1
LT1270A/LT1270
1270afc
DESCRIPTIO
U
TYPICAL APPLICATIO
U
APPLICATIO S
U
FEATURES
8A and 10A High Efficiency
Switching Regulators
■
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
■
Can be Externally Synchronized
(See LT1072 Data Sheet)
■
Comes in Standard 5-Pin TO-220 Package
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.
V
SW
V
IN
FB
V
C
GND
MBR1035*
LT1270*
0.047µF
1k
0.047µF1µF
35µH, 8A*
ANODE
LT1431CZ
CATHODE
REF
1N4158
9.1k
10k
+
470µF
35V
V
IN
(8V TO 25V)
2N3904
360Ω560Ω1000µF
25V
5V*
5A
+
†
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.
LT1270 • TA01
1N5819
High Efficiency
†
Buck Converter Maximum Output Power*
INPUT VOLTAGE (V)
0
0
OUTPUT POWER (W)
20
40
60
510 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
1270afc
WU
U
PACKAGE/ORDER I FOR ATIO
ELECTRICAL C CHARA TERISTICS
The ● denotes specifications which apply over the full operating tempera-
ture range, otherwise specifications are at TA = 25°C. VIN = 15V, VC = 0.5V, VFB = VREF, switch pin open, unless otherwise noted.
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 ●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
= ±25µA 3000 4400 6000 µmho
Transconductance ●2400 7000 µmho
Error Amplifier Source of V
C
= 1.5V 150 200 350 µA
Sink Current ●120 400 µA
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 ●0.03 %/V
A
V
Error Amplifier Voltage Gain 0.9V ≤ V
C
≤ 1.4V 500 800 V/V
Minimum Input Voltage ●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 0.70 0.90 1.08 V
●0.50 1.25 V
Normal/Flyback Threshold 0.40 0.45 0.54 V
on Feedback Pin
V
FB
Flyback Reference Voltage I
FB
= 50µA 15.0 16.3 17.6 V
●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
= 50µA 0.01 0.03 %/V
Line Regulation 3V ≤ V
IN
≤ V
MAX
Flyback Amplifier ∆l
C
= ±10µA 150 300 650 µmho
Transconductance (gm)
Flyback Amplifier Source V
C
= 0.6V Source ●15 32 70 µA
and Sink Current I
FB
= 50µA Sink ●25 40 70 µA
BV Output Switch Breakdown 3V ≤ V
IN
≤ V
MAX
●60 75 V
Voltage I
SW
= 1.5mA
V
SAT
Output Switch ON T
J
≤ 100°C 0.12 0.18 Ω
Resistance (Note 2, 4) T
J
≤ 125°C 0.22 Ω
Control Voltage to Switch 12 A/V
Current Transconductance
LIM
Switch Current Limit (LT1270) Duty Cycle = 50%, T
J
≤ 100°C●816A
(Note 4) Duty Cycle = 80%, T
J
≤ 100°C●614A
A
U
G
W
A
W
U
W
ARB
S
O
LUTEXI T
I
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
ORDER PART
NUMBER
LT1270CT
LT1270ACT
T PACKAGE
5-LEAD PLASTIC TO-220
V
IN
V
SW
GND
FB
V
C
FRONT VIEW
TAB
IS
GND
5
4
3
2
1
T
JMAX
= 100°C, θ
JA
= 2°C/W, θ
JA
= 50°C/W
(Note 1)
Consult LTC Marketing for parts specified with wider operating temperature ranges.
3
LT1270A/LT1270
1270afc
ELECTRICAL C CHARA TERISTICS
The ● denotes specifications which apply over the full operating tempera-
ture range, otherwise specifications are at TA = 25°C. VIN = 15V, VC = 0.5V, VFB = VREF, switch pin open, unless otherwise noted.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
I
LIM
Switch Current Limit (LT1270A) Duty Cycle = 50%, T
J
≤ 100°C●10.0 16.0 A
(Note 4) Duty Cycle = 80%, T
J
≤ 100°C●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
●50 70 kHz
DC
MAX
Maximum Switch Duty Cycle 80 92 95 %
Flyback Sense Delay Time 1.5 µs
Shutdown Mode Supply Current 3V ≤ V
IN
≤ V
MAX
, V
C
= 0.05V 100 400 µA
Shutdown Mode Threshold Voltage 3V ≤ V
IN
≤ V
MAX
100 150 250 mV
●50 300 mV
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: Measured with V
C
in Hi Clamp, V
FB
= 0.8V.
