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LP5912: LP5912 input voltage requirements

Sean Howson
Prodigy 30 points
Part Number: LP5912
Other Parts Discussed in Thread: TPS7A94, TPS7A90

Hi,

I am currently in the process of sourcing an LDO chip with the view to taking input power from a 3.7V(nominal) LiPo battery.

I am after a low noise (powering op-amps etc), 3.3V output chip with at least 0.5A output capability. No Enable pin required (but would be useful).

Initially i had narrowed the search down to the LP5912 (3.3V fixed output version, of course).

The datasheet however, has a statement i find confusing...

"10 Power Supply Recommendations This device is designed to operate from an input supply voltage range of 1.6 V to 6.5 V. The input supply must be well regulated and free of spurious noise. To ensure that the LP5912 output voltage is well regulated and dynamic performance is optimum, the input supply must be at least VOUT + 0.5 V. A minimum capacitor value of 1 µF is required to be within 1 cm of the IN pin"

Yet this is a low dropout regulator? Surely i should be able to take the input voltage to within the specified dropout limit, of the output voltage, before the output voltage suffers at all. Which is exactly what the earlier statement in the data sheet says (below).

"8.4.2 Minimum Operating Input Voltage (VIN) The LP5912 device does not include any dedicated UVLO circuit. The device internal circuit is not fully functional until VIN is at least 1.6 V. The output voltage is not regulated until VIN has reached at least the greater of 1.6 V or (VOUT + VDO)"

Any recommendations on low noise regulators for 3.3V supplies from LiPo's would be appreciated, or ideally, any comments on the suitablility of the LP5912.

Many thanks,

Sean

  • +1
    TI__Mastermind 41865 points

    Hi Sean,

    You are correct in that the output voltage will not fall so long as the input is at least VOUT + VDO. However, PSRR is affected by the proximity to dropout - operating closer to dropout reduces effective PSRR - so I would imagine that that is what this statement is referring to. 

    There are better alternatives to LP5912 for low-noise performance. TPS7A90 would be a good alternative, and the best alternative is the upcoming TPS7A94, which will be the lowest noise LDO on the market when it launches next year (samples are available now). 

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 30 points

    Hi Nick,

    Brilliant, thank you very much for the answer, I had a feeling the answer was in the wording >> "dynamic performance is optimum", kind of implies it's a recommendation, as does the section title... "Power Supply Recommendations", so a second opinion is very helpful, especially given most of the characteristics graphs are given using VIN = VOUT+0.5V.

    Thanks for the clarification and the recommendation!

    Kind regards,

    Sean