Post Go back to editing

# Decide soft-start cap of LT8643S

Category: Hardware
Product Number: LT8643S

Hi,

I'm a DFAE in Japan. Our customer evaluates LT8643S's soft-start and gives us a question. We don't find the equation for the relationship between output rise time and TR/SS capacitance on the data sheet. Could you give us it?

Regards

Hiroyuki

Parents
• Hello Hiroyuki,

You can use the formula below,

C_{ss}=\frac{tss\cdot1.9uA}{1.6V}

Css = TR/SS Capacitance in Farads(F)

tss = Soft-start timing in seconds(s)

1.9uA is the SS pin internal pull-up for LT8643S

1.6V is the reference voltage for soft-start voltage.

Regards,

Nino

• Hello Hiroyuki,

You can use the formula below,

C_{ss}=\frac{tss\cdot1.9uA}{1.6V}

Css = TR/SS Capacitance in Farads(F)

tss = Soft-start timing in seconds(s)

1.9uA is the SS pin internal pull-up for LT8643S

1.6V is the reference voltage for soft-start voltage.

Regards,

Nino

Children
• Hello Nino,

Thank you very much for your quick reply. I think the reference voltage is 0.97V. Please see below LTspice.

Regards,

Hiroyuki

• Hello Hiroyuki,

The reference voltage should be 1.67V as stated on the datasheet. Kindly use 1.67V on your calculations as this may have been an issue on our device model.

I already forwarded the issue with LTSpice modelling team. An internal team will correct this model and you will need to update your library via SYNC RELEASE under TOOLS.

If you can, please provide your project information and estimated annual usage (EAU).

We hope to give you an update as soon as possible.

Regards,

Nino

• Hello Nino,

I don't find the spec on the electrical characteristics of LT8640S/LT8643S data sheet.

Regards,

Hiroyuki

• Hello Hiroyuki,

I was referring to 1.6V as the reference voltage for soft-start voltage. If you refer from the Soft-Start Tracking figure and the TR/SS Pin function from the datasheet, the FB voltage shows that it should only regulate to internal reference voltage, 0.97V when the TR/SS Voltage reaches 1.6V.

Upon simulating LT8643S,

The FB pin already regulates to 0.97V when TR/SS pin reach 0.97V.

Regards,

Nino