UCC28250_15 Datasheet, PDF (34/54 Pages) Texas Instruments – Advanced PWM Controller With Pre-Bias Operation

UCC28250
SLUSA29C – APRIL 2010 – REVISED JULY 2011
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Voltage Mode Control and Input Voltage Feed-Forward
For voltage mode control, a resistor R_CS and a capacitor C_CS 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 R_CS 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 C_CS value must be small
enough to discharge RAMP/CS from the peak to ground within T_D(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
R_CS 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.
UCC28250
VREF
V_IN 36 V to 75 V
UCC28250
R_CS
RAMP/CS
R_CS
RAMP/CS
C_CS
BLANK
GND
C_CS
BLANK
GND
Figure 42. External Configuration of RAMP/CS Pin With/Without Feed-Forward Operation
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