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ATL431: Stability considerations for 4V output

Marco Degano
Prodigy 180 points
Part Number: ATL431
Other Parts Discussed in Thread: TL431, LM4041-N, LM4040

Hi,

I want to use the ATL431 for a 4V reference, and from the datasheet Fig 14 and 15 it seems that there will be some cases where it will be unstable.

The cathode current should be between 0.1mA and 5mA, excluding the start-up where it could have a peak up to 100mA.

To overcame the instability problems, as suggested. I am planning on using a very high capacitance.

Do you think 4  x 47uF will be enough?

Do you have some graphs that will show the stability for C > 10uF, and possibly 4V reference? (something like slva482a that is for TL431)

Thankjs

  • 0
    TI__Mastermind 19370 points
    Hi Marco,

    It depends on the application. While I believe there should be no issues with that much capacitance, it would make your system have a very slow start up.
    Can you tell me more about your system?
    Why is there such a high variance of current consumption?
    Is it possible to have a schematic?

    -Marcoo Z
  • 0 in reply to Marcoo Z
    Prodigy 180 points

    Hi Marcoo,

    I don't mind a slow start-up, in fact the initial inrush is to charge some capacitance elsewhere in the circuit, that will reach steady state in about 50ms.

    During operation there is a pulsating load, that can be switched off: this causes the variation in current.

    Unfortunately the application is confidential, so I can not share the schematic.

    Do you have any documentation for a 4V reference and so much capacitance, for my peace of mind?

    Thanks,

    Marco

  • 0 in reply to Marco Degano
    TI__Mastermind 19370 points
    Hi Marco,

    The ATL431 is a very low Iq device but its stability range is probably not suited for your application. I would recommend looking into the TL431 as its stability region is more flexible and it would make it so you just need a 10uF cap.

    -Marcoo
  • 0 in reply to Marcoo Z
    Prodigy 180 points
    Hi Marcoo,

    We are using the TL431 on another project, however for this one we would like to use the new version that has lower Iq and better dynamic impedance; also the datasheet states "The ATL43x has > 20x improvement cathode current range over it's TL43x predecessor. It also is stable with a wider range of load capacitance types and values."

    Is it possible to know which conditions are not ideal for it, maybe we can do something e.g. add some bias current to improve its stability.


    Regard,
    Marco
  • 0 in reply to Marco Degano
    TI__Mastermind 19370 points

    Hi Marco,

    The conditions it is not ideal with are outlined in the fig 14 - Fig 21 on the ATL431 DS. You have to keep in mind the current going through the device when you are using the standard configuration which is shown as the simplified schematic. When your load is on, the device will only consume X current Ik and the load will consume Y current,  but when the load is off the device will now consume X +Y. Ideally you want both of those states to be stable. In your case, the inrush current will not go through the device so I will not consider that current, but I think this current will be limited by the input resistor or else the TL431 will always be consuming 100mA which is not ideal. The draw back of a switching load is that the current will always be drawn, and it can either be used or wasted.

    The ATL431 is great when you have applications where the current Ik is not changing. In these situations you can target a very low Ik and stable CLoad and always have the device working there. For example if we use the schematic from figure 34 from the TL431 DS the device current will never change so we can target a very low Ik. In this schematic the load current will be drawn through the FET when on and not drawn at all when off so no wasted current. If you are considering adding more current to make it more stable, It would work but it would be a waste of power. I would recommend to use the schematic shown in the TL431 DS.

    If stability is becoming an issue, we also offer devices such as the LM4040 and LM4041-N family which are stable across all capacitors. Their device draw back is that they can't handle a high current.

    -Marcoo

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