STCMB1 datasheet(8/59 Pages) STMICROELECTRONICS

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Pin connections and description

STCMB1

8/59

DocID028958 Rev 4

STBY

Burst mode operation threshold for the HB. The pin senses some voltage related to the

feedback control, which is compared to an internal reference (1.25 V). If the voltage on the pin

is lower than the reference, the IC stops switching, enters an idle state and its quiescent current

is reduced. The chip restarts switching as the voltage exceeds the reference by

40 mV. Soft-start is not invoked. When the HB is stopped the PFC is stopped as well; while the

HB is switching the PFC stage is enabled and it switches or not depending on the level of the

COMP pin.

Tie the pin to RFMIN if the burst mode operation is not used.

An RC filter is mandatory, with R > 100

 and C > 100 pF. Please note that the time constant of

the RC filter enters in the loop transfer function, when the pin is directly fed by the

phototransistor of the optocoupler.

ISEN_PFC

The inductor current is sensed through a resistor RS on the current return side and the resulting

negative voltage is applied to this pin through a limiting resistor.

When the power MOSFET is turned on an internal comparator enables the PWM ramp to start

only when the voltage on the pin is lower than -25 mV (typical value), e.g.: when the inductor

current is slightly positive.

If the voltage on the pin goes below -0.5 V the internal overcurrent comparator is triggered and

terminates the conduction cycle of the external power MOSFET before the normal PWM circuit

does. In this way, the peak inductor current is limited at a maximum of 0.5/RS.

A voltage on this pin higher than -25 mV (typical value) enables both power MOSFET's turn-on

when a cycle is initiated by the demagnetization sensing circuit and when it is initiated by the

internal starter. In this way, the unit stops during the current surges occurring at power up or

after a mains dip or a missing cycle and restarts switching only when the surge is over.

An RC filter is mandatory at the pin, R > 100

 and C > 100 pF. The overall time constant

should give a negligible delay with respect to the typical operating frequencies of the PFC. The

value of the resistor also defines the effect of the THD optimizer (enhanced COT): THD is

usually optimized with a value between 100

 and 300 

ISEN_HB

Current sense input for the HB. The pin senses the resonant current through a sense resistor or

a capacitive divider for lossless sensing. If the voltage exceeds a 0.8 V threshold the soft-start

capacitor connected to the CSS pin is internally discharged: the frequency increases hence

limiting the power throughput. Under the output short-circuit, this normally results in a nearly

constant peak primary current. This condition is allowed for a maximum time internally set at 40

ms minimum. If the current keeps on building up despite this frequency increase, a second

comparator referenced to 1.5 V disables switching immediately. In both cases VCC is recycled

before restarting (see the HV pin). As long as the voltage on the CSS pin is lower than 0.3 V,

this second comparator is inhibited.

The pin is used also by the hard switching prevention (HSP) function. Do not short the pin to

ground; this would prevent the device from operating correctly.

An RC filter is recommended to reduce the noise level at the pin, with R > 50

 and C > 100 pF.

The overall time constant should give a negligible delay with respect to the typical operating

frequencies of the LLC.

GND

Ground. Current return for signal parts of the IC, the PFC gate driver and the low-side gate

driver of the half-bridge. Keep the PCB trace that goes from this pin to the sources of the PFC

and the low-side MOSFETs separate from the trace that collects the grounding of the bias

components.

GD_PFC

PFC gate driver output. The totem pole output stage is able to drive power MOSFETs and

IGBTs. It is capable of a 0.6 A source current and a 0.8 A sink current (minimum values). The

pin is actively pulled to GND during UVLO.

Table 1. Pin description (continued)

no.

Name

Function

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