Application Note
21
2011-07-06
2.4.3
Advanced Turn off delay
delay
off
T
_
- Delay pin
Advanced turn-off delay time of the SR MOSFET
delay
off
T
_
, normally is determined by the propagate
delay and transition time in the actual converter system. The value of
delay
off
T
_
can be set by the Delay
pin. For example, if the delay time required is 220ns, a
Ω
=
k
R
delay
33
need to connect at Delay pin
according to the curve below.
Figure 11
Turn-off delay time versus Rdelay
2.4.4
A review of the control scheme
After all the SR related parameters have been set, such as maximum on-time
max
_
on
T
, turn-on
delay
delay
on
T
_
, advanced turn-off delay
delay
off
T
_
, simplified typical waveforms can be drawn in Figure 12
for the two conditions when
sw
f
>
r
f
and
sw
f
<
r
f
.
From the waveforms on the left, the switch-on of the SR MOSFET is
delay
on
T
_
after the switch-on of the
primary side switch; while switch-off of the SR MOSFET is in advance with
delay
off
T
_
to the switch-off of
primary side switch. Under this operation condition, the SR MOSFET’s on-time changes with the primary
side MOSFET gate switching.
From the waveforms on the right, the SR MOSFET on-time is almost
constant
and equal to
max
_
on
T
,
which is independent of the primary side MOSFET turn-off.
In actual operation, the
sw
f
doesn’t have to be monitored. SR MOSFET will be turned off by whichever
signal comes first – the turning-off of the primary gate, or the falling edge of
max
_
on
T
.