PM8801 datasheet(23/37 Pages) STMICROELECTRONICS

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PM8801

PD interface

Figure 9. Line transient response

Ch1: RTN - VSS, Ch2: VDD - VSS, Ch3: I input, Ch4: 5Vout (with offset)

4.6

Sleep mode / MPS mode

It is possible to put the PM8801 in sleep mode for applications connected to either a PoE

network or auxiliary sources. In both cases the DC/DC converter is turned off. The device

draws a minimum current and the LED is activated (see Section 4.7 for details).

The sleep mode is activated by pulling ENSL below its internal threshold. The ENSL pin is a

logic level active-low input. The ENSL pin may be connected to a system microcontroller

through an optocoupler.

When working from a PoE network or from a frontal auxiliary source, the internal hot-swap

MOSFET is opened and the DC/DC section shuts down. In this condition the PD

consumption is very low and for this reason it may be disconnected by the PSE.

To avoid this disconnection the PM8801 features a Maintain Power Signature (MPS) current

control. The device will keep on drawing a current whose profile is determined by sampling

the level of ENDC when entering sleep mode. The ENDC pin is designed to receive the

signal from a system microcontroller connected through an optocoupler.

According to the IEEE802.3at standard, continuous or pulsed MPS profiles are available.

Pulsed MPS is obtained by 100 ms pulses of 12 mA current, followed by 200 ms of MPS

dropout as shown in Figure 10. This profile is obtained with ENDC = low when entering

sleep mode.

Constant current MPS is a 12 mA constant current drawn during the entire duration of the

sleep mode. This profile is obtained with ENDC = high when entering sleep mode.

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