PWM section

PM8800A

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Figure 18.

Internal short circuit

Ch1 = 5 Vout, Ch2 = SoftStart, Ch4= I input

6.6

Leading edge blanking

The sense current waveform often observes large spikes on the leading edge. This is due to

rectifier recovery and/or interwinding capacitance in the power transformer. If not

attenuated, this transient can prematurely terminate the output pulse. A simple RC filter -

whose time constant is approximately equal to the spike duration - is usually adequate to

suppress this spike.

An internal blanking time has been anyway integrated in order to prevent false overcurrent

detection to terminate the PWM waveform. This is done by a masking logic, active for

duration of 70 ns after the turn-on time of the primary side MOSFET.

6.7

Thermal protection

PM8800A has a two level thermal protection referred to the internal junction temperature.

First limit is set to 130 °C and limits the current into the hot-swap MOSFET to 140 mA every

time the voltage difference between VSS and GND is higher than 3 V.

The second limit is set to 160 °C and is always active. When this threshold is overtaken, the

hot-swap is opened and the PWM controller is switched-off.

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