L6566B_08 Datasheet, PDF (27/51 Pages) STMicroelectronics – Multi-mode controller for SMPS

L6566B
Application information
Figure 15. Externally controlled burst-mode operation by driving pin COMP: timing
diagram
Vcc
(pin 5)
VccON
VccOFF
Vccrestart
Standby is commanded here
COMP
t
(pin 9)
GD
t
(pin 4)
Vcc_OK
t
Icharge
t
0.85 mA
Vout
t
t
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
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