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TPS40140 Datasheet(PDF) 31 Page - Texas Instruments |
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TPS40140 Datasheet(HTML) 31 Page - Texas Instruments |
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31 / 68 page  SS t t1 t2 = + t2 + C SS 12 mA 0.7 V * V OUT R BIAS R1 ) RBIAS t1 + C SS 6 mA V OUT R BIAS R1 ) RBIAS 3 SS SS t C 58 10 = ´ ´ TPS40140 www.ti.com SLUS660I – SEPTEMBER 2005 – REVISED JANUARY 2015 9.1.6 Track (Soft-Start Without PreBiased Output) A capacitor connected to the TRKx pins sets the power-up time. When UVLO_CEx is high and the internal power-on reset (POR) is cleared, the calibrated current source, starts charging the external soft start capacitor with 12- μA. The PGOOD pin is held low during the start-up. The rising voltage across the capacitor serves as a reference for the error amplifier, U10 and U14. When the soft start voltage reaches the level of the reference voltage, VREF = 0.7 V, the converter’s output reaches the regulation point and further voltage rise of the soft-start voltage has no effect on the output. When the soft start voltage reaches 1.4 V, the powergood (PGOOD) function is cleared to be reported on the PGOOD pin. Normally the PGOOD pin goes high at this time. Equation 2 is used to calculate the value of the soft-start capacitor. CSS is in Farads and tSS is given in seconds. (2) 9.1.7 Soft-Start With PreBiased Outputs For prebiased outputs the TPS40140 uses two levels of soft-start current that charge the soft-start capacitor connected to the TRKx pin(s). PWM switching begins when the TRKx voltage rises to the voltage present on the FBx pin. When the first PWM pulse occurs, the charging current is increased to 12 μA. Figure 30 shows the typical waveforms present on the TRKx pin and the output voltage, VOUT when VOUT is prebiased. TRKx rises due to the 6 μA current, until at 1 the voltage on TRKx equals the prebiased voltage on the FBx pin, at time t1. At this time, the soft-start current is increased to 12 μA and TRKx rises with an increase in the slope. When TRKx reaches 0.7 V, at time t2, the output should be in regulation. The voltage on the TRKx pin continues to rise. When the TRKx voltage is 1.4 V, at time t3, the PGOODx signal is enabled. The TRKx voltage continues to rise to 2.4 V where it is clamped internally. This approach provides for an accurate detection of the threshold where FBx = TRKx. Figure 31 is a block diagram of the implementation. The calculation for the soft start time, due to prebias, includes the time from t0 to t1, plus the time from t1 to t2, as shown in Equation 3 through Equation 5. (3) where • CSS is in Farads • tSS is in seconds (4) (5) If there is no prebias (VOUT = 0 V), the equation reduces to case without prebias. Copyright © 2005–2015, Texas Instruments Incorporated Submit Documentation Feedback 31 Product Folder Links: TPS40140 |
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