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UCC28251 Datasheet(PDF) 32 Page - Texas Instruments |
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UCC28251 Datasheet(HTML) 32 Page - Texas Instruments |
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32 / 55 page 
( ) COMP( final) MAX V SR _ D 0.5 3 V 2 = - ´ ´ IN PRE BIAS RAMP SR(ramp) PRE BIAS V V 2 n V V 2 V - - - ´ = ´ ´ BLANK C CS R CS RAMP/CS GND UCC28251 V IN 36 V to 75 V BLANK C CS R CS RAMP/CS GND UCC28251 V CHARGE UCC28251 SLUSBD8E – FEBRUARY 2013 – REVISED DECEMBER 2014 www.ti.com Application Information (continued) To realize a fixed amplitude ramp, connect RCS to the constant voltage source, 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 41. External Configuration Of RAMP/CS Pin With/Without Feed-Forward Operation The input voltage feedforward also helps on pre-biased start up. When doing primary-side control to pre-biased start up, three conditions need to be considered: 9.1.3.1 Condition 1 At initial start up the primary side needs to provide enough energy to prevent output voltage dip; 9.1.3.2 Condition 2 At the end of soft start, it is required to keep the SR duty cycle change to be as small as possible. With input voltage feedforward, the COMP pin voltage is virtually fixed for different input voltages. Therefore, before the end of soft start, the duty cycle is the same for different input voltages. Choose the RCS and CCS following the procedure below. Considering initial start up, the RAMP peak voltage should be: (22) In this equation, VIN is the input voltage because of the feedforward any input voltage should be fine; VPRE-BIAS is the highest pre-bias start-up voltage required by the system; n is the tranformer primary to secondary turns ratio and VSR(ramp) is the internal SR ramp peak voltage 3 V. Another consideration is at the end of soft start, the SR duty cycle changes from controlled by the soft start, to complimentary to the primary-side duty cycle. The design should keep the transition as smooth as possible. Considering this, based on the output voltage and input voltage range, as well as the transformer turns ratio, calculate the SR duty cycle at different line voltages. Next, based on the maximum duty cycle on the SR_DMAX, and the internal fixed ramp amplitude 3 V, the COMP voltage at regulation can be chosen as: (23) 32 Submit Documentation Feedback Copyright © 2013–2014, Texas Instruments Incorporated Product Folder Links: UCC28251 |
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