Datasheet LT1070, LT1071 (Linear Technology) - 7

LT1070/LT1071
U OPERATION
midfrequencies in the energy storage inductor. This transformer coupled flyback topology regulator. By regu- greatly simplifies closed-loop frequency compensation lating the amplitude of the flyback pulse, the output under widely varying input voltage or output load condi- voltage can be regulated with no direct connection be- tions. Finally, it allows simple pulse-by-pulse current tween input and output. The output is fully floating up to limiting to provide maximum switch protection under the breakdown voltage of the transformer windings. output overload or short-circuit conditions. A low drop- Multiple floating outputs are easily obtained with addi- out internal regulator provides a 2.3V supply for all tional windings. A special delay network inside the LT1070/ internal circuitry of the LT1070/LT1071. This low drop- LT1071 ignores the leakage inductance spike at the out design allows input voltage to vary from 3V to 60V leading edge of the flyback pulse to improve output with virtually no change in device performance. A 40kHz regulation. oscillator is the basic clock for all internal timing. It turns The error signal developed at the comparator input is “on” the output switch via the logic and driver circuitry. brought out externally. This pin (V Special adaptive antisat circuitry detects onset of satura- C) has four different functions. It is used for frequency compensation, current tion in the power switch and adjusts driver current limit adjustment, soft starting and total regulator shut- instantaneously to limit switch saturation. This mini- down. During normal regulator operation this pin sits at mizes driver dissipation and provides very rapid turn-off a voltage between 0.9V (low output current) and 2.0V of the switch. (high output current). The error amplifiers are current A 1.2V bandgap reference biases the positive input of the output (gm) types, so this voltage can be externally error amplifier. The negative input is brought out for clamped for adjusting current limit. Likewise, a capacitor output voltage sensing. This feedback pin has a second coupled external clamp will provide soft start. Switch function; when pulled low with an external resistor, it duty cycle goes to zero if the VC pin is pulled to ground programs the LT1070/LT1071 to disconnect the main through a diode, placing the LT1070/LT1071 in an idle error amplifier output and connects the output of the mode. Pulling the VC pin below 0.15V causes total flyback amplifier to the comparator input. The LT1070/ regulator shutdown, with only 50µA supply current for LT1071 will then regulate the value of the flyback pulse shutdown circuitry biasing. See AN19 for full application with respect to the supply voltage. This flyback pulse is details. directly proportional to output voltage in the traditional
U TYPICAL APPLICATIONS (Note that maximum output currents are divided by 2 for the LT1071) Driving High Voltage NPN
C1
Driving High Voltage FET (for Off-Line Applications, See AN25)
D2 G D R2** Q1 R1* S Q1 D1 V D1 IN VSW VIN VSW LT1070/LT1071 *SETS IB(ON) + LT1070/LT1071 10V SETS I ** B(OFF) TO 20V GND GND 1070/71 TA03 1070/71 TA03 1070/71 TA16 10701fe 7