LP38691-ADJ: Programmable output voltage

Art Mecina

Part Number: LP38691-ADJ
Other Parts Discussed in Thread: LP38691

Hello Guys,

Can LP38691-ADJ output be programmable through digital potentiometer? If yes, appreciate your advice on how it should be done and any advise on the digital potentiometer to use?

Thanks in advance!

Art

over 4 years ago

0 JCHK over 4 years ago

TI__Mastermind 44820 points

HI Are,

Sure. You can use a digital potentiometer as R2

The key is to make sure that the total series resistance (R1+R2) is less than 100K.

Does this answer your question?

0 JCHK over 4 years ago

TI__Mastermind 44820 points

HI Are,

Sure. You can use a digital potentiometer as R2

The key is to make sure that the total series resistance (R1+R2) is less than 100K.

Does this answer your question?

0 Art Mecina over 4 years ago in reply to JCHK

TI__Genius 16585 points

Thanks for looking into this John!

The customer schematic will be the one below. He want to place a POT on the LP3869 instead of R2, if you can sketch that for me I would appreciate it, I’m not sure were the wiper pin of the POT goes.

Thanks!

Art

0 JCHK over 4 years ago in reply to Art Mecina

TI__Mastermind 44820 points

Hi Art,

To be safe, the sum total of R1+R2 should be less than 100K. If it is more than this the input leakage into the ADJ pin could impact regulation accuracy. I would recommend using R3, R5, R6(R2), and R8 at 1/10th the current value.

Regards,

John

0 Art Mecina over 4 years ago in reply to JCHK

TI__Genius 16585 points

Thanks John for the advise on the resistor values.

How about the connection for the digital potentiometer? I’m not sure were the wiper pin of the potentiometer goes.

Thanks!

Art

0 JCHK over 4 years ago in reply to Art Mecina

TI__Mastermind 44820 points

Hi Art,

I think this is how you would hook it up. This should allow you to sweep the output voltage from 1.25V up to 5.5V:

If VIN of the LP38691 is greater than 5.5V, then the H pin of the potentiometer should be connected to a 5V rail or lower.

You may want to confirm with the data converter team or at a minimum order an EVM for both to try it out.

I hope this helps.

0 Art Mecina over 4 years ago in reply to JCHK

TI__Genius 16585 points

Hello John,

Thank you very much for this!

If the customer were to use an analog potentiometer, will the connection be the same?

I might direct the customer to this ticket personally for him to explain his use case better.

Art

0 JCHK over 4 years ago in reply to Art Mecina

TI__Mastermind 44820 points

Hi Art,

No. An analog potentiometer can be connected directly between the FB pin and GND. I think this is a better approach.

If you want simple digital control, the Any-Out(TM) feature with a device like the TPS7A83. The Any-Out pins connected to GND simply add to the reference. So with an open-drain MCU or GPIO expander, you can sweep VOUT from 0.8 to 3.85V in 50mV steps.

Let me know what you think.

0 Art Mecina over 4 years ago in reply to JCHK

TI__Genius 16585 points

Hello John,

Can you help provide a sketch of that?

It will be greatly appreciated.

Art

0 JCHK over 4 years ago in reply to Art Mecina

TI__Mastermind 44820 points

Hi Art,

Pins 1 & 3 should connect between VOUT and GND. Pin 2 would connect to the FB pin.

The Digital potentiometers have a limited minimum VIN range which makes them harder to use.

I hope this helps.

0 Art Mecina over 4 years ago in reply to JCHK

TI__Genius 16585 points

Thanks John for the excellent support!

I will set the ticket to resolved now. In case the customer have further clarification, I will let you know.

Best regards,

Art

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LP38691-ADJ: Programmable output voltage