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TPS40322RHBT Datasheet(PDF) 19 Page - Texas Instruments |
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TPS40322RHBT Datasheet(HTML) 19 Page - Texas Instruments |
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19 / 47 page  ´ = SS SS SS t I C 600 mV 0.4 0.8 1.3 Time VEN1/SS1 VSS_INT TPS40322 www.ti.com SLUSAF8E – JULY 2011 – REVISED JANUARY 2016 Feature Description (continued) 7.3.7 Start-Up and Shutdown 7.3.7.1 Start-Up Sequence When the ENx/SSx pin is pulled below 0.3 V, the respective channel is disabled. When ENx/SSx is released, the controller starts automatically and an internal 40-µA current source begins to charge the external soft-start capacitor. When the voltage across the soft-start capacitor is over 0.7 V, the internal BP regulator is enabled. The ENx/SSx voltage is clamped to 1.3 V while waiting for signals indicating that BP6, VDD, and the oscillator clock are good. After all the signals are confirmed, ENx/SSx is discharged to 0.4 V with a 140-µA current source, and then charged again with the internal 10-µA current source. The operation is described by the waveform shown in Figure 20. VSS_INT is an internal signal level shifted from ENx/SSx and then connected to the non- inverting terminal of the error amplifier. Figure 20. EN/SS Start-Up Waveform The soft-start time is determined by the internal charge current and the external capacitance. The actual output ramp-up time is the time for the internal current source to charge the capacitor through a 600-mV range. There is some initial lag time due to the offset (800 mV typical) from the actual ENx/SSx pin voltage to VSS_INT. The soft- start sequence takes place in a closed loop fashion, meaning that the error amplifier controls the output voltage constantly during the soft-start period and the feedback loop is never open (as occurs in duty cycle limit soft-start designs). The error amplifier has two non-inverting inputs, one connected to the 600-mV reference voltage, and the other connected to the offset VSS_INT. The error amplifier controls the FB pin to the lower of these two voltages. As the voltage on the ENx/SSx pin ramps up past approximately 1.4 V (800-mV offset voltage plus the 600-mV reference voltage), the 600-mV reference voltage becomes the dominant input and the converter has reached its final regulation voltage. Equation 4 is the calculation for soft-start capacitance. where • CSS is the soft-start capacitance connected to ENx/SSx pin • tSS is the desired soft-start time • ISS is the internal soft-start current (typically 10 µA) (4) Copyright © 2011–2016, Texas Instruments Incorporated Submit Documentation Feedback 19 Product Folder Links: TPS40322 |
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