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

Raza Qazi
Prodigy 150 points
Part Number: LM27762
Other Parts Discussed in Thread: TPS7A39, TPS62802, TPS62808, TPS62840, LM2776, TPS7A20, LP5907

We have previously used LM27762DSSR to supply power to our analog multi-board system. However, we found that it is unreliable when far away from a certain baord or to variable demands in power. Recently, talking with TI, we were asked to put putting a dual-rail LDO in front to stabilize the power. In that case, we were considering to have 2 options:

- Used LM27762DSSR and input it to a dual rail LDO like TPS7A39 to stabilize power rails for 2.5V and -2.5V

- Use simple LDO for 2.5V and a charge pump only for -2.5V followed by a negative rail LDO

This is a LiPo battery-powered application our main PMU consists of BQ25155YFPR Battery Charger, TPS62802 Buck Regulator to generate 1.8V rails and TPS745-3v3 LDO to generate 3v3 rails. 

Can you please suggest the best reliable circuit and components we can use from your ecosystem. 


  • 0
    TI__Mastermind 23500 points

    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

  • 0 in reply to Febin
    Prodigy 150 points

    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:

  • 0 in reply to Raza Qazi
    TI__Mastermind 23500 points

    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

  • 0 in reply to Febin
    Prodigy 150 points

    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

  • 0 in reply to Raza Qazi
    TI__Mastermind 23500 points

    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

  • 0 in reply to Febin
    Prodigy 150 points

    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.


  • 0 in reply to Raza Qazi
    Expert 1540 points

    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.

  • 0
    TI__Mastermind 45650 points

    Hi Raza,  

    The key feature that the TPS7A39 has is startup tracking as shown below:

    For the LDO to properly regulate and filter any ripple from a DC/DC or charge-pump, It is best to have |VIN|>|VOUT|+/-0.5. Here are the PSRR plots that show how a wider delta between VIN and VOUT impacts performance. 

    I hope this answers your question.

  • 0 in reply to JCHK
    Prodigy 150 points

    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

  • 0 in reply to Raza Qazi
    TI__Mastermind 45650 points

    Hi Raza,

    Thanks. To confirm you recommend running the charge pump at 3V to get 2.5V rails from the LDO, right? Yes, 3V at a minimum to ensure the best performance. 

    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? I am not sure what the best recommendation is, It could depend on your actual load current. Charge pumps tend to have some variation over load. So if a charge pump is used, please be sure it can supply at least 3V under full load. 
    - Do you have better part recommendations for positive separate 2.5V LDO and negative charge pump + negative rail LDO? Not at this time. This is something I have been kicking around doing for quite some time now. 

    Also, which of the two options would provide better performance and stability I think that the TPS7A39 approach is more stable as the startup tracking ensures that the positive and negative rail come up at the same time. This helps to mitigate startup issues. 

  • 0 in reply to Raza Qazi
    Prodigy 150 points

    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

  • 0 in reply to Raza Qazi
    TI__Mastermind 45650 points

    Hi Raza,

    I think the simplest device for the positive rail could be the TPS7A20. For the negative rail, it would be the TPS723. And while not in the LDO product line, the LM2776 might work for creating the negative rail. 

    Let me know what you think.

  • 0 in reply to JCHK
    Expert 1540 points

    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.

  • 0 in reply to Matt Gaidica1
    TI__Mastermind 45650 points

    Hi Matt,

    The X2SON package is sampling now. All package options should be released by the end of October.