LT1268B/LT1268
7.5A, 150kHz
Switching Regulators
FEATURES
s
s
s
s
s
s
DESCRIPTIO
s
Wide Input Voltage Range: 3.5V to 30V
Low Quiescent Current: 7mA
Internal 7.5A Switch
Very Few External Parts Required
Self-Protected Against Overloads
Available in Standard and Surface Mount
5-Pin Packages
Can Be Externally Synchronized
(See LT1072 Data Sheet)
APPLICATI
s
s
s
s
S
High Efficiency Boost Converter
PC Power Supply with Multiple Outputs
Battery Upconverter
Negative-to-Positive Converter
The LT1268B and LT1268 are monolithic high power
switching regulators. Identical to the popular LT1070,
except for switching frequency (150kHz) and higher
switch current, they can be operated in all standard
switching configurations 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. Integration of all functions allows
the LT1268 to be built in standard 5-pin power packages.
This makes it extremely easy to use and provides “bust
proof” operations similar to that obtained with 3-pin
linear regulators.
The LT1268 operate with supply voltages from 3.5V to
30V and draw only 7mA quiescent current. By utilizing
current mode switching techniques, it provides excellent
AC and DC load and line regulation.
The LT1268 use an adaptive anti-sat switch drive to allow
very wide ranging load currents with no loss in efficiency.
An externally activated shutdown mode reduces total
supply current to 100µA typical for standby operation.
USER NOTE:
This data sheet is only intended to provide specifications, graphs, and a general functional
description of the LT1268B/LT1268. Application circuits are included to show the capability of the
LT1268B/LT1268. A complete design manual (AN19) should be obtained to assist in developing new
designs. This manual 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. The manual can also be used for the LT1268B/LT1268 factoring in the higher switch
current rating and higher operating frequency.
TYPICAL APPLICATI
V
IN
4.7V TO
5.3V
L1
8µH, 5A*
D2, D3
†
1N4001
Boost Regulator with 5.3V
±1%
Output
D1
MBR1035
L2
2µH, 5A**
100
V
OUT
5.3V ±1%
3.75A
EFFICIENCY (%)
90
V
IN
= 4.7V
+
C3
390µF
16V
C5
0.02
+
C1
220µF
16V
V
IN
V
SW
LT1268B FB
R1
4.043k
0.1%
+
C4
390µF
16V
80
CASE GND
V
C
C2
0.068
R3
220Ω
R2
1.24k
0.1%
70
60
LT1268 • TA01
*COILTRONICS CTX8-5-52
**COILTRONICS CTX2-5-52
†
OPTIONAL. D2 AND D3 ARE USED TO INCREASE
UNDERVOLTAGE LOCKOUT FROM 2.7V TO
≈4V.
U
Efficiency of 5.3V Boost Converter
V
IN
= 5.3V
V
IN
= 5V
0
1
2
LOAD CURRENT (A)
LT1268 • TA02
UO
UO
3
4
1
LT1268B/LT1268
ABSOLUTE
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
FRONT VIEW
5
4
3
2
1
Q PACKAGE
5-LEAD DD
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
Operating ............................................... 0°C to 125°C
Short-Circuit ......................................... 0°C to 140°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LT1268CQ
LT1268BCQ
T
JMAX
= 125°C,
θ
JC
= 2°C/ W,
θ
JA
= See Notes 5 and 6
FRONT VIEW
5
4
3
2
1
T PACKAGE
5-LEAD TO-220
T
JMAX
= 125°C,
θ
JC
= 2°C/W,
θ
JA
= 50°C/W
V
IN
V
SW
GND
FB
V
C
ORDER PART
NUMBER
LT1268CT
LT1268BCT
Consult factory for Industrial and Military parts
ELECTRICAL CHARACTERISTICS
SYMBOL
V
REF
PARAMETER
Reference Voltage Measured at
Feedback Pin (Note 4)
Reference Voltage
I
B
g
m
Feedback Input Current
Error Amplifier Transconductance
Error Amplifier Source or 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
BV
V
SAT
Output Switch Breakdown Voltage
Output Switch-ON Resistance
(Note 1, 3)
Control Voltage to Switch
Current Transconductance
I
LIM
Switch Current Limit (Note 3, 6)
V
IN
= 15V, V
C
= 0.5V, V
FB
= V
REF
, switch pin open, unless otherwise noted.
MIN
q
CONDITIONS
LT1268B
LT1268
q
TYP
1.244
1.244
1.244
1.244
350
MAX
1.253
1.264
1.264
1.274
750
1100
6000
7000
350
400
2.30
0.52
0.03
UNITS
V
V
V
V
nA
nA
µmho
µmho
µA
µA
V
V
%/V
V/V
V
mA
V
V
V
Ω
Ω
A/V
1.235
1.224
1.224
1.214
V
FB
= V
REF
q
∆I
C
=
±25µA
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
0.18
0.22
3V
≤
V
IN
≤
V
MAX
, V
C
= 0.6V
Duty Cycle = 0
q
0.7
0.5
60
0.9
75
0.12
12
3V
≤
V
IN
≤
V
MAX
, I
SW
= 1.5mA
T
J
≤
100°C
T
J
≤
125°C
q
Duty Cycle = 50%, T
J
≤
100°C
Duty Cycle = 65%, T
J
≤
100°C
q
q
7.50
6.50
15
14
2
U
A
A
W
U
U
W W
W
LT1268B/LT1268
ELECTRICAL CHARACTERISTICS
SYMBOL
∆I
IN
∆I
SW
f
DC
MAX
PARAMETER
Supply Current Increase
During Switch-ON Time
Switching Frequency
V
IN
= 15V, V
C
= 0.5V, V
FB
= V
REF
, switch pin open, unless otherwise noted.
