AN2487 datasheet(19/32 Pages) STMICROELECTRONICS

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AN2487

Electrical characteristics

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4.3

Powered device supply solution

The following properties were verified by testing the demonstration board against IEEE

802.3af, section 33.C.5 specifications.

4.3.1

Current draw at low input voltage

The system is not allowed to draw significant power if its input is below 30 V.

4.3.2

Power Good signal and capacitor charging current

●

Upper trace, channel 1: Power Good signal relative to power converter input negative

●

Lower trace, channel 2: Input current to PD

4.3.3

Discussion

On the lower trace, two distinct regions are visible.

The first is the charging of the power converter input capacitors. The current is limited to a

low value (around 140 mA) by the STHS4257A1 chip. After these capacitors charge, the

Power Good (channel 1) signal drops, allowing the converter's Vcc capacitor to begin

charging. A short time later, the converter starts, drawing significant current (approximately

350 mA) while it charges its output capacitors.

4.4

Startup loading on the power sourcing equipment

According to the specification, once power is applied to the PD, the PSE must shut down if

the load current exceeds 400 mA for more than 50 milliseconds. After this startup transient,

the overcurrent trip point is reduced to 350 mA.

Table 4.

Input current at 30V

Input voltage

Input current

Specification Imax

30.0

0.273 mA

1.14 mA

Figure 9.

42 V input

Figure 10.

57 V input

AN2487 Datasheet(PDF) 19 Page - STMicroelectronics