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+5V \ 100mA LDO

Shmuel Davis
Expert 2920 points
Other Parts Discussed in Thread: TPS7A20, LP5912, TPS7A90
In my design I have very little room on my PCB and I am looking for a LDO to connect to the output of my Buck-Boost MAX77827 to give me the cleanest possible supply for my ADC circuit as close to +5V as possible.
My specs are as follows :-

1. Smallest possible package with a pitch not less than 0.5mm
2. Smallest number \ size of external components
3. The Vin to the LDO I expect to range between 4.98V - 5.017 (based on simulation results - but this range could be greater in the real design)
4. Vout not to drop below 4.8V(I would like to take into account a little ripple - my maximum lowest possible voltage is 4.7V)
5. The highest possible PSRR
6. Max Iout expected 100mA
Do you have something that can come close these requirements ?
Thank you in advanced 
  • 0
    TI__Mastermind 44820 points

    HI Shmuel,

    Please see below:

    1. Smallest possible package with a pitch not less than 0.5mm *The TPS7A20 in the X2SON package has a pin pitch of 0.65mm
    2. Smallest number \ size of external components * If this is placed right at the output of the DC/DC converter, there is no need for additional input capacitance so you would only need one external capacitor at the output for stability. 
    3. The Vin to the LDO I expect to range between 4.98V - 5.017 (based on simulation results - but this range could be greater in the real design) *For any LDO, as the difference between VIN and VOUT decreases, its ability to filter is diminished. If you plan to use the 5V version of this device with a 5V input, it is working as a load switch and will not be helping to filter. If you could raise the input voltage to 5.5V or use the 4.5VOUT version of the TPS7A20 with 5V on the input, then this will do a great job at filtering the DC/DC ripple. 
    4. Vout not to drop below 4.8V(I would like to take into account a little ripple - my maximum lowest possible voltage is 4.7V). *At 100mA and 5VOUT, the TPS7A20 would have a maximum drop-out of ~47mV.  So VIN-47mV would be the minimum VOUT.
    5. The highest possible PSRR *Here is an example of the PSRR performance with a 500mV drop-out. 
    6. Max Iout expected 100mA
    Do you have something that can come close these requirements ? * I think that the TPS7A20 might be the best option here. 
  • 0 in reply to JCHK
    Expert 2920 points

    Dear John,

    Thank you for your quick, detailed response. 

    I will look into see if I can raise the vin.

    The TPS7A20 with the X2SON, even though it does have 0.65mm Pitch, its non standard formation and the 0.22mm clearance between the Pad and middle pad \ Heat Sink \ Exposed pad is too small for us (please note I tried to find the info on X2SON package and I couldn't find it in the data sheet nor on the site for the TPS7A20. I had to go to another Data sheet to get the details. It would probably be a good idea to update this info in the data sheet for the PTS7A20)

    As an alternative - do you have anything(LDO) that will give and output between 4.8V-4.9V ?

    Do you have any device that that combines a Buck-Boost and LDO together to get a single smaller solution ?

    Best regards

    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Mastermind 44820 points

    HI Shmuel,

    If you want our lowest possible drop-out LDO, the TPS7A52 is it. with 100mA load current the drop-out would only be 11.25mV max. However, the package is larger and only comes with an adjustable output so you would need to add two external resistors at a minimum. 

    I will see if someone from our DC/DC team can recommend a suitable Buck-boost solution.

    Regards,

  • 0 in reply to JCHK
    Expert 2920 points

    Dear John,

    Thanks for your quick reply. 

    I just realized the the MAX77827 can output 5.3V ! 

    Thus I can afford a "little" more drop out voltage. I am really looking for an LDO the smallest package that is possible (with a "standard footprint" - as I mentioned I am concerned that we might have trouble with the production of device with a footprint similar to X2SON, and a pitch not less than 0.5mm), fixed voltage(+5V - but we can go a little lower if needed - our limit is 4.7V but I don't really want to approach this voltage), to give the highest possible PSRR !  

    Thanks for all your help 

    SHmuel 

  • 0 in reply to Shmuel Davis
    TI__Mastermind 44820 points

    HI Shmuel,

    300mV of drop-out should help a lot with filtering. Here is a plot showed PSRR with a 20mA load and 300mV drop-out:

    The SOT-23 package of this device will release by the end of September which would also meet your minimum pitch requirements.

    Does this work for you?

  • 0 in reply to JCHK
    Expert 2920 points

    Dear John,

    Thanks for getting back to me. 

    The package of the TPS7A20 are not really suitable for us so I am leaning more towards the device MIC5219-5.0YMT-TR as I think it will fit our requirements a little better ! 