Note 3: 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 4: Minimum current limit is reduced by 0.5A at 125°C. 100°C test
limits are guaranteed by correlation to 125°C tests.
TYPICAL APPLICATIO S
U
Boost Converter (5V to 12V)
1k
MBR1020
V
IN
V
SW
FB
V
C
GND
LT1270
10.7k
+
55µH*
1µF
1.24k
+
1000µF
25V
5V
12V, 2A
PULSE ENGINEERING #PE-92116
LT1270 • TA05
TYPICAL PERFOR A CE CHARACTERISTICS
UW
Switch Saturation VoltageSwitch Current Limit vs Duty Cycle
DUTY CYCLE (%)
0
SWITCH CURRENT (A)
18
24
80
LT1270 • TA03
12
6
020 40 60 100
–55°C
125°C
25°C
SWITCH CURRENT (A)
0
SWITCH SATURATION VOLTAGE (V)
1.2
1.6
8
LT1270 • TA04
0.8
0.4
024610
T
J
= 25°C
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 representation that the
interconnection of circuits as described herein will not infringe on existing patent rights.
4
LT1270A/LT1270
1270afc
PART NUMBER DESCRIPTION COMMENTS
LT1074/HV 4.4A (I
OUT
), 100kHz, High Efficiency Step-Down V
IN
: 7.3V to 45/64V, V
OUT
(min) = 2.21V, I
Q
= 8.5mA, I
SD
= 10µA,
DC/DC Converter DD-5/7, TO220-5/7
LTC3414 4A (I
OUT
), 4MHz, Synchronous Step-Down V
IN
: 2.3V to 5.5V, V
OUT
(min) = 0.8V, I
Q
= 64µA, I
SD
<1µA,
DC/DC Converter TSSOP20E
LT3430/LT3431 60V, 2.75A (I
OUT
), 200/500kHz, High Efficiency V
IN
: 5.5V to 60V, V
OUT
(min) = 1.2V, I
Q
= 2.5mA, I
SD
= 30µA,
Step-Down DC/DC Converter TSSOP16E
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507
●
www.linear.com
LINEAR TECHNOLOGY CORPORATION 1992
LT/TP 1203 1K REV C • PRINTED IN USA
RELATED PARTS
PACKAGE DESCRIPTIO
U
T5 (TO-220) 0801
.028 – .038
(0.711 – 0.965)
.067
(1.70) .135 – .165
(3.429 – 4.191)
.700 – .728
(17.78 – 18.491)
.045 – .055
(1.143 – 1.397)
.095 – .115
(2.413 – 2.921)
.013 – .023
(0.330 – 0.584)
.620
(15.75)
TYP
.155 – .195*
(3.937 – 4.953)
.152 – .202
(3.861 – 5.131)
.260 – .320
(6.60 – 8.13)
.165 – .180
(4.191 – 4.572)
.147 – .155
(3.734 – 3.937)
DIA
.390 – .415
(9.906 – 10.541)
.330 – .370
(8.382 – 9.398)
.460 – .500
(11.684 – 12.700)
.570 – .620
(14.478 – 15.748)
.230 – .270
(5.842 – 6.858)
BSC
SEATING PLANE
* MEASURED AT THE SEATING PLANE
T Package
5-Lead Plastic TO-220 (Standard)
(Reference LTC DWG # 05-08-1421)
TYPICAL APPLICATIO S
U
R3
VIN VSW
FB
VC
GND
LT1270
L1
100µH**
C1 R2
1.24k
*REQUIRED IF INPUT LEADS ≥ 2"
**PULSE ENGINEERING #PE-92115
LT1270 • TA07
C3
100µF*
–5.2V, 7.5A
VIN
–20V
OPTIONAL
INPUT FILTER
C2
1000µF
D1 R1
4.64k
LOAD
12k
L3
10µH
+
C4
200µF
OPTIONAL
OUTPUT
FILTER
L2
4µF
+
+
Negative Buck ConverterNegative-to-Positive Buck-Boost Converter
R3
2.2k
V
IN
V
SW
FB
V
C
GND
LT1270
L1
55µH**
C1
0.22µF
1.24k
*REQUIRED IF INPUT LEADS ≥ 2"
**PULSE ENGINEERING #PE-92116
LT1270 • TA06
+
C2
2000µF
D1
R1
11.3k
+
C4
200µF*
V
OUT
12V, 3A
L2
C3OPTIONAL
OUTPUT
FILTER
V
IN
–12V
OPTIONAL
INPUT
FILTER
L3
