L6566A View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6566A' PDF : 52 Pages View PDF
Application information
L6566A
Figure 15. Externally controlled burst-mode operation by driving the COMP pin:
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
Vcc_PFC
t
(pin 6)
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 instant when the external MOSFET must be switched off. Its output resets the PWM
latch, previously set by the oscillator or the ZCD triggering block, which asserts the gate-
driver output low. The use of PWM latch avoids spurious switching of the MOSFET that may
result from the noise generated (“double-pulse suppression”).
Cycle-by-cycle current limitation is realized with a second comparator (OCP comparator)
that also senses the voltage across the current sense resistor Rs 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). In this way, if the programming signal delivered by the
feedforward block and sent to the PWM comparator exceeds VCSX, it is the OCP comparator
that first resets 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 considerably with the input voltage. Obviously, this is
not a problem if the flyback converter runs off a fixed voltage bus generated by the PFC pre-
regulator; however, with a tracking boost PFC (a “boost follower” PFC), the regulated output
voltage at maximum mains voltage can be even twice the value at minimum mains voltage.
In this case the issue remains, although it is not as great as without PFC and wide-range
mains. With a 1: 2 voltage change, the maximum transferable power at maximum line can
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Doc ID 13794 Rev 4
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