IR22771S/IR21771S(PbF)
9
     www.irf.com
1     DEVICE DESCRIPTION
1.1 SYNC input
Sync input clocks the whole device. In order to
make   the   device   work   properly   it   must   be
synchronous with the triangular PWM carrier as
shown in Figure 8.
SYNC pin is internally pulled-down (10 k&) to V
SS
.
1.2 PWM Output (PO)
PWM output is an open collector output (active low).
It must be pulled-up to proper supply with an
external resistor (suggested value between 500&
and 10k&).
Figure 7: PO rising and falling slopes
PO pull-up resistor determines the rising slope of
the PO output and the lower value of PO as shown
in Figure 7, where     RC
=    , C is the total PO pin
capacitance and R is the pull-up resistance.
up
pull
on
on
low
R
R
R
Supply
V
+
?/DIV>
=
where R
on
is the internal open collector resistance
and R
pull-up
is the external pull-up resistance.
PO duty cycle is defined for active low logic by the
following formula:
Eq. 1
n
cycle
n
cycle
off
n
T
T
D
_
1
_
_
+
=
PO duty cycle (D
n
) swings between 10% and 30%.
Zero input voltage corresponds to 20% duty cycle.
A residual offset can be read in PO duty cycle
according    to    V
POs
   (see    Static    electrical
characteristics).
According to Figure 8, it can be assumed that odd
cycles are represented by SYNC at high level
(channel 1) and even cycles represented by SYNC
at low level (channel 2).
The two channels are independent in order to
provide the correct duty cycle value of PO even for
non-50% duty cycle of SYNC signal. Small variation
of   SYNC   duty   cycle   are   then   allowed   and
automatically corrected when calculating the duty
cycle using Eq. 1.
However, channel 1 and channel 2 can have a
difference in offset value which is specified in
V
POS
(see Static electrical characteristics).
To implement a correct offset compensation of PO
duty cycle, each channel must be compensated
separately.
1.3 Over Current output (OC)
OC output is an open drain pin (active low).
A simplified block diagram of the over current circuit
is shown in the Figure 9.
Over current is detected when |V
in
|=|V
inp
-V
inm
|>V
OCth
.
If an event of over current lasts longer than t
dOCon
,
OC pin is forced to V
SS
and remains latched until
PO is externally forced low for at least t
OCoff
(see
timing on Figure 4). During an over current event
(OC is low), PO is off (pulled-up by external
resistor).
If OC is reset by PO and over current is still active,
OC pin will be forced low again by the next edge of
SYNC signal.
To reset OC state PO must be forced to V
SS
for at
least T
OCoff
.
"    Auto reset function
The auto reset function consists in clearing
automatically the OC fault.
To enable the auto reset function, simply short
circuit the OC pin with the PO pin.
?
V
low
Supply