s
Flyback-Regulated Mode Has Fully Floating Outputs
s
Comes in Standard 5-Pin Package
s
Can be Externally Synchronized
(See LT1072 Data Sheet)
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 100
A typical for standby
operation.
U
S
A
O
PPLICATI
s
High Efficiency Buck Converter
s
PC Power Supply with Multiple Outputs
s
Battery Upconverter
s
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.
U
A
O
PPLICATI
TYPICAL
V
SW
V
IN
FB
V
C
GND
MBR1035*
LT1270*
0.047
F
1k
0.047
F
1
F
35
H, 8A*
ANODE
LT1431CZ
CATHODE
REF
1N4158
9.1k
10k
+
470
F
35V
V
IN
(8V TO 25V)
2N3904
360
560
1000
F
25V
5V*
5A
+
EFFICIENCY IS
88% @ 2A, 86% @ 5A.
FOR
POWER 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.
LT1270 TA01
1N5819
High Efficiency
Buck Converter
Maximum Output Power*
INPUT VOLTAGE (V)
0
0
OUTPUT POWER (W)
20
40
60
5
10
15
20
LT1270 TA02
25
80
100
30
BOOST
FLYBACK
BUCK-BOOST, V
O
= 5V
(INVERTING)
BUCK-BOOST,
V
O
= 12V
(INVERTING)
*MULTIPLY BY 1.2 FOR LT1270A.
BUCK MODE OUTPUT POWER
(7.5A)(V
OUT
)
TRANSFORMER TURNS RATIO MUST BE
OPTIMUM TO ACHIEVE FULL POWER
2
LT1270A/LT1270
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
REF
Reference Voltage
Measured at Feedback Pin
1.224
1.244
1.264
V
V
C
= 0.8V
q
1.214
1.244
1.274
V
I
B
Feedback Input Current
V
FB
= V
REF
350
750
nA
1100
nA
gm
Error Amplifier
l
C
=
25
A
3000
4400
6000
mho
Transconductance
q
2400
7000
mho
Error Amplifier Source of
V
C
= 1.5V
150
200
350
A
Sink Current
q
120
400
A
Error Amplifier Clamp Voltage
Hi Clamp, V
FB
= 1V
1.80
2.30
V
Lo Clamp, V
FB
= 1.5V
0.25
0.38
0.52
V
Reference Voltage Line Regulation
3V
V
IN
V
MAX
, V
C
= 0.8V
q
0.03
%/V
A
V
Error Amplifier Voltage Gain
0.9V
V
C
1.4V
500
800
V/V
Minimum Input Voltage
q
2.8
3.0
V
I
Q
Supply Current
3V
V
IN
V
MAX
, V
C
= 0.6V
7
10
mA
Control Pin Threshold
Duty Cycle = 0
0.80
0.90
1.08
V
q
0.60
1.25
V
Normal/Flyback Threshold
0.40
0.45
0.54
V
on Feedback Pin
V
FB
Flyback Reference Voltage
I
FB
= 50
A
15.0
16.3
17.6
V
q
14.0
18.0
V
V
FB
Change in Flyback Reference
0.05
I
FB
1mA
4.5
6.8
8.5
V
Voltage
Flyback Reference Voltage
I
FB
= 50
A
0.01
0.03
%/V
Line Regulation
3V
V
IN
V
MAX
Flyback Amplifier
l
C
=
10
A
150
300
650
mho
Transconductance (gm)
Flyback Amplifier Source
V
C
= 0.6V Source
q
15
32
70
A
and Sink Current
I
FB
= 50
A Sink
q
25
40
70
A
BV
Output Switch Breakdown
3V
V
IN
V
MAX
q
60
75
V
Voltage
I
SW
= 5mA
V
SAT
Output Switch ON
T
J
100
C
0.12
0.18
Resistance (Note 1, 3)
T
J
125
C
0.22
Control Voltage to Switch
12
A/V
Current Transconductance
A
U
G
W
A
W
U
W
A
R
BSOLUTE
XI
TI
S
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
W
U
U
PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
LT1270CT
LT1270ACT
T PACKAGE
5-LEAD TO-220
V
IN
V
SW
GND
FB
V
C
FRONT VIEW
5
4
3
2
1
T
JMAX
= 100
C,
JA
= 2
C/W,
JA
= 50
C/ W
ELECTRICAL CHARACTERISTICS
V
IN
= 15V, V
C
= 0.5V, V
FB
= V
REF
, switch pin open, unless otherwise noted.
