PM8805 datasheet(29/45 Pages) STMICROELECTRONICS

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DS12813 Rev 1

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PM8805

PM8805 overview

For these reasons, high power systems may prefer using the Rear auxiliary connection as

shown in Figure 9, which also has the feature of taking priority over PoE regardless of the

order of connection and the voltage applied.

The simplified application schematics in Figure 9 shows an auxiliary source connected after

the internal hot swap MOSFET. This connection, together with the resistor divider on the

RAUX, ensures priority of the external source with respect to the PoE. Indeed, if the voltage

of this pin is above RAUX_thr (1.25 V typ.), the internal hot swap MOSFET is opened while

PGD is asserted. See Table 12 for more details on rear aux operation.

The minimum operative voltage for a rear aux operation is 9 V - which allows the use of 12

V +/-10% adapters - even though for high power systems it is recommended using high-

voltage power adapters, e.g. 48 V, to facilitate the operation of DC/DC converter and to

avoid voltage transients higher than VDS_fail (14 V typ.) across the hot swap MOSFET in

the switchover between PoE and auxiliary source.

Depending on the value of the auxiliary source, the resistor divider on RAUX must be sized

taking into account the following:

– RAUX_thr in order to provide the PGD to the system when the auxiliary voltage is

good enough for a stable operation;

– The AMR of RAUX pin (6 V), which never has to be reached at the maximum auxiliary

voltage;

– The RAUX command pin internally presents a 400 kΩ typ. equivalent resistance to

AGND (RAUX_res) which causes a typical sink current of about 3 uA at the threshold

value of 1.25 V;

– For very low RAUX voltage (12 V or lower) that still falls in the PoE detection range it

is suggested to replace the resistor divider with a resistor (up) and Zener (down)

network.

Following the above guidelines, the transition between PoE power and rear aux is typically

smooth, with PGD staying asserted during the switchover.

The removal of the auxiliary source is typically followed by a system reboot instead, since

the PD consumption is below the MPS limits, and the PSE typically disconnects the port

while the aux source is connected. In order to keep the connection with the PSE active, it is

possible to enable the MPS circuitry while a rear aux connection is operating. See

Section 4.14 for more details.

If RAUX or FAUX pin are asserted during classification, the class buffers are turned off and

the Class Event counter is reset. If FAUX or RAUX are asserted after UVLO, the counter

removal. Regardless of the classification, when FAUX or RAUX are asserted, the T0, T1, T2

are set as described in Table 13.

When plugging the DC jack of a rear auxiliary source, it is recommended to limit dV/dt

values on VOUT (i.e > 20 V/us), which may cause large current surges. In case the dV/dt

cannot be limited, a fast switching diode must be added between VOUT and VCP as a

protection for the PM8805.

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