Rev. G 08/16

5

TOP221-227

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VIN

VOUT

0

IOUT

0

DRAIN

0

VIN

VC

0

•••

•••

0

IC

•••

•••

VC(reset)

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 trig-

gering 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

C regulation changes from

shunt mode to the hysteretic auto-restart mode described

above. When the fault condition is removed, the power sup-

ply output becomes regulated, V

C regulation returns to shunt

mode, and 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

C is regulated in hysteretic mode and a

4.7 V to 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

T)

during start-up or hysteretic operation. Hysteretic opera-

tion 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

C) and

discharge currents (I

CD1 and ICD2). This current source is

turned off during normal operation when the output MOSFET

is switching.

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