7

LT1070/LT1071

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

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

clamped for adjusting current limit. Likewise, a capacitor

coupled external clamp will provide soft start. Switch

through a diode, placing the LT1070/LT1071 in an idle

regulator shutdown, with only 50

µA supply current for

shutdown circuitry biasing. See AN19 for full application

details.

R1*

R2**

1070/71 TA16

D1

*

**

D2

C1

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

G

D

S

Q1

LT1070/LT1071

GND

+