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UA78M: Minimum Load for Regulation

Faizul Bacchus
Intellectual 1875 points
Part Number: UA78M
Other Parts Discussed in Thread: LM317

Hello,

Does the UA78M33 device require a minimum load current to maintain regulation? The datasheet does not mention anything about this but I was thinking about devices like the LM317 that require 10mA. Is that the same case here for the UA78M33 or does it regulate fine with 0A. This question is from one of our customers.

Thanks very much

Faizul Bacchus

  • 0
    TI__Guru 55940 points
    Hi Faizul,

    The datasheet should always say if a minimum load is required for regulation. In this case as you point out there is no stated requirement; however, the minimum load used in the Electrical Characteristics table is 5 mA. As such, the minimum load to achieve the Electrical Characteristics specifications is 5 mA.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    TI__Intellectual 1875 points
    Hi Ryan,

    Thanks for pointing out the 5mA in the electrical characteristics table, I missed that. I understand what this means but I think the customer is more concerned what will happen if the output is not loaded. Do we know if it will go unstable. They have 3 x 0.1uF ceramic caps on the output.

    Thanks,
    Faizul
  • 0 in reply to Faizul Bacchus
    TI__Guru 55940 points
    Hi Faizul,

    Stability involves several application specific conditions such as board layout, capacitors and specific transient events. The best way to determine if the LDO is stable in the customer's application is to force a worst case transient for the application and watch the transient response. Unfortunately as this particular device was released in 1976 we do not have an EVM or design databases for simulation to aid the customer in such a test. As you stated in your original post, the datasheet has no mention of a minimum load condition.

    Very Respectfully,
    Ryan