LT1
170/LT1
171/LT1172
BLOCK DIAGRA
FB
†
ALWAYS CONNECT E1 TO THE GROUND PIN ON MINIDIP, 8- AND 16-PIN SURFACE MOUNT PACKAGES.
E1 AND E2 INTERNALLY TIED TO GROUND ON TO-3 AND TO-220 PACKAGES.
OPERATIO
The LT1170/LT1171/LT1172 are current mode switchers.
This means that switch duty cycle is directly controlled by
switch current rather than by output voltage. Referring to
the block diagram, the switch is turned “on” at the start of
each oscillator cycle. It is turned “off” when switch current
reaches a predetermined level. Control of output voltage is
obtained by using the output of a voltage sensing error
amplifier to set current trip level. This technique has
several advantages. First, it has immediate response to
input voltage variations, unlike ordinary switchers which
have notoriously poor line transient response. Second, it
reduces the 90° phase shift at midfrequencies in the
energy storage inductor. This greatly simplifies closed-
loop frequency compensation under widely varying input
voltage or output load conditions. Finally, it allows simple
pulse-by-pulse current limiting to provide maximum switch
W
V
IN
16V
SWITCH
OUT
2.3V
REG
FLYBACK
ERROR
AMP
5A, 75V
SWITCH
100kHz
OSC
LOGIC
DRIVER
LT1172
MODE
SELECT
COMP
ANTI-
SAT
–
+
ERROR
AMP
V
C
+
SHUTDOWN
CIRCUIT
1.24V
REF
CURRENT
AMP
GAIN
≈
6
0.15V
(LT1170 AND LT1171 ONLY)
–
0.02
Ω
(0.04
Ω
LT1171)
(0.16
Ω
LT1172)
0.16
Ω
E1
†
E2
1170/1/2 BD
U
protection under output overload or short conditions. A
low dropout internal regulator provides a 2.3V supply for
all internal circuitry on the LT1170/LT1171/LT1172. This
low dropout design allows input voltage to vary from 3V to
60V with virtually no change in device performance. A
100kHz oscillator is the basic clock for all internal timing.
It turns “on” the output switch via the logic and driver
circuitry. Special adaptive anti-sat circuitry detects onset
of saturation in the power switch and adjusts driver
current instantaneously to limit switch saturation. This
minimizes driver dissipation and provides very rapid turn-
off of the switch.
A 1.2V bandgap reference biases the positive input of the
error amplifier. The negative input is brought out for
output voltage sensing. This feedback pin has a second
7
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