LMQ66430: Reduce power consumption with adjustable output 3.8V

Alexis LATHIERE

Part Number: LMQ66430

Tool/software:

Hello,

I'm using the regulator LMQ66430R5RXBR to generate 3.8V from 7.2V following the chapter 9.2.3.2.1 (datasheet), I selected Rfbt = 38.03kOhms and Rfbb 13.3KOhms and with this configuration, I obtain well Vout = 3.8V.

But, I wonder if there's a way to reduce the current drawn by these resistance. Basically, I want to limit the current effect of voltage divider.

Is it possible to increase the values of Rfbt and Rfbb while keep an output voltage of 3.8V ?

As you can see in the chapter 9.2.3.2.1, possible values don't allow a wide choice.

Thanks for your help !

over 1 year ago

0 Frank De Stasi over 1 year ago

TI__Guru 73235 points

Hello

It is OK to use larger values of FB divider resistors to save current.

The max value shown in the equation should be 100k.

You can use Rfbt = 100k and Rfbb = 35.7k

You may need a feed-forward capacitor across the 100k to improve your loop phase margin.

You should leave a place for this on your PCB layout.

Thanks

0 Alexis LATHIERE over 1 year ago in reply to Frank De Stasi

Prodigy 10 points

Hello Frank,

With your advice, I've gain of 30uA on my design.

Do you know what the problem would be if I increased resistances again (with the same ratio) ?
I've a slot for a feed-forward capacitor.

Thanks

0 Frank De Stasi over 1 year ago in reply to Alexis LATHIERE

TI__Guru 73235 points

Hello

I would limit it to 1M.

As you mentioned the usual parasitic capacitance and leakage is the

major limiting factor.

I will speak with the design engineer to see if there are any other concerns

with this particular device.

Thanks

0 Alexis LATHIERE over 1 year ago in reply to Frank De Stasi

Prodigy 10 points

Hello Frank,

I tested with Rfbt = 1M, Rfbb = 383k and feed forward capacitor = 10pF, it seems to work.

Vout is correct (3.8V) and the voltage is stable.

Before validating this solution, I'm waiting for your feedback from the design engineer.

Thanks.

0 Frank De Stasi over 1 year ago in reply to Alexis LATHIERE

TI__Guru 73235 points

Hello

The Designer is looking at it now and we will get back to you this week.

Thanks

0 Frank De Stasi over 1 year ago in reply to Frank De Stasi

TI__Guru 73235 points

Hello

My Designer informs me that 1M is the largest value for Rfbt.

Of course, as you mentioned, you will need to leave a place for a Cff across Rfby.

Thanks

+1 Alexis LATHIERE over 1 year ago in reply to Frank De Stasi

Prodigy 10 points

Perfect !

Thanks for your help.

It's solved.

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LMQ66430: Reduce power consumption with adjustable output 3.8V