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UCC28251 Datasheet(PDF) 21 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) 21 Page - Texas Instruments

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CS
OUTA
OUTB
COMP
B
MAX
3 V
R
I
/ n
=
BLANK
C
CS
R
CS
VREF
RAMP /CS
GND
UCC28251
BLANK
C
CS
R
CS
RAMP /CS
GND
UCC28251
VIN 36 V to 75 V
UCC28251
www.ti.com
SLUSBD8E – FEBRUARY 2013 – REVISED DECEMBER 2014
Feature Description (continued)
Figure 29. Fixed Ramp Generation/Ramp Generation With Input Voltage Feedforward
Voltage feed-forward can be achieved by driving RCS from line input VIN. The peak of RAMP/CS is proportional
to VIN and output will have much faster line transient response. When the UCC28251 is used for the primary-
side control, RAMP parameters are critical for the optimal pre-biased start up performance. Refer to the ‘Voltage
Mode Control and Input Voltage Feed-Forward’ section of the Functional Description section for a detailed design
procedure of choosing RCS.
If the line input cannot be easily accessed due to limited board area or other limitation, a RAMP signal with fixed
peak voltage can be implemented by simply driving RCS from 3.3 V VREF (Figure 29).
8.3.7.2 CS: Current Mode Control
For current mode control, the RAMP/CS pin is driven by a signal representative of the transformer primary-side
current. The current signal has to have compatible input range of the COMP pin. As shown in Figure 30, the
COMP pin voltage is used as the reference for peak current. The primary-side signals OUTA and OUTB are
turned on by the internal clock signal and turned off when sensed peak current reaches the COMP pin voltage.
Choose the current sense transformer turns ratio (1:n) and the burden resistor value (RB) based on the peak
current at maximum load IMAX. Refer to the Functional Description section for more details on the current mode
control.
(6)
Figure 30. Peak Current Mode Control and PWM Generation
8.3.8 REF/EA+ (1/8)
REF/EA+ is the non-inverting input of the UCC28251’s internal error amplifier.
Copyright © 2013–2014, Texas Instruments Incorporated
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