L6566BH datasheet(21/51 Pages) STMICROELECTRONICS

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L6566BH

Application information

Doc ID 16610 Rev 2

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If the MOSFET is enabled to turn on but the amplitude of the signal on the ZCD pin is

smaller than the arming threshold for some reason (e.g. a heavy damping of drain

oscillations, like in some single-stage PFC topologies, or when a turn-off snubber is used),

MOSFET turn-on cannot be triggered. This is identical to what happens at startup: at the

end of the next oscillator cycle the MOSFET is turned on, and a new switching cycle takes

place after skipping no more than one oscillator cycle.

The operation described so far does not consider the blanking time TBLANK after MOSFET

turn-off, and actually TBLANK does not come into play as long as the following condition is

met:

Equation 3

where D is the MOSFET duty cycle. If this condition is not met, there are no substantial

changes: the time during which MOSFET turn-on is inhibited is extended beyond Tosc by a

fraction of TBLANK. As a consequence, the maximum switching frequency is a little lower

than the programmed value fosc and valley-skipping mode may take place slightly earlier

than expected. However this is quite unusual: setting fosc = 150 kHz, the phenomenon can

be observed at duty cycles higher than 60%. See

Section 5.11: OVP block for further

implications of TBLANK.

If the voltage on the COMP pin (9) saturates high, which reveals an open control loop, an

internal pull-up keeps the ZCD pin close to 2 V during MOSFET OFF-time to prevent noise

from false triggering the detection block. When this pull-up is active, the ZCD pin may not be

able to go below the triggering threshold, which would stop the converter. To allow auto-

restart operation, however ensuring minimum operating frequency in these conditions, the

oscillator frequency that retriggers MOSFET turn-on is that of the external oscillator divided

by 128. Additionally, to prevent malfunction at converter startup, the pull-up is disabled

during the initial soft-start (see the relevant section). However, to ensure a correct startup, 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 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 is exactly equal to that of a standard current-mode

PWM controller. It works at a frequency fsw = fosc; both DCM and CCM transformer

operations 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 to

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).

osc

BLANK

−

≤

ZCD

Z I

Naux

Vout

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