Applications information

PM8804

12/27

DocID031629 Rev 1

5

Applications information

5.1

High-voltage start-up regulator and VC pin

The device embeds a high-voltage startup regulator to provide the supply voltage to the

internal current mode PWM controller during its startup phase and if needed, also during the

normal operation.

The regulated voltage is typically 8 V and it is internally connected to the VC pin as well as

to the controller 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 PM8804.

More in detail, when the auxiliary voltage surpasses 8.3 V, the HV regulator resets its

regulation level to less than 6.0 V. In this way, a loosely regulated voltage from the external

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

voltage drops under 6 V, the internal regulator is turned on in order to supply the controller

and maintain the output regulation.

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

winding, it must be also lower than 15.3 V for all operating conditions, to avoid the

intervention of the internal OV protection.

A ceramic capacitor at least 2.2

F must be connected at the controller output to guarantee

the stability of the HV regulator.

For applications with high current drawn from VC pin, a large capacitance should be

connected on VC pin in order to avoid converter switch-off during the startup phase.

A UV/OV mechanism monitors the level of voltage on the VC pin.

When VC voltage overcomes the UVLO rising threshold (VCUV_R), the PWM controller is

enabled by the assertion of PGD signal and it remains enabled until the VC voltage drops

below the UVLO falling threshold (VCUV_F) or the PGD is de-asserted. The VCUV_F is set to

5.0 V typically: below this voltage the PWM controller operations are stopped and the

outputs frozen in default state.

The overvoltage circuitry detects a feedback fault condition on the output voltage by sensing

the VC voltage. If VC exceeds the OV rising threshold (VCOV_R), the PWM controller is

stopped, the soft-start capacitor discharged, then a new start up is attempted.

In case of persistent overvoltage, the control logic tries 4 cycles of fast hiccup before

definitively shutting down the PWM controller. To restart the device the main supply VIN

must be lowered to ground and then powered up again.

The HV regulator internal current limit is set at 22 mA typ. This value includes the current

internally drawn to bias the controller, the gate drivers, and the external components that

may be connected to the VC pin.

The use of an external auxiliary source is strongly recommended: the amount of power

dissipated inside the device will be significantly reduced.