STCMB1 datasheet(7/59 Pages) STMICROELECTRONICS

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STCMB1

Pin connections and description

3FB

Being the pin uncommitted, its voltage is proportional to the instantaneous output voltage of the

PFC stage. Under steady state conditions the voltage on the pin sits at the internal reference of

the error amplifier (2.5 V). If the voltage exceeds the steady state value by 7% (e.g.: due to an

output voltage overshoot) switching is stopped until it gets back close to it.

If the FB voltage falls below 0.5 V, a failure of the output divider is assumed and both the PFC

stage and the half-bridge are latched off. The HV start-up generator is intermittently turned on

to keep the device supplied. To restart the device it is necessary to disconnect the unit from the

input source.

As the input voltage monitor for the half-bridge, the pin handles power-on and power-off

sequencing. As the device is turned on, the PFC stage starts first and the half-bridge is kept

disabled until the FB voltage exceeds 2.4 V. In case the voltage is already above 2.4 V as the

PFC starts, the half-bridge waits 0.2 ms before starting, so to have always consistent power-on

sequencing. The half-bridge is again inhibited as the voltage on the pin falls below 1.75 V

(dc brown-out).

4COMP

To avoid an uncontrolled rise of the output voltage at the zero load, when the voltage on the pin

falls below 1 V (typical value) the gate driver output will be inhibited (burst mode operation).

5TON

Maximum on-time of the PFC MOSFET. A resistor and a capacitor in parallel are connected

from this pin to ground. When the MOSFET of the PFC stage is ON an internal current

generator produces a ramp that, along with the voltage on the COMP pin, determines the turn-

off instant of the MOSFET.

6ZCD

Boost inductor's demagnetization sensing input. A negative-going edge triggers the PFC

MOSFET's turn-on. The pin can be connected to an auxiliary winding of the boost inductor

through a resistor or to the drain of the MOSFET via an RC series.

7CF

Timing capacitor for HB oscillator. A capacitor connected from this pin to GND is charged by an

internal current generator programmed by the external network connected to the RFMIN pin. In

each half cycle the ramp starts as the tank current (sensed through the ISEN_HB pin) and the

applied square wave voltage has the same sign ( e.g.: ISEN_HB positive during the high-side

MOSFET on-time and ISEN_HB negative during low-side MOSFET on-time ). The ramp is

reset as a fixed peak value is reached. This also causes the HB to be toggled.

8CSS

HB soft-start. This pin connects an external capacitor to GND and a resistor to the RFMIN pin

that set both the initial oscillator frequency and the time constant for the frequency shift that

occurs as the chip starts up (soft-start). An internal switch discharges this capacitor every time

the chip turns off (VCC < UVLO, FB < 1.75 V, ISEN_HB > 1.5 V) to make sure it will be soft-

started next. Additionally the switch is activated when the voltage on the current sense pin

(ISEN) exceeds 0.8 V or when the converter is working in the capacitive mode operation. As

long as the voltage on the pin is lower than 0.3 V, the burst mode operation and second level

OCP protection are inhibited.

9RFMIN

Timeshift setting. This pin provides an accurate 2 V reference, and a resistor connected from

this pin to GND defines a current that is used to set the maximum timeshift. To close the

feedback loop that regulates the converter output voltage by modulating the timeshift, the

phototransistor of an optocoupler will be connected to this pin through a resistor. The value of

this resistor will set the minimum timeshift. An R-C series connected from this pin to GND sets

the timeshift at the start-up to prevent an excessive energy inrush (soft-start).

Table 1. Pin description (continued)

no.

Name

Function

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