LM5021: Timing of hicuup mode Ton/Toff if parallel resistor and capacitor in SS pin.

Ray Chang

Intellectual 1345 points

Part Number: LM5021

Hi, Team

We have a question, if my customer parallel resistor and capacitor in SS pin.

Please refer to as below:

How to calculate the Toff/Ton timing for hiccup mode?

over 5 years ago

0 Hong Huang over 5 years ago

TI__Mastermind 47415 points

What is the resistor value R connected from SS to GND?

0 Ray Chang over 5 years ago in reply to Hong Huang

TI__Intellectual 1345 points

Hi, Hong

Is it possible to provide the formula for this connection?

0 Hong Huang over 5 years ago in reply to Ray Chang

TI__Mastermind 47415 points

The resistor Rss connected between SS and GND should be > 350k-ohm to allow SS capacitor charge up to 5.2V. With Rss, the SS pin voltage rise time can be calculated with the equivalent circuit of a constant current source Iss (22uA typical) connected to Rss//Css. The equation is:

Css *Rss * dVss/dt + Vss = Iss * Rss

From there, the time ton for SS pin voltage charge from 0 to 5.2V,

ton = -(Rss*Css) * ln (1- 5.2V/(Rss*0.022mA))

where, Rss in k-ohm, Css in uF, ton in ms.

I will provide toff later.

0 Hong Huang over 5 years ago in reply to Hong Huang

TI__Mastermind 47415 points

Here is toff calculation:

toff = (4.6V - 0.3V)/((4.6V/Css*Rss) + (0.25uA/Css))

where, Rss in Mega ohm, Css in uF, toff in seconds

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LM5021: Timing of hicuup mode Ton/Toff if parallel resistor and capacitor in SS pin.