L6566B datasheet(27/51 Pages) STMICROELECTRONICS

27 / 51 page

L6566B

Application information

27/51

Figure 15. Externally controlled burst-mode operation by driving pin COMP: timing

diagram

5.6

PWM comparator, PWM latch and voltage feedforward blocks

The PWM comparator senses the voltage across the current sense resistor Rs and, by

comparing it to the programming signal delivered by the feedforward block, determines the

exact time when the external MOSFET is to be switched off. Its output resets the PWM

latch, previously set by the oscillator or the ZCD triggering block, which will assert the gate

driver output low. The use of PWM latch avoids spurious switching of the MOSFET that

might result from the noise generated (“double-pulse suppression”).

Cycle-by-cycle current limitation is realized with a second comparator (OCP comparator)

that senses the voltage across the current sense resistor Rs as well and compares this

voltage to a reference value Vcsx. Its output is or-ed with that of the PWM comparator (see

the circuit schematic in Figure 17 on page 29). In this way, if the programming signal

delivered by the feedforward block and sent to the PWM comparator exceeds Vcsx, it will be

the OCP comparator to reset first the PWM latch instead of the PWM comparator. The value

of Vcsx, thereby, determines the overcurrent setpoint along with the sense resistor Rs.

The power that QR flyback converters with a fixed overcurrent setpoint (like fixed-frequency

systems) are able to deliver changes with the input voltage considerably. With wide-range

mains, at maximum line it can be more than twice the value at minimum line, as shown by

the upper curve in the diagram of Figure 16 on page 28. The device has the Line

Feedforward function available to solve this issue.

It acts on the overcurrent setpoint Vcsx, so that it is a function of the converter’s input voltage

Vin sensed through a dedicated pin (15, VFF): the higher the input voltage, the lower the

Vcc

(pin 5)

(pin 4)

Vcc_OK

VccON

VccOFF

Vccrestart

Icharge

0.85 mA

Standby is commanded here

COMP

(pin 9)

Vout

L6566B Datenblatt(PDF) 27 Page - STMicroelectronics