Datasheet
L6566B Application information
23/51
start-up, at the end of the soft-start phase the output voltage of the converter must meet the
condition:
Equation 4
where Ns is the turn number of the secondary winding, Naux the turn number of the
auxiliary winding and I
ZCD
the maximum pull-up current (130 μA).
The operation described so far under different operating conditions for the converter is
illustrated in the timing diagrams of
Figure 10.
If the FF option is selected the operation will be exactly equal to that of a standard current-
mode PWM controller. It will work at a frequency fsw = fosc; both DCM and CCM
transformer's operation are possible, depending on the operating conditions (input voltage
and output load) and on the design of the power stage. The MOSFET is turned on at the
beginning of each oscillator cycle and is turned off as the voltage on the current sense pin
reaches an internal reference set by the line feedforward block. The maximum duty cycle is
limited at 70 % minimum. The signal on the ZCD pin in this case is used only for detecting
feedback loop failures (see
Section 5.11: OVP block on page 35).
ZCD1Z
IR
Naux
Ns
Vout >
Figure 10. Operation of ZCD, triggering and oscillator blocks (QR option active)
a) full load
GD
(pin 4)
ZCD
blanking
tim e
ZCD
(pin 11)
Oscillator
ramp
PWM latch
Set
PWM latch
Reset
ON-enable
Arm / Tr i g ge r
50 mV
100 mV
armed trigger
b) light load
ZCD
blanking
time
PWM latch
Set
PWM latch
Reset
ON-enable
Ar m /Tr ig ge r
50 mV
100 mV
Oscillator
ramp
GD
(pin 4)
ZCD
(pin 11)
c) start-up
ZCD
blanking
time
PWM latch
Set
PWM latch
Reset
ON-enable
Arm /Tri g ge r
50 mV
100 mV
Oscillator
ramp
GD
(pin 4)
ZCD
(pin 11)