3
LT1270A/LT1270
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
I
LIM
Switch Current Limit (LT1270)
Duty Cycle = 50%, T
J
100
C
q
8
16
A
(Note 3)
Duty Cycle = 80%, T
J
100
C
q
6
14
A
I
LIM
Switch Current Limit (LT1270A)
Duty Cycle = 50%, T
J
100
C
q
10.0
16.0
A
(Note 3)
Duty Cycle = 80%, T
J
100
C
q
7.5
14.0
A
I
IN
Supply Current Increase
25
40
mA/A
I
SW
During Switch-ON Time
f
Switching Frequency
50
60
70
kHz
q
50
70
kHz
DC
MAX
Maximum Switch Duty Cycle
80
92
95
%
Flyback Sense Delay Time
1.5
s
Shutdown Mode Supply Current
3V
V
IN
V
MAX
, V
C
= 0.05V
100
400
A
Shutdown Mode Threshold Voltage
3V
V
IN
V
MAX
100
150
250
mV
q
50
300
mV
ELECTRICAL CHARACTERISTICS
V
IN
= 15V, V
C
= 0.5V, V
FB
= V
REF
, switch pin open, unless otherwise noted.
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.
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.
C
C
HARA TERISTICS
U
W
A
TYPICAL PERFOR
CE
Switch Saturation Voltage
Switch Current Limit vs Duty Cycle
DUTY CYCLE (%)
0
SWITCH CURRENT (A)
1.8
2.4
80
LT1270 TA03
1.2
6
0
20
40
60
100
55C
125C
25C
SWITCH CURRENT (A)
0
SWITCH SATURATION VOLTAGE (V)
1.2
1.6
8
LT1270 TA04
0.8
0.4
0
2
4
6
10
T
J
= 25C
4
LT1270A/LT1270
TYPICAL APPLICATIO S
U
Boost Converter (5V to 12V)
Negative-to-Positive Buck-Boost Converter
R3
V
IN
V
SW
FB
V
C
GND
LT1270
L1
100
H**
C1
R2
1.24k
*REQUIRED IF INPUT LEADS
2"
**PULSE ENGINEERING #PE-92115
LT1270 TA07
C3
100
F*
5.2V, 7.5A
V
IN
20V
OPTIONAL
INPUT FILTER
C2
1000
F
D1
R1
4.64k
LOAD
12k
L3
10
H
+
C4
200
F
OPTIONAL
OUTPUT
FILTER
L2
4
F
+
+
Negative Buck Converter
PACKAGE DESCRIPTIO
U
Dimensions in inches (millimeters) unless otherwise noted.
LT/GP 1292 5K REV B
LINEAR TECHNOLOGY CORPORATION 1992
T5 (FORMED) 0392
0.028 0.035
(0.711 0.889)
0.057 0.077
(1.448 1.956)
0.139 0.153
(3.531 3.886)
DIA
0.079 0.135
(2.007 3.429)
0.380 0.420
(9.652 10.668)
0.970 1.050
(24.64 26.67)
0.560 0.650
(14.224 16.510)
0.866 0.913
(21.996 23.190)
0.327 0.335
(8.306 8.509)
0.700 0.728
(17.780 18.491)
0.035 0.055
(0.889 1.397)
0.169 0.185
(4.293 4.699)
0.170 0.185
(4.318 4.699)
0.620 0.020
(15.75 0.508)
0.460 0.500
(11.68 12.70)
0.210 0.240
(5.334 6.096)
0.055 0.090
(1.397 2.286)
0.015 0.025
(0.381 0.635)
0.079 0.115
(2.007 2.921)
1k
MBR1020
V
IN
V
SW
FB
V
C
GND
LT1270
10.7k
+
55
H*
1
F
1.24k
+
1000
F
25V
5V
12V, 2A
PULSE ENGINEERING #PE-92116
LT1270 TA05
R3
2.2k
V
IN
V
SW
FB
V
C
GND
LT1270
L1
55
H**
C1
0.22
F
1.24k
*REQUIRED IF INPUT LEADS
2"
**PULSE ENGINEERING #PE-92116
LT1270 TA06
+
C2
2000
F
D1
R1
11.3k
+
C4
200
F*
V
OUT
12V, 3A
L2
C3
OPTIONAL
OUTPUT
FILTER
V
IN
12V
OPTIONAL
INPUT
FILTER
L3
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900
q
FAX
: (408) 434-0507
q
TELEX
: 499-3977
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