L6566B View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6566B' PDF : 51 Pages View PDF
L6566B
Application information
start-up, at the end of the soft-start phase the output voltage of the converter must meet the
condition:
Equation 4
Vout
>
Ns
Naux
R Z1
IZCD
where Ns is the turn number of the secondary winding, Naux the turn number of the
auxiliary winding and IZCD 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 ).
Figure 10. Operation of ZCD, triggering and oscillator blocks (QR option active)
ZCD
(pin 11)
100 mV
50 mV
Oscillator
ramp
ZCD
(pin 11)
100 mV
50 mV
Oscillator
ramp
ZCD
(pin 11)
100 mV
50 mV
Oscillator
ramp
ZCD
blanking
time
Arm/Trigger
armed
trigger
ZCD
blanking
time
Arm/Trigger
ZCD
blanking
time
Arm/Trigger
ON-enable
ON-enable
ON-enable
PWM latch
Set
PWM latch
Set
PWM latch
Set
PWM latch
Reset
PWM latch
Reset
PWM latch
Reset
GD
(pin 4)
GD
(pin 4)
GD
(pin 4)
a) full load
b) light load
c) start-up
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