TOP221-227

D

7/01

5

PI-2030-042397

VIN

VOUT

0

IOUT

0

1

2

1

3

1

DRAIN

0

VIN

VC

0

• • •• • •

12

12

81

0

IC

• • •• • •

12

8

81

2

8

1

VC(reset)

compensated to minimize variation of the effective peak current

limit due to temperature related changes in output MOSFET

R

The leading edge blanking circuit inhibits the current limit

comparator for a short time after the output MOSFET is turned

on. The leading edge blanking time has been set so that current

spikes caused by primary-side capacitances and secondary-side

rectifier reverse recovery time will not cause premature

termination of the switching pulse.

The current limit can be lower for a short period after the leading

edge blanking time as shown in Figure 12. This is due to

dynamic characteristics of the MOSFET. To avoid triggering

the current limit in normal operation, the drain current waveform

should stay within the envelope shown.

Shutdown/Auto-restart

To minimize TOPSwitch power dissipation, the shutdown/

auto-restart circuit turns the power supply on and off at an auto-

restart duty cycle of typically 5% if an out of regulation

condition persists. Loss of regulation interrupts the external

current into the CONTROL pin. V

shunt mode to the hysteretic auto-restart mode described above.

When the fault condition is removed, the power supply output

becomes regulated, V

normal operation of the power supply resumes.

Overtemperature Protection

Temperature protection is provided by a precision analog

circuit that turns the output MOSFET off when the junction

temperature exceeds the thermal shutdown temperature

(typically 135

°C). Activating the power-up reset circuit by

removing and restoring input power or momentarily pulling the

CONTROL pin below the power-up reset threshold resets the

latch and allows TOPSwitch to resume normal power supply

operation. V

5.7 V (typical) sawtooth waveform is present on the CONTROL

pin when the power supply is latched off.

High-voltage Bias Current Source

This current source biases TOPSwitch from the DRAIN pin and

charges the CONTROL pin external capacitance (C

start-up or hysteretic operation. Hysteretic operation occurs

during auto-restart and overtemperature latched shutdown.

The current source is switched on and off with an effective duty

cycle of approximately 35%. This duty cycle is determined by

the ratio of CONTROL pin charge (I

(I

operation when the output MOSFET is switching.

Figure 6. Typical Waveforms for (1) Normal Operation, (2) Auto-restart, and (3) Power Down Reset.