NCP1252 ON Semiconductor, NCP1252 Datasheet - Page 13

• Manufacturer Part Number: NCP1252
• Description: Flyback Applications
• Manufacturer: ON Semiconductor

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Brown−Out Protection
the forward converter against low input voltage conditions.
When the BO pin level falls below the V
controllers stops pulsing until the input level goes back to
normal and resumes the operation via a new soft start sequence.
resistors for BO pin.
controller must start switching (V
voltage for shutdown (V
Where:
•
•
•
•
reference), the internal current source (I
following equation can be written:
The following figure illustrates a soft start sequence.
By monitoring the level on BO pin, the controller protects
The following equations show how to calculate the
First of all, select the bulk voltage value at which the
When BO pin voltage is below V
V
V
V
I
BO
bulkon
bulkoff
BO
= 10 mA
= 1 V
= 370 V
= 350 V
shutdown
bulkoff
) the controller.
Vbulk
RB O u p
RB Olo
bulkon
BO
BO
) is activated. The
(internal voltage
) and the bulk
BO
Figure 31. Soft Start Example
IBO
BO
Figure 32. BO Pin Setup
level, the
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T
SS
= 13 ms
VBO
−
+
13
reference level (V
using the internal current connected between the BO pin and
the ground when the BO pin is below the internal voltage
reference (V
current source is now disabled. The following equation can
be written:
From Equation 2 it can be extracted the R
R
BOlo
The brown−out comparator features a fixed voltage
When BO pin voltage is higher than V
Equation 3 is substituted in Equation 1 and solved for
UVLO r eset
BOK
, yields:
V
V
SS
bulkON
= 4 V
BO
R
BOup
).
+ R
V
BO
BO
+
R
S
BOup
Q
). The hysteresis is implemented by
Q
+
V
R
bulkoff
I
BO
V
BOlo
Soft Start pin
(2 V/div)
CS pin
(0.5 V/div)
Time
(4 ms/div)
bulkoff
V
)
BO
* V
) R
R
V
R
BOlo
Grand
Reset
BO
BOlo
BO
BOup
R
) V
BOlo
BOup
BO
BO
, the internal
:
(eq. 1)
(eq. 2)
(eq. 3)