TPS62740: Delay start-up using EN pin

Nikos Antoniou

Part Number: TPS62740

Hi everyone,

I've chosen to use the TPS62740 for my application due to its ultra-low power consumption and its great performance and characteristics. I want to take advantage of the EN pin by adding a start-up delay. While searching I found this great technical guide that was really helpful.

http://www.ti.com/lit/an/slvaea3/slvaea3.pdf?&ts=1589560061579

I do not need a preceise delay so I am ok with the loose EN pin. In my case, I am just needed a delay of around 1s. That requires a big capacitor and in order to reduce capacitor's size, I have to use the largest possible value of the resistor. What I do not know is how to calculate the resistor...

Basically, the guide explains how the resistor is calculated but I cannot understand it.. Also, I cannot find what is the EN leakage current of TPS62740..

The guide sais:

Any advice?

Thanks in advance

Nick

over 5 years ago

0 Davor Pavlovski over 5 years ago

TI__Expert 3611 points

Hi Nick,

I will get back to you on this matter on Monday.

Regards,

Davor

0 Nikos Antoniou over 5 years ago in reply to Davor Pavlovski

Intellectual 820 points

Thanks Davor!!

0 Davor Pavlovski over 5 years ago in reply to Nikos Antoniou

TI__Expert 3611 points

Hi Nick,

The value of the leakage current in the EN pin is mentioned on page 5, section 8.5 labeled as an "Input bias current" in the TPS62740 datasheet found here: https://www.ti.com/lit/ds/symlink/tps62740.pdf?&ts=1589804071008.

Kind regards,

Davor

0 Nikos Antoniou over 5 years ago in reply to Davor Pavlovski

Intellectual 820 points

Hi Davor and thank you for your support,

Could you explain me why the author choose resistor current of 1uA?

0 Davor Pavlovski over 5 years ago in reply to Nikos Antoniou

TI__Expert 3611 points

Hi Nick,

The reason why the current through the resistor is chosen to be at least 100 times larger than the leakage current, is because when calculating the start up delay the leakage current through the EN pin is not considered. The leakage current decreases the charging time of the capacitor, however if it is much smaller than the total current through the resistor then the error in the charging time is negligible.

Regards,

Davor

0 Nikos Antoniou over 5 years ago in reply to Davor Pavlovski

Intellectual 820 points

Thank you Davor!!

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TPS62740: Delay start-up using EN pin