PM8800 STMicroelectronics, PM8800 Datasheet - Page 28

• Manufacturer Part Number: PM8800
• Description: Integrated IEEE 802.3af compliant PoE-PD interface and PWM controller with support of external source
• Manufacturer: STMicroelectronics

PWM section
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6.7
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Figure 18. Internal short circuit
Ch1 = 5 Vout, Ch2 = SoftStart, Ch4= I input
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.
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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PM8800A