TPS62177DQCT Inductor Selection for low current applications

Dhoresh Adabala1

Hello,

I am trying to design Buck converter using TPS62177DQCT. But across the datasheet they are suggesting maximum 10uH. Actually required current in our design is 80mA maximum. Input is 24V and output is 3.3V. With these specifications, if we are calculating for inductor value we are getting close to 180uH. Please let me know Do I need to follow the graphs given in datasheet and selected an inductor of value 10uH or do I need to follow calculations?

Also regarding the current rating for inductor, while choosing an inductor do I need to select inductor with current rating 2x or 3x times of the rate current of IC or 2x or 3x times the application maximum current? I want to use the following circuit in my design, shall I proceed with this or not?

Thanks,

Dhoresh.

over 11 years ago

0 Chris Glaser over 11 years ago

TI__Guru**** 150820 points

You can use 10uH for all designs. I'm not sure what calculations you refer to that are giving a higher value. 10uH is recommended.

We recommend sizing the saturation current of the inductor to be higher than the current limit of the IC in order to tolerate a fault condition (output short, for example).

0 Dhoresh Adabala1 over 11 years ago in reply to Chris Glaser

Prodigy 220 points

Hello Chris Glaser,

Thank you for the reply.

But considering Equation No:7 on page 24 of datasheet (Inductor Selection Section), I am getting a higher value of Inductor.

In that equation used parameters are Load Current is 80mA, Peak to Peak inductor ripple current is 20% of Load Current that is 16mA, efficiency of 72% and switching frequency of 1MHz. With these values if I am calculating for Inductor value it is coming close to 180uH.

Please clarify on this equation conflict with recommended inductor value.

Thanks,

Dhoresh

0 Chris Glaser over 11 years ago in reply to Dhoresh Adabala1

TI__Guru**** 150820 points

That equation just lets you calculate the ripple. There is no need to keep the ripple at some percentage for these TPS62k devices. Just use the recommended application circuit. They are easy to use.

0 Dhoresh Adabala1 over 11 years ago in reply to Chris Glaser

Prodigy 220 points

Hello Chris Glaser,

Thank your advice.

Dhoresh

0 Dhoresh Adabala1 over 11 years ago in reply to Chris Glaser

Prodigy 220 points

Hello Chris Glaser,

For 10uH inductor, we are getting around 320mA of ripple current by using the equation 7 suggested in the datasheet, where as our application load current is only 80mA maximum. Still is it ok?

Following parameters are considered as per our requirement for estimating inductor ripple current.

Parameter	Value	Units
VIN (Input Voltage)	24	Volts
Vout (Output Voltage)	3.3	Volts
η is the converter efficiency	0.72
Switching Frequency (fsw)	1000000	Hz

Thanks,

Dhoresh.

0 Chris Glaser over 11 years ago in reply to Dhoresh Adabala1

TI__Guru**** 150820 points

Yes, this is perfectly fine. The IC will operate in DCM, which is ok.

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TPS62177DQCT Inductor Selection for low current applications