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UCC28250 Datasheet(PDF) 34 Page - Texas Instruments

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Part No. UCC28250
Description  Advanced PWM Controller With Pre-Bias Operation
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Manufacturer  TI [Texas Instruments]
Direct Link  http://www.ti.com
Logo TI - Texas Instruments

UCC28250 Datasheet(HTML) 34 Page - Texas Instruments

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BLANK
C
CS
R
CS
VREF
RAMP/CS
GND
UCC28250
BLANK
C
CS
R
CS
RAMP/CS
GND
UCC28250
V
IN 36 V to 75 V
UCC28250
SLUSA29B – APRIL 2010 – REVISED OCTOBER 2010
www.ti.com
Voltage Mode Control and Input Voltage Feed-Forward
For voltage mode control, a resistor RCS and a capacitor CCS are connected externally at RAMP/CS pin as
shown in Figure 42. A ramp signal is generated on the RAMP/CS pin, at a rate of two times that of the switching
frequency. The generated ramp signal is used to control the duty cycle for both the primary-side switches and
secondary-side synchronous rectifiers. The ramp amplitude can be fixed or variable with the input voltage (input
voltage feedforward).
To realize a fixed amplitude ramp, connect RCS to the VREF pin, so that the ramp capacitor charging voltage is
fixed regardless of line and load condition. The RAMP/CS pin is clamped internally to 4 V for internal device
protection. Because the internal pull-down switch has about 40-
Ω on-resistance, the CCS value must be small
enough to discharge RAMP/CS from the peak to ground within TD(sp) + 70 ns (i.e. the pulse width of BLANK
signal).
To achieve the input voltage feedforward, the slope of the ramp needs to be proportional to the input voltage. Tie
RCS to the input line voltage. Because the ramp voltage is much lower than the input voltage, the ramp capacitor
charging current is considered to be proportional to the input voltage. With input voltage feedforward, the COMP
pin voltage should only move slightly even with large input voltage variation. This will provide much better line
transient response for the converter.
Figure 42. External Configuration of RAMP/CS Pin With/Without Feed-Forward Operation
34
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Copyright © 2010, Texas Instruments Incorporated
Product Folder Link(s): UCC28250


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