AN3142 datasheet(33/44 Pages) STMICROELECTRONICS

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AN3142

Designing a fixed-off-time PFC

Doc ID 17005 Rev 3

33/44

operation of the DC-DC converter supplied by the PFC preregulator by disabling its PWM

controller. A typical use of this function is brownout protection in systems where the PFC

preregulator is the master stage. If not used, the pin is left floating.

Pin 12 (GND). This pin acts as the current return both for the signal internal circuitry and for

the gate drive current. When layouting the printed circuit board, these two paths should run

separately.

Pin 13 (GD) is the output of the driver. The pin is able to drive an external MOSFET with a

600 mA source and an 800 mA sink capability.

The high-level voltage of this pin is clamped at about 12 V to avoid excessive gate voltages

in case the pin is supplied with a high Vcc. To avoid undesired switch-on of the external

MOSFET because of some leakage current when the supply of the L6563S is below the

UVLO threshold, an internal pull-down circuit holds the pin low. The circuit guarantees 1.1 V

maximum on the pin (@ Isink = 2 mA), with Vcc > VCC_ON. This allows omitting the “bleeder”

resistor connected between the gate and the source of the external MOSFET used for this

purpose.

Pin 14 (Vcc) is the supply of the device. This pin is externally connected to the startup circuit

(usually one resistor connected to the rectified mains) and to the self-supply circuit.

Whatever the configuration of the self-supply system, a capacitor is connected between this

pin and ground.

To start the L6563S, the voltage must exceed the startup threshold (12 V typ.). Below this

value the device does not work and consumes less than 90 µA (typ.) from Vcc. This allows

the use of high value startup resistors (in the hundreds k

Ω), which reduces power

consumption and optimizes system efficiency at low load, especially in wide range mains

applications.

When operating, the current consumption (of the device only, not considering the gate drive

current) rises to a value depending on the operating conditions but never exceeding 6 mA.

The device keeps on working as long as the supply voltage is over the UVLO threshold (13

V max). If the Vcc voltage exceeds 22.5 V an internal Zener diode, 20 mA rated, is activated

in order to clamp the voltage. Please remember that during normal operation the internal

zener does not have to clamp the voltage, because in that case the power consumption of

the device increases considerably and its junction temperature increases too. The

suggested operating condition for safe operation of the device is below the minimum

clamping voltage of the pin.

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