PM8801 datasheet(33/37 Pages) STMICROELECTRONICS

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PM8801

Auxiliary sources

The internal logic will enable operations of the pwm controller only if the input voltage is over

the signature disable threshold (10.8 V typ, 10.3 - 11.3 V range).

Note that inrush current in this case is not limited and an external solution must be found.

The simplest solution is to put a low-value resistor in-series, but this lowers the converter’s

efficiency. A more efficient solution is the use of a MOSFET as the power switch, able to

limit the current during the charging phase, and to add only a few m

Ω in-series during

normal operation.

No DC current limit is foreseen for rear connection, since the current will not be flowing

through the hot-swap MOSFET. Cycle-by-cycle, overcurrent protection and thermal

protection are instead always active, as well as when the voltage on SA pin is above the

threshold 1.20 V.

The T2P signal will remain high (de-asserted) if a rear auxiliary source is connected.

If the T2P signal was asserted, when the the auxiliary source is connected, it will be turned

off; its status is stored unless the input voltage drops. So if the PSE stays connected until

the auxiliary source is removed, the T2P indication turns on again when the wall adapter is

disconnected.

The removal of the external auxiliary source such as a wall adapter usually is followed by a

system reboot because the PSE needs some time to re-detect the PD.

If a rear auxiliary connection (using SA pin) is foreseen in the converter design, it is

suggested to move the 0.1 µF capacitor (required by the IEEEE802.3at standard) from the

input (VDD to VSS) to the internal hot-swap MOSFET (VSS to GND). Alternatively, split the

0.1 uF in two capacitors of 47 nF: one placed at the input terminal, the second one across

the hot-swap MOSFET.

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