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+5V Clean power supply - very small area

Shmuel Davis
Expert 2930 points
Other Parts Discussed in Thread: TPS7A20

We have a design where we receive +5V supply from a PC power supply. We need to regulate this supply to provide a very clean +5V to our ADC \ DAC front end. We expect the absolute maximum current draw will be 50mA.

We have very little room so we are looking for, lets say, a LDO or something similar that can provide us we the cleanest possible +5V in the smallest possible PCB area. Possibly we should take a device that has a spec of 70-80 mA just for headroom but if there is a difference in solution area we could go down to 50mA ? Can you help us ?

  • 0
    TI__Guru 50170 points

    Hi Shmuel,

    For an LDO to be work as a filter there needs to be a bit of a delta between VIN and VOUT. If you consider a device like the TPS7A20, if you are using the 5V version with 5VIN, the device will be in drop-out. In this mode, with a 50mA load, VOUT would be ~4.977V and there is no effective PSRR as it is working like a load switch.

    If VIN could be raised to ~5.3V, then the LDO would have close to 50dB PSRR at 1MHz.

    Do you think this could work?

  • 0
    TI__Guru 50170 points

    Hi Shmuel,

    For an LDO to be work as a filter there needs to be a bit of a delta between VIN and VOUT. If you consider a device like the TPS7A20, if you are using the 5V version with 5VIN, the device will be in drop-out. In this mode, with a 50mA load, VOUT would be ~4.977V and there is no effective PSRR as it is working like a load switch.

    If VIN could be raised to ~5.3V, then the LDO would have close to 50dB PSRR at 1MHz.

    Do you think this could work?

  • 0 in reply to JCHK
    Expert 2930 points

    Dear JCHK,

    Thanks for your reply. 

    One of our components needs a min. supply of 4.7V and since my supply might be coming from a PC +5V I can't guarantee that is won't droop or even surpass this min supply for short periods of time. So I am not sure if an LDO will do the job. I might need to move to a Buck-Boost or something similar ? What is the smallest solution size do you have for 50-100mA load ? Also I was worried about this solution as this DC-DC will be in the vicinity of my ADC circuitry and I am a little concerned about the noise issues ? 

    Thanks fro your help 

    Best regards

    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Guru 50170 points

    Hi Smuel,

    It is quite common to follow a DC/DC converter with an LDO to filter the noise. The TPS7A20's smallest package is a 1mmx1mm X2SON. 

    A boost or buck-boost might help to ensure you can properly regulate a clean 5V rail. I am not sure what the smallest recommendation is, this would be from our DC/DC converter team. 

    I hope this helps.

  • 0 in reply to JCHK
    Expert 2930 points

    Dear John,

    Thanks for your reply.

    I just checked that the min voltage from the +5V of a PC is 4.75V. Since the TPS7A20 has a drop out of 300mA - this would give me an output of 4.45V which is out of the required min voltage of my component - 4.7V ! Unless there is such a thing as a LDO with 50mA dropout (I am not sure if there is) I think my only choice is a Buck-Boost ??

    Best regards

    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Guru 50170 points

    HI Shmuel,

    For 5VOUT the maximum drop-put is 140mV at 300mA:

    So if your load is 50mA, the worst-case drop-out is 140/6 or 23.3mV.

    Does this help to clarify?

  • 0 in reply to JCHK
    Expert 2930 points

    Dear John,

    Sorry that was a TYPO.

    What I meant that we would need a LDO with a max drop out voltage of 50mV (4.75V - 0.05V => 4.7V which is my limit !). I figured such a LDO probably doesn't exits ? 

    Best regards

    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Guru 50170 points

    Hi Smuel,

    Many LDOs can do this. With a 50mA load, the TPS7A20 meets this criteria. In the above example, the 140mV drop-out is given at 300mA, so this translates to an effective RDSON of 0.46Ohms. So for a maximum 50mA load, the drop-out would be only 23mV.

    Does this help to clarify?

  • 0 in reply to JCHK
    Expert 2930 points

    Dear John,

    Thanks for your reply once again.

    I just realised that the the actual input voltage may even be less than 4.75V and in fact we just measured a loaded supply and measured 4.4V. So unfortunately I am going to have to drop the LDO and go for a Buck-Boost DC-DC ! 

    Thanks for your help all the same

    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Guru 50170 points

    Hi Shmuel,

    You're welcome. If you need a low noise power supply, you want to still use an LDO to follow the Buck-Boost to minimize noise. on the ADC/DAC power supply.