PM8805 datasheet(36/48 Pages) STMICROELECTRONICS

36 / 48 page

PM8805 overview

PM8805

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DS12813 Rev 2

Figure 10. MPS current profile

4.14

Special operation modes

Beyond the standard operation modes, the combination of the input commands described in

Table 10 activate three special operation modes that can be useful in certain cases; all the

special modes have as a basic assumption that the PD system has been successfully

detected and classified; in other words, the PD system has already been powered by a

PSE.

Rear Aux with MPS

This operation mode allows the PD to maintain an active connection with the PSE even if a

wall adapter is connected in rear configuration.

The behavior of the system is similar to a rear aux mode described in Section 4.10, but as

the MPS pulses are active, the PD load is powered by the auxiliary supply while the PoE line

is maintained active with the sole MPS current consumption.

This operation mode occurs under certain conditions:

the auxiliary source voltage is lower than / but close to the PoE voltage (12 V max. in

order to avoid activation of VDS_fail protection: for example, PoE at 54 V and Aux at 48 V).

A smooth transition to PoE power is also possible when removing the auxiliary source, while

the PoE to rear aux smooth transition is always guaranteed if the following conditions are

satisfied:

the Cout, which must provide enough energy to the load for all the time period

between the Aux removal and the closure of the hot swap MOSFET: as a general

rule of thumb use 1 uF for each watt on the load;

the inrush current must be lower than the OC2 threshold; a proper selection of the

capacitor on the CTRL pin permits to control the peak current at the closure of the

hot swap MOSFET: as a general rule of thumb a value between 1 and 2.2 nF

covers most of the applications.

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