Soft-Starting TPS758A01 for Super Cap Charging

Harold Taylor

Other Parts Discussed in Thread: LMP8646

I'm trying to devise a way to soft-start the 758A01 (5V in 4V out) adjustable LDO for charging a 5F SuperCap over a 30- to 60-second interval. Traditional adjustable regulators with NPN pass devices would allow a large capacitor from the reference/feedback pin to ground. However, I don't know if the 758A01 would be stable during the interval where the reference pin is forced to be <0.8V during the initial charging of the soft-start capacitor. Is this method feasible, or do I need to use an adjunct current-limit op-amp and sense resistor such as LMP8646. If the latter, how do I ensure stability?

over 12 years ago

0 Harold Taylor over 12 years ago

Prodigy 190 points

Correction: Soft-starting the TPS758A01 adjustable regulator with a single cap would require the cap to be connected in parallel with the gain setting resistor, i.e., the resistor which connects from the output pin to the FB pin. The reference voltage would be unaffected as the cap charges up, and the output voltage would vary according to Vout = Vref * (1 + Zfb/Rshunt), where Zfb is the parallel combination of the feedback resistor and the soft-start capactor, and Rshunt is the resistor between the feedback pin and ground (typically 9.1 Kohm). So the initlial Vout would be 0.8V and the final Vout would be Vref * (1 + Rfb/Rshunt).

0 Darwin Fernandez over 12 years ago in reply to Harold Taylor

TI__Mastermind 41790 points

Hi Harold,

Adding a parallel capacitor is the simplest method for increasing the startup of the LDO. Below is another alternative

http://www.ti.com/analog/docs/litabsmultiplefilelist.tsp?literatureNumber=slva156&docCategoryId=1&familyId=659

Regards,

Darwin

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Soft-Starting TPS758A01 for Super Cap Charging