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14
Figure 28. BO Input Functionality − V
bulk
< V
bulk2
, PFC Start Follows
The I
BO
current sink is turned ON for 50 ms after any
controller restart to let the BO input voltage stabilize (there
can be connected big capacitor to the BO input and the I
BO
is only 18.2 m A so it will take some time to discharge). Once
the 50 ms one shoot pulse ends the BO comparator is
supposed to either hold the I
BO
sink turned ON (if the bulk
voltage level is not sufficient) or let it turned OFF (if the bulk
voltage is higher than V
bulk1
). See Figures 25 through 28 for
better understanding on how the BO input works.
Latched−Off Protection
There are some situations where the converter shall be
fully turned−off and stay latched. This can happen in
presence of an overvoltage (the feedback loop is drifting) or
when an overtemperature is detected. Due to the addition of
a comparator on the BO Pin, a simple external circuit can lift
up this pin above V
latch
(2 V typical) and permanently
disable pulses. The V
CC
needs to be cycled down below
6.5 V typically to reset the controller.
Figure 29. Adding a Comparator on the BO Pin Offers a Way to Latch−Off the Controller
−
+
−
+
BO_OK
to Permanent Latch
SW
V
refBO
High Level for 50 ms After V
CC
On
V
ref
latch
R
upper
BO
R
lower
20ms
Filter
V
bulk
I
BO
V
CC
Q1
NTC
V
out
20ms
Filter
To PFC Delay
+
−
+
−
On Figure 29, Q1 is biased off and does not affect the BO
measurement as long as the NTC and the optocoupler are not
activated. As soon as the secondary optocoupler senses an
OVP condition, or the NTC reacts to a high ambient
temperature, Q1 base is biased on and the BO Pin goes up,
permanently latching off the controller.
The High−Voltage Driver
Figure 30 shows the internal architecture of the
high−voltage section. The device incorporates an upper
UVLO circuitry that makes sure enough V
gs
is available for
the upper side MOSFET. The V
CC
for floating driver section
is provided by C
boot
capacitor that is refilled by external
bootstrap diode.
