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UCC28250 Datasheet(PDF) 34 Page - Texas Instruments |
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UCC28250 Datasheet(HTML) 34 Page - Texas Instruments |
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34 / 50 page  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 Submit Documentation Feedback Copyright © 2010, Texas Instruments Incorporated Product Folder Link(s): UCC28250 |
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