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LP2980-N: Output voltage out of specifications at about 100 °C

FC1
Prodigy 100 points
Part Number: LP2980-N

Hello.

We use the LP2980AIM5-5.0/NOPB on our boards to genearte 5 V from 12 V, to power a digital circuit which draws about 3.5 mA.
During recent tests in the climatic chamber, we noticed a wrong behaviour of this regulator.
When the temperature of the IC (measured on its plastic body) reaches 99 °C, the output voltage starts to increase and is out of specifications.
I provide our measurements below.

T(IC) [°C] Output voltage [V] Notes
<99 5.014
99 5.180 after 1 minute
99 5.614 after 1 additional minute
100 5.870 after 1 additional minute
100 6.150 rises quickly 
<99 5.014 in about 18 seconds

Then this cycle repeats again and again, affecting the function of our boards.

Other info: at the output of the regulator we have a 10 uF, X7R capacitor.

Q1. What could be the reason of this behaviour? 
Q2. In the datasheet of this regulator it is stated that there is a thermal shutdown, but we expect that in this case the output voltage would drop to 0 V. So, what we observed is not related to thermal shutdown of the IC, is this correct?

Thank you.

  • 0
    TI__Mastermind 43610 points

    Hi Flavio,

    When testing in the chamber, do you have the 3.5mA load?

    To answer your questions:

    Q1 - I suspect that the leakage from VIN to VOUT is increasing as a function of temperature. Can you add a dummy load to confirm this is the case? Maybe around 20mA?

    Q2 - I don't believe that you are seeing thermal shutdown. If this were happening, you would see the output drop to 0V and then retry.

    We do now have the TPS7A24 which is much lower Iq. This has a no-load Iq of ~2uA whereas the LP2980-N is ~65uA. 

    I hope this helps to answer your question. 

  • 0 in reply to JCHK
    Prodigy 100 points

    Hello.

    Thank you very much for your reply.

    We tested the complete board in the climatic chamber, so the regulator was loaded.

    If the leakage is the cause, it is relevant to say that there is a diode (anode to OUT, cathode to IN) to protect the regulator, IR = 0.01 uA at VR=5 V, 25 °C but increases to 500 uA at 85 °C.

    We will make tests and communicate our results.

    Thanks again.

  • 0 in reply to FC1
    TI__Mastermind 43610 points

    Hi Flavio,

    if the diode used is a Schottky diode, then this makes a lot of sense. These show a lot of leakage at higher temperatures. 

    I hope this helps.

  • 0 in reply to JCHK
    Prodigy 100 points

    Hello John.

    Yes, it was a Schottky diode.

    I replaced the Schottky with a standard diode, which has a leakage current which is negligible compared to the one of the LP2980. Now 5 V are stable and the problem is solved. Same footprint is possible on the existing PCB.

    With a multimeter I measured a diode from pin 5 to pin 1 of the LP2980. So I wonder if there is already an internal diode on the chip. In other words, is the external diode necessary with this reagulator or could we save this component (with the goal to minimize the total leakage current), still ensuring safe operation (protection)?

    Thank you very much for your help!

  • 0 in reply to FC1
    TI__Mastermind 43610 points

    Hi Flavio,

    You're most welcome!

    You might be seeing an ESD diode from VOUT to VIN. If you have a lot of capacitance at COUT and you have situations where VIN can quickly fall to GND then the external diode is recommended. That being said the datasheet does show that VOUT has an absolute maximum rating of 9V and it is not with respect to VIN, so you may be able to remove the external diode. 

    Regards,

  • 0 in reply to JCHK
    Prodigy 100 points

    Hello John.

    For the LP2980AIM5-5.0/NOPB, do you have a max. value of the leakage current (over the entire temperature range), so that we can optimize our circuit accordingly?

    Thanks again.

    Regards

  • 0 in reply to FC1
    TI__Mastermind 43610 points

    Hi Flavio,

    I do not have this data, but eh design examples in the datasheet hint at a minimum load current of 1mA (minimum):

    Can you try with a minimum load of 1mA?

    I hope this answers your question.