PM8805 datasheet(27/45 Pages) STMICROELECTRONICS

27 / 45 page

DS12813 Rev 1

27/45

PM8805

PM8805 overview

Figure 7. Simplified general schematic with PM8805 and CM filter before the DC/DC

The introduction of the CM filter separates in high frequency the ground reference of

PM8805 from the ground reference of the PWM controller, but this has no effect on the

behavior of the converter; in fact the only signal exchanged is PGD that can be pulled up

close to the PWM controller at the internal reference voltage and, as a further precaution,

can also be filtered with a small ceramic capacitor.

4.10

Auxiliary sources

Some Powered Devices are designed to work with power from either a PoE network or an

auxiliary source such as a wall adapter. This allows PDs to be used also when the PoE is

not available or when the assigned power is not sufficient.

Two different configurations are typically chosen to connect an auxiliary source to the

DC/DC section of a PD device: after the bridge, but before the hot swap MOSFET or after

both of those; the PM8805 is designed to facilitate the design of both configurations,

allowing the coexistence with the PoE interface.

Figure 8 and Figure 9 show simplified application schematics where auxiliary sources can

be connected either prior (front) or after (rear) the internal hot swap MOSFET, making use of

a resistor divider between the external source and FAUX or RAUX pins.

PM8805 Fiches technique(PDF) 27 Page - STMicroelectronics