• State
  • Locked Locked
  • Replies 4 replies
  • Subscribers 62 subscribers
  • Views 570 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM4132 Input Capacitor >= Output Capacitor Requirement -Damage If Violated?

Andrew Gibson
Intellectual 640 points
Other Parts Discussed in Thread: LM4132

Hello.  I am reviewing a circuit in which the LM4132BMF-3.0/NOPB 3 Volt reference has been used with a larger output capacitor than input capacitor.  The datasheet is explicit that this is not allowed:

The value of CIN must be sized according to the output capacitor value. The value of CIN must satisfy the relationship CIN ≥ COUT.
When no output capacitor is used, CIN must have a minimum value of 0.1 µF.

Is this restriction related to the question asked in <https://e2e.ti.com/support/power_management/linear_regulators/f/321/t/467130>, where the question was whether the reference would suffer reverse current if the output voltage was higher than the input voltage?

Is the LM4132 apt to be damaged if the output capacitor is 2 - 3 times larger than the input capacitor?

Thank you in advance,

Andrew

  • 0
    Intellectual 640 points
    Can anyone answer this question or discuss the possibilities? It refers to a real world situation and has had no attention for 2 weeks.
  • 0 in reply to Andrew Gibson
    TI__Intellectual 1745 points

    Hi Andrew,

    Sorry for your question getting missed.  As a side note, we have a new forum specifically for voltage references now, which is a great place for these types of questions in the future.  

    To answer your question, the most likely cause for the input / output capacitor requirement is to maintain regulator stability.  I can't comment on possibility of damage from this condition, the part may be survive but may not have the stable output that is expected from it.  You can test the design out yourself, but of course TI recommends adhering to what is written in the datasheet to meet the performance stated in the datasheet.

    Chris

    here is a link to the voltage references forum:  

  • 0 in reply to Christopher Dean
    Intellectual 640 points

    Hi Chris,

    Thank you for looking into this question.  I have seen a few defective LM4132's.  2 had no output and one had an internal short, pulling down the 5.2 V line supplying it.  There were no obvious transients during power up and the reference IC's load is well behaved, through hundreds of Ohms.  The only thing that is "off base" is that the output capacitance is larger than the input capacitance.  I would be interested in instability, too, but the first big question is to understand why the units failed in what appears to be a gentle circuit application. 

    Is there anyone you can pull in who might have more detail on the reason behind the comment in the datasheet or experience with this sort of reference failure?

    Andrew

  • 0 in reply to Andrew Gibson
    TI__Intellectual 1745 points

    Hi Andrew,

    There isn't anyone to pull in unfortunately; the team that created the LM4132 is no longer around.  The two things that come to mind are, the output capacitance larger than the input could result in instability, or perhaps could cause reverse current flowing back into the device if VIN is pulled below VOUT, where the extra capacitance on VOUT could maintain a voltage after VIN drops.  I do not believe there is reverse protection built into the device.  You could try adding an external diode between VOUT and VIN to take any reverse current in order to protect the device.

    Chris