STCMB1 datasheet(17/59 Pages) STMICROELECTRONICS

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STCMB1

Application information

7.3

Ac brown-out protection

The STCMB1 device is equipped with brown-out protection to prevent the operation at too

low ac input voltage (typically when it falls below 70 V ac). When the brown-out protection is

activated, the PFC and HB sections are both turned off, the VCC start falling because no

more sustained by the on-board self-supply. When VCC reaches the turn-off threshold

(VVCCoff ), the HV start-up is turned on and VCC is charged up to VCCon. The HV start-up is

then turned off and VCC falls again toward VCCoff, since the switching activity is forbidden

by the brown-out protection. The charge and discharge of VCC goes on as long as the

abnormal ac mains condition lasts. The brown-out protection is deasserted when the ac

voltage goes above the brown-in threshold (typically 80 V ac).

In order to avoid unexpected intervention of AC brown-out protection in case of missing

cycles or short dips of the ac mains, a debounce time, TDB, has been introduced: in practice,

an AC brown-out condition has to last a time equal to TDB (32 ms, typ.) to stop the converter

operation.

To allow correct behavior of the brown-in and brown-out protections, accordingly to

the threshold values given in Table 4 on page 11, a resistor with value between 3 k



and 9 k

, in series with the HV pin is mandatory.

The brown-in threshold has to be intended as the voltage at the HV pin right before the

adapter turn-on (first GDPFC pulse, in general), when the mains voltage is slowly increased.

In detail, before the brown-in, VCC is charged by the HV start-up generator and naturally

discharged. During the charge phase, the voltage at the HV pin is about the actual mains

voltage minus the drop on the series resistor. When VCC = VCCon, the HV start-up

generator is turned off, the voltage drop is reduced to zero and the voltage at the HV pin

becomes equal to the actual mains voltage. So, the converter turn-on by brown-in will occur

when the mains voltage is above the threshold and the charge phase of VCC is finished.

Conversely, because of the debounce time, the brown-out threshold has to be intended as

the last peak voltage at the HV pin at about one debounce time before the adapter turns off

(PFC and LLC simultaneously turn off, in general), when the mains voltage is slowly

decreased. Note that before the brown-out, the converter is working, so VCC is generated

on the board and the HV start-up generator is off; consequently, the voltage at the HV pin is

about the actual mains voltage. When the brown-out is triggered, the converter is turned off

and the VCC is naturally discharged up to VCCoff when the HV start-up generator is turned

on and the charge / discharge operation of VCC starts to maintain the protected condition.

Figure 6. Brown-in and brown-out operation

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