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LM2931-N: 2931 VS 2936, direct replacement or no

Michael Wood
Prodigy 10 points
Part Number: LM2931-N
Other Parts Discussed in Thread: LM2936

Hi, 

I am comparing the use of LM2931 vs LM2936. Both are being used with a 15V input and a 5V output, well within range of both. 

It was noticed that a series input resistance of 1.8k could be used on the 2936 (for protection), but not on the 2931. I am having trouble identifying the portion of the datasheets that shows where this is / is not allowed. 

The 2936 has a lower Iq than the 2931, but considering that LDO is unloaded I did not expect the 2931 to be unable to maintain 5V (maintains 1.67V at 15V input). 

When reducing the input resistance to half (900ohm) on the 2931, the LDO regulates slightly higher (2.2V). When removed completely it does achieve 5V regulation. 

The output capacitance between these is different (10uF VS 100uF) and I did not change these between tests, could the 10uF allow regulation with no input resistance, but not allow it with such an input resistance? 

  • 0
    TI__Mastermind 18466 points

    Hey Michael, 

    Are you trying to do a reverse current protection with the resistor? or what is this meant to achieve?

    Are you following this guidance for the 2931:

    Also, this device was characterized with  a load, which could be explaining the odd behaviors without one. 

    Do you have a waveform of the device not regulating? can you please share your schematics?

    Best,

    Juliette

  • 0 in reply to Juliette
    Prodigy 10 points

    Hello, 

    The purpose of the resistor is suspected to be current limiting as it is legacy and not standard from the app notes i've found. Some protection is desirable as the input power is coming from an external connection that could have a fault, with no easy diode pull up/down available. The schematic cant be shared, but it is a typical setup as shown below, but with the addition of a series input resistor and incorrect input / output capacitance. The reason i suspect it to be an input resistance issue is that the part works fine without this additional resistor added AND because I can not find any example of this used with one elsewhere. 

    From LM2936

    Not waveforms currently exist. The only measurements done were by eye from the power supply. 2936 can have input series resistance for protection and create a stable 5V output, but the 2931 cannot. 

    Below is the datasheet for the 2936 with many of the same test parameters. It is not practical to say you only get a stable 5V output when it is loaded by a certain amount, but I can say that I did try regulation with / without the load circuit and it did not work in either case. The only time 2931 did work was with the input resistors removed, when this was changed then it regulated with / without the load just fine. 

  • 0 in reply to Michael Wood
    TI__Mastermind 18466 points

    Hey Michael, 

    I can't say for certain, but there may be an internal node that the resistors are interacting with an causing a voltage divider at the input of the LDO. it is also possible that the device is not capable of regulating outside a certain ESR range on the input, causing the output to regulate incorrectly - meaning that the resistors is affecting something in an internal control loop.

    We have an app not with other configurations for reverse current protection if you would like to change your design.

    https://www.ti.com/document-viewer/lit/html/SSZT658

    Best,

    Juliette

  • 0 in reply to Juliette
    Prodigy 10 points

    Hi. We may do something like the app note if needed. 

    My main concern is I could not find anything in the two datasheets showing a series input resistor was not allowed. 

    Did I miss something, or is it not a common practice for these LDOs? 

  • +1 in reply to Michael Wood
    TI__Mastermind 18466 points

    yeah, a series input resistor is not common practice for these LDOs.