PM8800ATR datasheet(21/35 Pages) STMICROELECTRONICS

21 / 35 page

PM8800A

PD interface

21/35

Figure 12.

DC current vs RDC

This limitation is active after nPGD set and when the PD is supplied through the PoE or in

the Front connection (see Section 7)

The PM8800A useful programming range for the current limitation is between 150 mA and

800 mA. Practical resistor value ranges between 15 k

Ω and 75 kΩ.

5.6

HV regulator startup

PM8800A embeds a high voltage start-up regulator to provide a controlled reference voltage

of 8.3 V to the Current mode PWM during its start-up phase.

The regulator output is connected to the VCC pin as well as to the DC DC section

In normal isolated topology, the VCC pin is diode connected to the auxiliary winding of the

transformer used for the flyback or forward configuration. When the voltage from the

transformer exceeds the regulated voltage, the high voltage regulator is shut off, reducing

the amount of power dissipated inside the PM8800A.

The external auxiliary voltage must higher then 8.3 V but must be also lower than 15 V

under all working conditions, to avoid the intervent of the internal protection clamp.

A VCC UVLO mechanism monitors the level of voltage on the VCC pin. When VCC voltage

exceeds the VCCUVLO_R the PWM controller is enabled and it remains enabled until the

VCC voltage drops under its VCCUVLO_F value.

100

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500

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800

900

RDC [kohm]

PM8800ATR Fiches technique(PDF) 21 Page - STMicroelectronics