7
LT1070/LT1071
OPERATION
U
midfrequencies in the energy storage inductor. This
greatly simplifies closed-loop frequency compensation
under widely varying input voltage or output load condi-
tions. Finally, it allows simple pulse-by-pulse current
limiting to provide maximum switch protection under
output overload or short-circuit conditions. A low drop-
out internal regulator provides a 2.3V supply for all
internal circuitry of the LT1070/LT1071. This low drop-
out design allows input voltage to vary from 3V to 60V
with virtually no change in device performance. A 40kHz
oscillator is the basic clock for all internal timing. It turns
“on” the output switch via the logic and driver circuitry.
Special adaptive antisat circuitry detects onset of satura-
tion in the power switch and adjusts driver current
instantaneously to limit switch saturation. This mini-
mizes 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, it
programs the LT1070/LT1071 to disconnect the main
error amplifier output and connects the output of the
flyback amplifier to the comparator input. The LT1070/
LT1071 will then regulate the value of the flyback pulse
with respect to the supply voltage. This flyback pulse is
directly proportional to output voltage in the traditional
transformer coupled flyback topology regulator. By regu-
lating the amplitude of the flyback pulse, the output
voltage can be regulated with no direct connection be-
tween input and output. The output is fully floating up to
the breakdown voltage of the transformer windings.
Multiple floating outputs are easily obtained with addi-
tional windings. A special delay network inside the LT1070/
LT1071 ignores the leakage inductance spike at the
leading edge of the flyback pulse to improve output
regulation.
The error signal developed at the comparator input is
brought out externally. This pin (V
C
) has four different
functions. It is used for frequency compensation, current
limit adjustment, soft starting and total regulator shut-
down. During normal regulator operation this pin sits at
a voltage between 0.9V (low output current) and 2.0V
(high output current). The error amplifiers are current
output (g
m
) types, so this voltage can be externally
clamped for adjusting current limit. Likewise, a capacitor
coupled external clamp will provide soft start. Switch
duty cycle goes to zero if the V
C
pin is pulled to ground
through a diode, placing the LT1070/LT1071 in an idle
mode. Pulling the V
C
pin below 0.15V causes total
regulator shutdown, with only 50µA supply current for
shutdown circuitry biasing. See AN19 for full application
details.
TYPICAL APPLICATIONS
U
(Note that maximum output currents are divided by 2 for the LT1071)
R1*
R2**
1070/71 TA16
D1
SETS I
B(ON)
SETS I
B(OFF)
*
**
D2
C1
V
SW
V
IN
LT1070/LT1071
GND
Q1
Driving High Voltage NPN
Driving High Voltage FET (for Off-Line
Applications, See AN25)
1070/71 TA03
10V
TO
20V
D1
V
SW
G
D
S
Q1
V
IN
LT1070/LT1071
GND
+
