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UCC28251 Datasheet(PDF) 25 Page - Texas Instruments

Part No. UCC28251
Description  UCC28251 Advanced PWM Controller With Prebias Operation
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Maker  TI [Texas Instruments]
Homepage  http://www.ti.com
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UCC28251 Datasheet(HTML) 25 Page - Texas Instruments

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OC(delay )
HICC
T
75 A
C
0.6 V
´
m
=
S
S
100 ns
C
R
=
S
PK
0.5 V
n
R
I
´
=
ILIM
HICC
UCC28251
CHICC
CS
RS
CT
1:n
Transformer Primary Side
Current
UCC28251
www.ti.com
SLUSBD8E – FEBRUARY 2013 – REVISED DECEMBER 2014
Feature Description (continued)
Figure 34. Current Limit Circuit
Once the over current protection level IPK is selected, the current transformer turns ratio and the burden resistor
value can be decided as:
(10)
In this equation, current transformer turns ratio is 1:n and RS is the burden resistor value.
Some filtering capacitance is required to reduce the sensing noise. Choose the RC constant at about 100 ns,
and calculate the capacitor value as:
(11)
The cycle-by-cycle current limit operation time before all four outputs shut down can be programmed by external
capacitor CHICC at HICC pin. (See HICC pin description)
8.3.14 HICC (10/17)
The cycle-by-cycle current limit operation time before all four outputs shut down can be programmed by an
external capacitor CHICC from HICC pin to ground, as shown in Figure 34. Once all four outputs are shutdown,
controller goes into hiccup cycle which is about 100 times the cycle-by-cycle current limit shut-down delay time. A
1-mA internal current source charges HICC pin up to 2.4 V, then the HICC pin is discharged by a 2.7-µA internal
current source to generate long hiccup restart time until HICC reaches 0.3 V. Based on the system requirement,
once the cycle-by-cycle current limit delay time TOC(delay) is selected, the HICC pin capacitor CHICC can be
selected based on the equation
(12)
Copyright © 2013–2014, Texas Instruments Incorporated
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