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Forum Post: TPS40304 Soft Start Feature

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Hello,

I'm designing a power supply to step down a voltage from 4V to 15V down to 3.3V.  I have the design in the lab working well, but I have an issue with the soft-start circuit where I believe the FSS resistor connected from EN/SS to BP is biasing the soft-start capacitor as well as the intentional 10µA current source.  This in turn is cutting my desired soft start time by at least 3, hence triggering the over-current upon start-up.  

Can someone verify that I am correct in this thinking?  With a 0.33µF capacitor on the EN/SS I expect ~20ms of soft start.  When the FSS 267k resistor is attached from EN/SS to BP this turns into ~7ms of soft-start.  

If this is the case, nothing is stated in the datasheet. I would expect somewhere in the datasheet to state, "Hey, if you use the FSS resistor use this equation for your soft start calculation"  I don't know the equation since I just simulated in SPICE instead to obtain my above example numbers.  

Lastly, the block diagram does not do a good job in showing the LDRV and HDRV over current sensing.   If I read the data sheet correctly this is a measurement of the VDS voltage on the upper and lower MOSFET during a switch cycle.  I'm taking it that is a measurement from VDD? - SW for the high side and SW - GND for the low side.  The block diagram doesn't show this well, and I can't imagine it being terribly accurate since VDD could be pretty far from the high side MOSFET.  The low side MOSFET should be a little a more accurate.

Thanks,

Adam


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