AN2782 datasheet(27/39 Pages) STMICROELECTRONICS

27 / 39 page

AN2782

Designing a fixed off-time PFC

Doc ID 14763 Rev 2

27/39

A commercial value of the limiting resistor of 1.8 k

Ω and a speed-up capacitor of 68 pF has

been selected for this application.

●

Pin 6 (GND): This pin acts as the current return both for the signal internal circuitry and

for the gate drive current. When laying out the printed circuit board, these two paths

should run separately.

●

Pin 7 (GD) is the output of the driver. The pin is able to drive an external MOSFET with

600 mA source and 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 L6562A is below the UVLO threshold, an internal pull-

down circuit holds the pin low. The circuit guarantees 1.1 V maximum on the pin (at

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 8 (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 L6562A, the voltage must exceed the startup

threshold (12.5 V typ). Below this value the device does not work and consumes less

than 30 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 3.75 mA. The device keeps

on working as long as the supply voltage is over the UVLO threshold (10.5 V max). If

the Vcc voltage exceeds 25 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, in which case the power consumption of the

device increases considerably and its junction temperature also increases. The

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

clamping voltage of the pin.

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AN2782 Datasheet(PDF) 27 Page - STMicroelectronics