LT1082
BLOCK DIAGRA
W
V
IN
16.2V
SWITCH OUT
2.3V
REG
FLYBACK
ERROR
AMP
OSC
60kHz
14kHz
LOGIC
DRIVER
MODE SELECT
COMP
FB
ANTI-SAT
1.24V
REF
OPERATIO
The LT1082 is a current mode switcher. 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 mid-frequencies 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
protection under output overload or short conditions. A
+
–
ERROR
AMP
V
C
GAIN
≈
5
SHUTDOWN
CIRCUIT
0.15V
GND
+
CURRENT
AMP
0.2Ω
0.2Ω
–
E1* E2
* ALWAYS CONNECT E1 TO GROUND PIN ON MiniDIP PACKAGE.
EMITTERS TIED TO GROUND ON TO-220 PACKAGE.
1082 BD
U
low dropout internal regulator provides a 2.3V supply for
all internal circuitry on the LT1082. This low dropout
design allows input voltage to vary from 3V to 75V with
virtually no change in device performance. A 60kHz
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
function: when pulled low with an external resistor and
with I
FB
of 60µA to 200µA, it programs the LT1082 to
sn1082 1082fas
7
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
