L6566A datasheet(23/51 Pages) STMICROELECTRONICS

23 / 51 page

L6566A

Application information

23/51

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)

a) full load

(pin 4)

ZCD

blanking

time

ZCD

(pin 11)

Oscillator

ramp

PWM latch

Set

PWM latch

Reset

ON-enable

Arm/Trigger

50 mV

100 mV

armed

trigger

b) light load

ZCD

blanking

time

PWM latch

Set

PWM latch

Reset

ON-enable

Arm/Trigger

50 mV

100 mV

Oscillator

ramp

(pin 4)

ZCD

(pin 11)

c) start-up

ZCD

blanking

time

PWM latch

Set

PWM latch

Reset

ON-enable

Arm/Trigger

50 mV

100 mV

Oscillator

ramp

(pin 4)

ZCD

(pin 11)

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