L6566A Application information
Doc ID 13794 Rev 4 21/52
Note: When the system operates in valley skipping-mode, uneven switching cycles may be
observed under some line/load conditions, due to the fact that the OFF-time of the MOSFET
is allowed to change with discrete steps of one ringing cycle, while the OFF-time needed for
cycle-by-cycle energy balance may fall in between. Therefore one or more longer switching
cycles is compensated by one or more shorter cycles, and vice versa. However, this
mechanism is absolutely normal and there is no appreciable effect on the performance of
the converter or on its output voltage.
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 case 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 T
BLANK
after MOSFET
turn-off, and actually T
BLANK
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, nothing changes
substantially: the time during which MOSFET turn-on is inhibited is extended beyond T
osc
by
a fraction of T
BLANK
. As a consequence, the maximum switching frequency is a little lower
than the programmed value f
osc
and valley-skipping mode may take place slightly earlier
than expected. However this is quite unusual: setting f
osc
= 150 kHz, the phenomenon can
be observed at duty cycles higher than 60%. See
Section 5.11 for further implications of
T
BLANK
.
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, while 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
Section 5.10). 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 I
ZCD
the maximum pull-up current (130 μA).
