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UCC28251 Datasheet(PDF) 23 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) 23 Page - Texas Instruments

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VSENSE
VREF
COMP
FB/EA-
REF /EA+
SS
UCC28251
+
+
C
SS
C
Z2
R
Z2
C
P1
C
Z3
R
Z3
R
O2
R
O1
R
S2
R
S1
VOUT
SS
SS
REF / EA
27 A
T
C
V
+
m
´
=
UCC28251
www.ti.com
SLUSBD8E – FEBRUARY 2013 – REVISED DECEMBER 2014
Feature Description (continued)
8.3.11 VSENSE (14/1)
The VSENSE pin is used to directly sense the output voltage and to feed it into a transconductance error
amplifier. The measured voltage allows the UCC28251 to achieve optimal pre-biased start up performance.
When configured as a secondary-side controller, the output voltage is sensed and fed into the FB/EA- pin. The
UCC28251 uses a conventional error amplifier approach to allow type III compensation. Therefore, the FB/EA-
pin voltage always follows the REF/EA+ voltage. The FB/EA- pin does not reflect the true output voltage and
therefore this dedicated VSENSE pin is required. The voltage divider connected to VSENSE is discussed in the
Pre-Biased Start-Up Section.
When UCC28251 is set up as primary-side control, connect VSENSE pin to VREF.
8.3.12 SS (Soft Start Programming Pin) (13/20)
The soft-start circuit gradually increases the converter’s output voltage until steady state operation is reached.
This reduces start-up stresses and current surge.
When the UCC28251 reaches its valid operating threshold, the SS pin capacitor is charged with a 27-µA current
source. The UCC28251’s internal error amplifier non-inverting terminal follows the SS pin voltage on REF/EA+
pin voltage depending on which one is lower. Hence, during soft start, the SS pin voltage is lower than REF/EA+.
The internal error amplifier then uses the SS pin as its reference voltage, until the SS pin voltage rises above the
REF/EA+ level. Once the SS pin voltage is above REF/EA+ voltage, soft-start time is considered finished.
The soft-start implementation scheme and timing is different, depending on the location of the UCC28251 with
respect to the isolation barrier.
For secondary-side control, the internal error amplifier is used to achieve the voltage regulation. The REF/EA+ is
connected to an external reference voltage, FB/EA- is connected to the voltage sensing divider, and the error
amplifier’s output pin (COMP) is connected through a compensation filter back to the FB/EA- pin (Figure 32). In
this case, the primary output’s start-up is a closed loop soft start (soft-start input reference of error amplifier). The
output soft-start time is determined by the external capacitor connected at SS pin based on the internal 27-µA
charging current and the voltage set at REF/EA+ pin.
Based on the soft-start time TSS, choose soft start capacitor CSS value as:
(8)
Figure 32. Error Amplifier EAMP Connections for Secondary-Side Control
Copyright © 2013–2014, Texas Instruments Incorporated
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Product Folder Links: UCC28251


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