PM8803

PD interface

Doc ID 018559 Rev 2

23/34

This limitation is active after the in-rush phase is completed. The useful programming range

for the current limitation is between 140 mA and 1 A. The practical resistor value range is

between 22 k

Ω and 150 kΩ.

4.6

High-voltage startup regulator

The PM8803 embeds a high-voltage startup regulator to provide a controlled reference

voltage of 8.0 V to the internal current mode PWM controller during its startup phase.

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

In normal isolated topology, the VC 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 PM8803.

In more detail, when the voltage from the auxiliary winding exceeds 8.0 V, the regulator

resets its intervention threshold to 7 V. In this way, a loosely regulated voltage from the

auxiliary winding is allowed without current-sharing with the internal regulator.

In the meantime, if the auxiliary voltage fails, the internal regulator takes over without losing

DC/DC control.

The UVLO threshold on VC is 6.0 V typically: at this voltage the DC/DC controller operations

are stopped and the outputs frozen in low state.

While the external auxiliary voltage has to be chosen higher than 8.0 V to take advantage of

the auxiliary winding, it must be also lower than 16 V for all operating conditions, to avoid the

intervention of the internal protection clamp.

A capacitor in the range of 220 nF-10 uF must be connected to DC/DC ground for stability.

For applications with high current drawn from VC, large capacitance should be used (e.g.

10 µF) in order to avoid converter switch-off during the startup phase.

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

When an auxiliary winding is not used, the internal HV regulator UVLO threshold is set at

6.6 V and the current limit is set at 20 mA typ. This value includes the current internally

drawn to bias the DC/DC controller, the gate drivers, the VB bias regulator and the external

components that may be connected to the VC and VB pins.

Notice that using the HV regulator without the auxiliary winding increases the internal power

dissipation, and, when operating at high ambient temperature, may lead the device to go

into thermal shutdown.