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TPS40061PWP Datasheet(PDF) 10 Page - Texas Instruments |
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TPS40061PWP Datasheet(HTML) 10 Page - Texas Instruments |
10 / 28 page www.ti.com PROGRAMMING SOFT START tSTART w 2p L CO (seconds) (4) CSS + 2.3 mA 0.7 V tSTART (Farads) (5) TPS40060 TPS40061 SLUS543D–DECEMBER 2002–REVISED SEPTEMBER 2004 APPLICATION INFORMATION (continued) The impedance of the input voltage can cause the input voltage, at the TPS4006x, to sag when the converter starts to operate and draw current from the input source. Therefore, there is voltage hysteresis that prevents nuisance shutdowns at the UVLO point. With RT chosen to select the operating frequency and RKFF chosen to select the start-up voltage, the amount of hysteresis voltage is shown in Figure 5. TPS4006x uses a closed-loop approach to ensure a controlled ramp on the output during start-up. Soft-start is programmed by charging an external capacitor (CSS) via an internally generated current source. The voltage on CSS minus 0.85 V, is fed into a separate non-inverting input to the error amplifier (in addition to FB and 0.7-V VREF). The loop is closed on the lower of the (CSS– 0.85 V) voltage or the internal reference voltage ( 0.7-V VREF). Once the (CSS– 0.85 V) voltage rises above the internal reference voltage, regulation is based on the internal reference. To ensure a controlled ramp-up of the output voltage the soft-start time should be greater than the L-CO time constant as described in Equation 4. There is a direct correlation between tSTART and the input current required during start-up. The faster tSTART,the higher the input current required during start-up. This relationship is describe in more detail in the section titled, Programming the Current Limit which follows. The soft-start capacitance, CSS, is described in Equation 5. For applications in which the VIN supply ramps up slowly, (typically between 50 ms and 100 ms) it may be necessary to increase the soft-start time to between approximately 2 ms and 5 ms to prevent nuisance UVLO tripping. The soft-start time should be longer than the time that the VINsupply transitions between 6 V and 7 V. 10 |
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