LM27762: Reliable Power Setup to Supply 2.5V/-2.5V for Multiple Analog Boards

Hi Raza,

I am working on your request.

Do you want to go ahead with charge pumps and LDOs or can we also propose bucks for the 2.5V and -2.5V rails.

Could you please share a block diagram of the system to better understand and propose the best solution?

Regards,

Febin

Hi Febin, 

We are open to any option as long as it provides a clean and stable power supply to multiple boards. Preferably we want a solution that occupies less space overall, low cost and high efficiency. Previously just using charge pump was unstable to variation in power demands, so we decided to use LDO in front to stabilize it. Here is the current block diagram:

Hi Raza,

I can recommend some buck converters for your application. The total load for the 2.5V and -2.5V rails is still not clear to me. Nevertheless, please find below some suggestions for our small size high efficiency buck converters: TPS62840, TPS6282x or TPS62808

Please have a look at the datasheet and let me know if you need further help.

https://www.ti.com/lit/ds/symlink/tps62840.pdf

https://www.ti.com/lit/ds/symlink/tps62825.pdf

https://www.ti.com/lit/ds/symlink/tps62808.pdf

Regards,

Febin

Thanks Febin, 

Using only buck converters, I am concerned about swithcing noise coupling to the senstiive analog signals. Do you think these are good to use for sensitive analog circuitry over LDO?

Raza

Hi Raza,

This depends on the noise requirements for the analog circuits. Unlike the LDOs, Buck converters have inherent switching noise but the efficiency and thermal performance is better than LDOs. What is the maximum load for the bucks/LDO?  The power dissipation of LDO would be higher with load compared to bucks.

If noise sensitivity is critical, it is better to use LDO or you will have to use Bucks with second order LC filter.

Regards,

Febin

Thanks, in that case, can you propose an LDO based solution to our problem? Does the option I suggested to use LDO in front of charge pump seems a viable option? Do you have any better suggestions? The total load driven is less than 50mA.

I'm also interested in this solution! Although, my use case is slightly different. I will, however, have components snugged very close together, so a buck solution is not ideal.

Thanks. To confirm you recommend running the charge pump at 3V to get 2.5V rails from the LDO, right?

I have 2 further questions:
- Do you have better part recommendations for dual charge pump + dual rail LDO? It seems that LM2776 already has a charge pump and LDO built in. Do you recommend  putting another external LDO to stabilize power?
- Do you have better part recommendations for positive separate 2.5V LDO and negative charge pump + negative rail LDO?

Also, which of the two options would provide better performance and stability? 

Best,

Raza

Can you provide best components for this solution:

- Use simple LDO for 2.5V and

- A charge pump only for -2.5V followed by a negative rail LDO

John, can you comment on the future availability of the TPS7A20? I see it is in preview, rather than active status at the moment. I'm currently using an LP5907 because it's active, but the TPS7A20 has superior qualities.