NCP1201
http://onsemi.com
13
Brownout Detect Protection
In order to avoid output voltage bouncing during
electricity brownout, a Bulk Capacitor Voltage Comparator
with programmable hysteresis is included in this device. The
non−inverting input, pin 1, is connected to the voltage
divider comprised of R
Upper
and R
Lower
as shown in
Figure 32, monitoring the bulk capacitor voltage level. The
inverting input is connected to a threshold voltage of 1.92 V
internally. As bulk capacitor voltage drops below the
pre−programmed level, i.e. Pin 1 voltage drops below
1.92 V, a reset signal will be generated via internal
protection logic to the PWM Latch to turn off the Power
Switch immediately. At the same time, an internal current
source controlled by the state of the comparator provides a
mean to setup the voltage hysteresis through injecting
current into R
Lower
. The equations below (Equations 5 and
6) show the relationship between V
BULK
levels and the
voltage divider network resistors.
Equations for resistors selection are:
RUpper
)
RLower
+
(VBULK_H
*
VBULK_L)
50
m
A
(eq. 5)
RLower
+
[1.92 V(VBULK_H
*
VBULK_L)]
(50
m
A
VBULK_H)
(eq. 6)
Assume V
BULK_H
= 90 Vdc and V
BULK_L
= 80 Vdc, by
using 4.3 k
W
for R
Lower
then R
Upper
is about 195.7 k
W
.
Figure 32. Brown−Out Protection Operation
R
Upper
V
BULK
R
Lower
BOK
V
REF
50
m
A
+
1.92 V
UVLO
−
