Electrical characteristics

AN2487

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4.2.2

Classification current

Table 2 shows the classification current for the values of classification resistors shown (R1 in

Figure 5).

All the currents are well within the specification limits.

4.2.3

Undervoltage lockout

The startup and shutdown voltages, at which the chip starts the load or turns the load off,

were measured. Results are shown in Table 3.

These values are the board input voltages at which the STHS4257A1 chip enables and

disconnects the power converter. The data was taken with no load on the power converter

output. Startup occurs shortly after the /Power Good signal goes low, when the Vdd

capacitor charges to the L6565's startup voltage (12.5 to 14.5 volts). Shutdown occurs when

the STHS4257A1 chip disconnects the converter from its power source and the input

capacitors discharge.

Table 2.

Classification current

Class

0

1234

Watts available at PSE

connector

15.4 W

4.0 W

7.0 W

15.4 W

(future)

Treat as

class 0

Specified R

Open

124

Ω

68.1

Ω

45.3

Ω

30.9

Ω

Measured R used

Open

124.1

Ω

67.8

Ω

45.3

Ω

30.8

Ω

Specified current range

0 to 5 mA

8 to 13 mA

16 to 21 mA

25 to 31 mA

35 to 45 mA

STSH4257-

A-1 test

results

Current at

15.5 V

0.243 mA

10.11 mA

18.28 mA

27.22 mA

39.74 mA

Current at

20.5 V

0.278 mA

10.15 mA

18.31 mA

27.25 mA

39.73 mA

Table 3.

PD Startup and shutdown voltages

Startup

Shutdown

37.01 V

31.9 V