AN2487

Electrical characteristics

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Figure 13.

No load current - DC maintain power signature

Note that most of the no-load current is due to the presence of the two LEDs on the board.

The AC maintain power signature is present by design. Small variations in input voltage will

draw capacitive current from the PD's large input capacitors.

Both the DC and AC maintain power signatures must be present for compliance with the

IEEE specification.

4.5

STHS4257A1 voltage drop under load

The load power was trimmed so that the input current was 350 mA and the voltage drop

between the STSH4257-A-1 Vout (pin 5) and Vin (pin 4) was measured.

Discussion

This represents an efficiency loss of 0.61%, which is quite negligible.

4.6

Input current limit vs. input voltage

The STHS4257A1 chip limits the input current to 350 mA. The power converter current limit

is set just slightly above that level, so that (if desired) the supply can be used at higher load

currents with the source voltage at the high end of its range. At 350 mA and 57 V, with no

Ethernet cable resistance present, the total input power available is almost 20 W.

The converter section's input current limit is flattened over the input voltage range by the

voltage feed-forward scheme. See the L6565 literature for a thorough explanation.

8.8

9

9.2

9.4

9.6

9.8

10

10.2

10.4

10.6

40

45

50

55

60

Input Voltage

Table 5.

Voltage drop under load, worst case input current

V drop measured

Power loss

0.35 A

260 mV

91 mW