PM8805 datasheet(28/48 Pages) STMICROELECTRONICS

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PM8805 overview

PM8805

28/48

DS12813 Rev 2

4.8

Hot swap protections

Fault conditions for the hot swap MOSFET can be:

 a severe hot swap MOSFET overcurrent;

 a voltage drop higher than 14 V across the hot swap MOSFET;

 a combination of the two above conditions, like a short-circuit between VOUT and GND.

Such fault conditions are internally detected monitoring the current flowing through the hot

swap MOSFET, and also the voltage across it, and signaled to the control logic.

If the overcurrent condition is above the level Idc_OC1 (2.5 A typ.) but does not reach the

2nd level (Idc_OC2, 5A typ.) and last for more than the time Td_OC1 (1 ms typ.), then the

gate voltage is shorted to VOUT and the PGD signal is deasserted for the time Tretry (10 ms

typ.); after that a new startup attempt is performed.

In case of a severe overcurrent, the Idc_OC2 current threshold (5 A typ.) is reached, the

gate voltage and the PGD signal are forced low after the time Td_OC2 (10 us typ.) and is

deasserted for the time Tretry; after that a new startup attempt is performed.

In case of a voltage drop higher than VDS_fail (14 V typ.) across the hot swap MOSFET, the

gate voltage is shorted to VOUT and the PGD signal is forced low after VDS fail delay (400

us typ.). If that voltage drop is combined with a 2nd level overcurrent condition, the delay is

reduced to Td_OC2 (10 us typ.).

The protections implemented in the PM8805 allow to overcome the temporary faulty

operating conditions and not catastrophic events like a very strong short-circuit on the Vout.

Note also that if the fault persists for a time longer than Tretry the new startup attempt fails

due to the limited current available during the startup phase; the fault needs to be removed

to be able to have a successful new startup attempt.

4.9

Interface with a DC/DC converter

In most of the PoE applications a common mode filter is required to allow the PoE converter

to be compliant with the applicable EMI standards.

The right position where such a filter is introduced is between PM8805 and the PoE

converter, as shown in Figure 7.

PM8805 датащи(PDF) 28 Page - STMicroelectronics