    Best regards

    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Mastermind 44820 points

    HI Shmuel,

    Here is a quick comparison between these two devices. From your application and drop-out, the MIC5219 will not be providing much filtering:

    Let me know what you think.

    Regards,

  • 0 in reply to JCHK
    Expert 2920 points

    Dear John,

    Thanks for getting back to me and pointing out this comparison ! 

    We are leaning now more towards the MAX38902AATA (TDFN-8) since it appears to give a better PSRR, Dropout Voltage and Headroom (c.f. MIC5219-5.0YMT-TR) while giving a "more conservative package" with pitch 0.5mm. The X2SON package of the TPS7A20 is not really relevant to the requirements of our project ! 

    Bests regards

    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Mastermind 44820 points

    HI Shmuel,

    Have you looked at our LP5912?

    Let me know what you think

  • 0 in reply to JCHK
    Expert 2920 points

    Dear John,

    Thanks for getting back to me.

    I had a look at the LP5912 and yes the package looks good. But as you outlined in your table and I double checked in the data sheets it appears that the MAX38902 has a better PSRR ?

    Best regards


    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Mastermind 44820 points

    Hi Shmuel,

    For equivalent PSRR and better noise, how about the TPS7A90?

    What do you think?

  • 0 in reply to JCHK
    Expert 2920 points

    Dear John,

    Thank you once again for a quick response ! 

    The  TPS7A90 has a comparable package but it does not come in a fixed voltage and thus requires a larger solution size due to the additional external components ! 

    Best Regards

    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Mastermind 44820 points

    HI Shmuel,

    The MAX77827 runs at 2.5MHz at this frequency the PSRR performance is mostly a result of layout and the output capacitor. 

    We have a video on PSRR here. 

    I hope this helps to clarify. 

  • 0 in reply to JCHK
    Expert 2920 points

    Dear John,

    Thanks for sending that video - it was very informative ! 

    But I am not sure how your answer related to my comment (Jul 21, 2020 4:39 PM) ?

    Best regards

    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Mastermind 44820 points

    Hi Shmuel,

    The TPS7A90 is the better device but as you point out, it is only available in an adjustable version. So at a minimum two external resistors are required to set Vout. 

    I think if you are able to test an LP5912 in your application, you would be happy with its performance. Do you need help getting an EVMZ or samples?

  • 0 in reply to JCHK
    Expert 2920 points

    Dear John,

    Thanks for that reply and all your help. 

    Why do you think that the LP5912 is so much greater than the MAX38902A ? 

    Best regards

    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Mastermind 44820 points

    HI Shmuel,

    The MAX38902 requires an external bypass capacitor to get low noise performance. While it does not seem to be a care about in your application, the MAX38902's IQ is >10x that of the LP5912. See more detailed comparison below:

    If you are looking to minimize PCB area, the LP5912 does the job with one less external component. 

    I hope this helps to clarify. 

  • 0 in reply to JCHK
    Expert 2920 points

    Dear John,

    I see your point about the reduction of the additional bypass capacitor ! It could be that the LP5912 could require smaller  input and output caps also ! 

    However it is hard for me to compare the output noise reduction benefits of the LP5912 over the MAX28902 ?

    Also I am going to use the MAX77827 and set its output to 5.3V. In the data sheets for the LP5912 it states "the input supply must be at least VOUT + 0.5 V" which I can't fit this requirement !?

    Best regards

    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Mastermind 44820 points

    Hi Shmuel,

    With respect to the output noise, I can only do the datasheet comparison. Based on this, the noise performance seems to be equivalent. 

    As for the 0.5V recommendation, this is written with the assumption you have a 500mA load and is a suggestion to have a lower noise well-regulated output:

    I do not think this applies when you plan on having a maximum load of 100mA. Note: the Typical application curves in the LP5912 datasheet are with a delta of 0.5V, the MAX38902 uses a delta of 1.1V. 

    Does this help to clarify?

  • 0 in reply to JCHK
    Expert 2920 points

    Dear John,

    Thank you for that rapid and clear answer ! 

    When I was trying to compare the PSRR, the data sheet it stated CIN = 1 µF, COUT = 1 µF - what is the max equivalent capacitance that would be recommended at a Vbias = 5 V - it would be a significant difference if I could go down from 0603 to 0402 caps ! 

    Best regards

    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Mastermind 44820 points

    HI Shmuel,

    You are most welcome!

    The LP5912 was characterized with a 1uF X5R/X7R capacitor. I see that on Digikey you can get a 1uF X5R dielectric in a 0402 package. 

    I am glad I could help you to minimize your solution size!