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UCC28251 Datasheet(PDF) 16 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) 16 Page - Texas Instruments

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Enable
Signal
EN
C1
R1
R2
UCC28251
EN
Enable
Signal
(a)
(b)
UVLO
EN
0.3V
SS
CLK
1
1
6 s
C
R
m
=
2
R
10 k
>
W
UCC28251
SLUSBD8E – FEBRUARY 2013 – REVISED DECEMBER 2014
www.ti.com
Feature Description (continued)
A pulse signal may also be applied to the EN pin. Pulse-enable operation is shown on Figure 22. If the EN falling
edge happens before the SS voltage reaches 0.3 V, the enable signal at EN pin is considered as a pulse. In this
case, the next rising edge at EN pin disables the controller. If the falling edge of the first pulse at EN pin happens
after SS rises to 0.3 V, the UCC28251 interprets the pulse enable as a level enable, and an external solution as
shown on Figure 23 (a) can be used to reduce the pulse width. In this circuit, R2 is used to limit the current
(especially the negative current) through the internal ESD cell. Figure 23 (b) illustrates the waveforms based on
this solution. To prevent false trigger by noises, the pulse at the EN pin must be at least 2.25 V (minimum) high
and 3 µs wide to be considered valid.
Choose the R1, R2, and C values based on the following equations:
Choose R2 based on the current limit requirement of under 0.1 mA .
(1)
Choose R1 arbitrarily but much smaller than R2 and choose C1 according to the time constant requirement to
generate longer than 3-µs pulse.
(2)
If enable function is not used, pull EN pin to VREF.
Figure 22. Pulse Enable at EN Pin
Figure 23. An External Solution to Generate Enable Pulses for Pulse Enable
16
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Product Folder Links: UCC28251


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