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UCC28251 Datasheet(PDF) 27 Page - Texas Instruments

Part No. UCC28251
Description  UCC28251 Advanced PWM Controller With Prebias Operation
Download  55 Pages
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Maker  TI [Texas Instruments]
Homepage  http://www.ti.com
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UCC28251 Datasheet(HTML) 27 Page - Texas Instruments

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VREF
OVP
UCC28251
+
0.7 V
8.5 ?A
R3
R1
PTC
VREF
OVP
UCC28251
+
0.7 V
8.5 ?A
R3
R1
R2
VIN or VOUT
(
)
(
)
PTC1
PTC2
PTC1
PTC2
3
PTC1
PTC2
2.6 V
0.7 V
R
R
8.5
A
R
R
R
8.5
A
2.6
R
0.7 V
R
é
ù
´
´
-
-
m ´
´
ë
û
<
m ´
´
+
´
1
PTC1
R
3.7
R
=
´
(
)
PTC1
PTC2
PTC1
PTC2
0.7 V
R
R
8.5
A
R
R
0
´
-
-
m ´
´
³
(
)
(
)
R
F
R
3
R
0.7 V
V
V
11 A
R2
V
0.7 V
R
8.5 A
V
´
-
-
m ´
´
-
<
m ´
UCC28251
www.ti.com
SLUSBD8E – FEBRUARY 2013 – REVISED DECEMBER 2014
Feature Description (continued)
(17)
If the external resistor divider runs from 3.3-V VREF, and replaces R2 with a positive temperature coefficient
(PTC) thermistor, an over temperature fault protection with programmable hysteresis is accomplished
(Figure 37). Choose an arbitrary PTC value, which has a resistance as RPTC1 at protection temperature and
resistance as RPTC2 at recovery temperature. Because of its positive temperature coefficient, RPTC1 is larger than
RPTC2. To ensure an available solution, RPTC1 and RPTC2 need to meet the criteria.
(18)
And resistors R1 and R3 can be calculated as:
(19)
(20)
Figure 36. Overvoltage Protection
Figure 37. Overtemperature Protection
Figure 38 shows an external configuration using the OVP/OTP pin to achieve both overvoltage and
overtemperature protection. Follow the same design procedure for the OVP setting to choose R1, R2, and R3.
Choose an NTC value at protection temperature much smaller than R1 and with the resistance at protection
temperature as RNTC1, and recover temperature as RNTC2. The R4 value can be calculated as:
Copyright © 2013–2014, Texas Instruments Incorporated
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