Datasheet
( )
( )
R F
2
R
0.7 V V V
R
11 A V 0.7 V
´ -
<
m ´ -
R
1 2
V 0.7 V
R R
0.7 V
-
= ´
( ) ( )
R F 2 R
3
R
0.7 V V V 11 A R V 0.7 V
R
11 A V
´ - - m ´ ´ -
=
m ´
( )
PTC1 PTC2 PTC1 PTC2
0.7 V R R 11 A R R 0´ - - m ´ ´ ³
1 PTC1
R 3.7 R= ´
( )
( )
PTC1 PTC2 PTC1 PTC2
3
PTC1 PTC2
2.6 V 0.7 V R R 11 A R R
R
11 A 2.6 V R 0.7 V R
é ù
´ ´ - - m ´ ´
ë û
=
m ´ ´ + ´
VREF
OVP
UCC28250
+
0.7 V
11 mA
R3
R1
R2
VIN or VOUT
UCC28250
SLUSA29C –APRIL 2010– REVISED JULY 2011
www.ti.com
OVP/OTP (19/6)
The OVP/OTP pin provides multiple fault protection functions. If the voltage on the OVP/OTP pin exceeds 0.7 V,
a fault shutdown occurs. All outputs stop switching and stay off (low) during the shutdown, and the SS pin is
pulled to ground internally. Once the fault condition is cleared (i.e. OVP/OTP voltage drops below 0.7 V), the
UCC28250 enters hiccup mode. A soft-start cycle begins after the hiccup cycle is finished. An internal 11-µA
switched current source is used to create hysteresis.
If the external resistor divider runs from line voltage VIN, a line over voltage protection is implemented.
If the external resistor divider runs from the output voltage, output over voltage fault protection is achieved.
Figure 17 shows the over-voltage protection external configuration at the OVP/OTP pin.
According to the protection threshold V
R
and recovery threshold V
F
, choose an arbitrary R
2
value. To ensure a
realistic solution, R
2
needs to meet the following:
(16)
The other two resistors, R
1
and R
3
can be calculated.
(17)
(18)
If the external resistor divider runs from 3.3-V VREF, and replaces R
2
with a positive temperature coefficient
(PTC) thermistor, an over temperature fault protection with programmable hysteresis is accomplished
(Figure 18). Choose an arbitrary PTC value, which has a resistance as R
PTC1
at protection temperature and
resistance as R
PTC2
at recovery temperature. Because of its positive temperature coefficient, R
PTC1
is larger than
R
PTC2
. To ensure an available solution, R
PTC1
and R
PTC2
need to meet the criteria.
(19)
And resistors R
1
and R
3
can be calculated as:
(20)
(21)
Figure 17. Over Voltage Protection
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