MIN
TYP
25
120
120
65
150
85
100
q
CONDITIONS
MAX
45
180
180
92
500
250
300
UNITS
mA/A
kHz
kHz
%
µA
mV
mV
q
Maximum Switch Duty Cycle
Shutdown Mode Supply Current
Shutdown Mode Threshold Voltage
3V
≤
V
IN
≤
V
MAX
, V
C
= 0.05V
3V
≤
V
IN
≤
V
MAX
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 65%, minimum guaranteed
switch current is given by I
LIM
= 6.25 (1.7 – DC).
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.
Note 4:
LT1268B reference voltage is specified at
±9mV
to guarantee
±1%
output voltage accuracy when 0.1% external resistors are used to set
output voltage. To maintain output accuracy under load, load current
should be taken from the case and the ground pin should be connected
separately to output ground. See AN19 for details.
Note 5:
The Q package is intended for surface mount without a separate
heat sink. See graph for thermal resistance as a function of the mounting
area. This curve assumes no other heat dissipators adjacent to package.
Note 6:
Maximum switch current may be limited by package power
dissipation, especially for the surface mount (Q) package. This package
has a thermal resistance of 20°C/W to 50°C/W (see graph). The following
formula will allow an estimate of maximum continuous switch current as a
function of power loss and duty cycle. See AN19 for more details.
I
MAX
=
P
R
SW
×
DC
P = Power dissipation due to switch current
R
SW
= Switch-ON resistance
≈
0.15Ω
DC = Switch duty cycle
In a typical application where thermal resistance is 30°C/W, maximum
power might be limited to 2W and power allocated to switch loss is 1.5W.
For a duty cycle of 40%, this yields
I
MAX
=
1.5
= 5A
0.15
×
0.4
Obviously, a combination of high thermal resistance and high duty cycle
may restrict switch current to a value well below the 7.5A electrical limit.
TYPICAL PERFOR A CE CHARACTERISTICS
Switch Current Limit vs Duty Cycle
24
1.6
SWITCH SATURATION VOLTAGE (V)
SWITCH CURRENT (A)
18
12
25°C
6
–55°C
125°C
0.8
0.6
0.4
0.2
0
0
1
2
3 4 5 6 7 8
SWITCH CURRENT (A)
9
10
θ
(°C/W)
0
0
10 20
30 40 50 60 70 80 90 100
DUTY CYCLE (%)
LT1268 • TPC01
U W
Switch Saturation Voltage
60
T
J
= 25°C
50
1.4
1.2
1.0
40
30
20
10
0
Q Package Thermal Resistance
SINGLE-SIDED BOARD, 1oz COPPER
DOUBLE-SIDED BOARD, 1oz COPPER
BOTTOM COPPER AREA >100cm
2
0
25
10
15
20
5
TOP SURFACE COPPER AREA (cm
2
)
30
LT1268 • TPC02
LT1268 • 603
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 its circuits as described herein will not infringe on existing patent rights.
3
LT1268B/LT1268
TYPICAL APPLICATI
PACKAGE DESCRIPTIO
0.060
(1.524)
Q Package
5-Lead DD
(
+0.012
0.331 –0.020
+0.305
8.407 –0.508
(
+0.012
0.143 –0.020
+0.305
3.632 –0.508
0.380 – 0.420
(9.652 – 10.668)
0.139 – 0.153
(3.531 – 3.886)
DIA
T Package
5-Lead TO-220
0.560 – 0.650
(14.224 – 16.510)
0.866 – 0.913
(21.996 – 23.190)
0.057 – 0.077
(1.448 – 1.956)
4
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900
q
FAX
: (408) 434-0507
q
TELEX
: 499-3977
U
UO
Boost Converter (5V TO 12V)
5V
20µH
V
IN
V
SW
MBR1020
LT1268CT
10.7k
FB
GND
V
C
1k
1.24k
12V, 1.5A
+
1000µF
25V
+
1µF
LT1268 • TA03
Dimensions in inches (millimeters) unless otherwise noted.
0.401 ± 0.015
(10.185 ± 0.381)
15° TYP
0.175 ± 0.008
(4.445 ± 0.203)
0.050 ± 0.008
(1.270 ± 0.203)
+0.008
0.004 –0.004
)
0.067 ± 0.010
(1.702 ± 0.254)
0.032 ± 0.008
(0.813 ± 0.203)
0.059
(1.499)
TYP
(
+0.203
0.102 –0.102
)
0.105 ± 0.008
(2.667 ± 0.203)
)
0.022 ± 0.005
(0.559 ± 0.127)
0.050 ± 0.012
(1.270 ± 0.305)
0.079 – 0.135
(2.007 – 3.429)
0.169 – 0.185
(4.293 – 4.699)
0.035 – 0.055
(0.889 – 1.397)
0.460 – 0.500
(11.68 – 12.70)
0.620 ± 0.020
(15.75 ± 0.508)
0.700 – 0.728
(17.780 – 18.491)
0.970 – 1.050
(24.64 – 26.67)
0.028 – 0.035
(0.711 – 0.889)
0.015 – 0.025
(0.381 – 0.635)
0.210 – 0.240
(5.334 – 6.096)
0.304 – 0.380
(7.722 – 9.652)
0.055 – 0.090
(1.397 – 2.286)
0.079 – 0.115
(2.007 – 2.921)
0.149 – 0.230
(3.785 – 5.842)
1268fs, sn1268 LT/GP 0194 10K REV 0 • PRINTED IN THE USA
©
LINEAR TECHNOLOGY CORPORATION 1994
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