STCMB1 datasheet(15/59 Pages) STMICROELECTRONICS

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DocID028958 Rev 4

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STCMB1

Application information

7.1

HV start-up

The STCMB1 is equipped with internal HV start-up circuitry dedicated to supply the IC

during the initial start-up phase, before the self-supply winding is operating. An external

electrolytic capacitor connected to the VCC pin is charged by the HV start-up circuitry,

connected to the HV pin.

As soon as the voltage on the HV pin is higher than VHVstart (18 V typ.), the HV start-up

system turns on and the external VCC capacitor is charged up to the turn-on threshold

(VCCOn, 16.5 V typ.), then to guarantee a reliable start-up of the STCMB1, the HV start-up

system is turned off only after the half-bridge section starts up. In case of a fault that

prevents the half-bridge section from starting up, the HV start-up system is automatically

shut down after a timeout TTOUT = 80 ms.

Figure 4. System wakeup waveforms

If for any reason the self-supply circuitry cannot power the STCMB1 the HV start-up will be

reactivated once the VCC reaches the turn-off threshold (VVCCOff, 9.5 V typ.).

The HV start-up system performs an R-C charge of the VCC pin. A double time constant

depending on the voltage value on the VCC pin is implemented to prevent damaging in case

of shorts to GND of the VCC circuitry.

The HV startup generator is activated also after detection of the PFC feedback failure, ac

brown-out or X-cap discharge condition. Different values of the charging current (IHV,ON)

are supplied to the VCC pin according to the state, see Table 4: Electrical characteristics on

page 11.

STCMB1 Datasheet(PDF) 15 Page - STMicroelectronics