PM8805 datasheet(22/45 Pages) STMICROELECTRONICS

22 / 45 page

PM8805 overview

PM8805

22/45

DS12813 Rev 1

4.4

Autoclass

During the first long classification finger, if the STBY pin is pulled above Autoclass_thr (0.5 V

typ.), the CLS1 buffer is turned off after Tcls_typ3 to advertise the PSE that the PD is

capable of Autoclass.

The purpose of Autoclass is to allow the PSE to determine the actual maximum power

drawn of the connected PD; after power-up, a PD that implements Autoclass draws its

highest required power throughout a time period bounded by 1350 ms as start time and

3650 ms as end time, both times measured from when PD rises above Vport 2P min.

4.5

Charge pump and VCP pin

Above UVLO voltage, a charge pump circuit is turned on to provide the correct driving

voltage to the upper side MOSFETs. Vcp is set about 8 V above the higher input voltage

VDD. An external small size ceramic cap, typically a 10 nF 50 V X7R, is suggested between

VDD and VCP to stabilize this voltage, as well as a fast switching protection diode with

reverse current lower than 1 uA; the current capability Icp of the charge pump is limited to a

50 uA typical and for this reason no other component or circuit is allowed to be connected to

VCP.

The charge pump is turned off during standby or sleep mode in order to reduce the power

consumption, as well as during RAUX operations, since neither the Active Bridge nor the hot

swap MOSFET are on.

See Table 12 PM8805 Control Signals description for more details.

Note that in the case of OT protection, the charge pump is not turned off in order not to

increase the power losses into the active bridge.

4.6

Input Bridges

The bridges integrated into the PM8805 are realized with N-ch MOSFETs in order to

minimize the losses due to the diode forward voltage, to improve the current balancing in 4-

pair PoE operations and, at the same time, to simplify the PD design allowing huge PCB

area savings with respect to discrete active bridges.

The PM8805 is capable of driving the internal N-ch MOSFETs in order to simulate the

behavior of the diodes: all the MOSFETs are continuously monitored to control the direction

of the current, and prevent a reverse current flow to the PSE or a fast input voltage

discharge due to a transient condition.

The MOSFETs are only enabled when the input voltage is greater than UVLO; this means

that during detection and classification phases the pass element of the bridges are the

MOSFETs body diodes, while the current consumption is reduced in order to avoid

corrupting the measurement of the detection resistor and the classification current.

Above UVLO voltage, the input active bridge state machine is activated, in order to discover

the polarity of the input bridges and select which bridge can be turned on.

On each high-side MOSFET of the bridges, two circuits work in parallel monitoring the drain

source voltage: the first circuit is an op-amp with slow response time, which is in charge to

amplify the voltage drop across the MOSFET when the current is lower 200 mA, increasing

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