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TPS7B7702-Q1: LDO UNSTABLE OUTPUT

Sreeram R
Prodigy 10 points
Part Number: TPS7B7702-Q1

Tool/software:

Hi Team, 

We have used tps7b7702qpwprq1 in our design to give power to an external amplifier.

We have an external ignition signal to put the overall circuit into sleep and wake-up .

In few conditions, when ignition is removed and given back immediately, the LDO is working and we are getting desired output. However for the above same condition, we also observe that the LDO's output is Zero.

Is there any minimum timing required to be maintained for on and off cycle of the LDO ?

This is happening randomly for 5 out of 10 boards which is not acceptable and I have attached the waveforms for your reference.

  • 0
    TI__Expert 3720 points

    Hi Sreeram,

    Could you please share a picture of your schematic? I am interested in seeing how the ignition input is provided and the nature of the load. It would be helpful if you can share the board layout as well. I have asked about any minimum timing required for this LDO to our systems engineer. I will add their response as soon as it's available.

    Best regards

    Ishaan

  • 0 in reply to Ishaan Shetye
    Prodigy 10 points

    Hi Ishaan,

    Here, we are connecting audio amplifier as an external load which draws current around 60 mA.

    Also, I have attached the LDO and IGN circuit image for your reference.

    Here, IGN is an external input from the vehicle which will put the overall circuit into sleep and wake-up.

    Thanks,

    Sreeram R

  • 0 in reply to Sreeram R
    TI__Expert 3720 points

    Hi Sreeram,

    I see that for the failing boards, the 'Short to battery and reverse current detection' feature is kicking in. Please see section 7.3.3 of the device datasheet.

    Each time the LDO switch is enabled on the rising edge of the EN pin or during the exiting of the thermal shutdown, the short-to-battery detection occurs. At this moment, if the device detects the short-to-battery fault, the LDO switch is latched off.

    As per the input voltage profile on the failing boards, the Vout-Vin differential lies outside the range specified for this feature in the EC table of the datasheet (as shown below)

    The device is functioning as intended. As a suggestion, I would avoid having Vin - Vout go below 500 mV. You can have some minimum load to drain the output faster when ignition is removed and that should resolve the issue.

    Best Regards

    Ishaan

  • 0 in reply to Ishaan Shetye
    Prodigy 10 points

    Hi Ishaan,

    Good day !

    The difference between output voltage and input voltage is within 500mV as the input voltage is 12 V.

    When we turn the ignition off and on, we can see that the short to battery condition gets occurs. However, it is occurring randomly in few boards.

    Also, it should be solved while turning the ignition back on correct . ( Toggling the ignition signal means toggling the enable signal)

    Should we toggle it multiple times to make it come back to normal state.

    Is there any timing requirements for it which we are not meeting which we asked earlier?

    Can we have a teams or webex meeting to discuss this issue in detail ?

    Thanks.

  • 0 in reply to Sreeram R
    TI__Expert 3720 points

    Hi Sreeram,

    From the scopeshot/ simulation window you provided above for the case in which the boards fail, you can see that Vin drops momentarily such Vout - Vin is within that -500mV to 110 mV range by the time ignition is toggled. This is not happening due to the device and needs to be investigated further at the board- level.

    Yes, the device should operate normally when the short-to-battery is removed and the EN pin is toggled. Could you please toggle the ignition signal again and re-check?

    There are no timing requirements to be maintained for on and off cycle of the LDO.

    Yes, we can definitely meet on Webex. My email is i-shetye@ti.com. I can also request a Systems engineer to join for more support.

    Best regards

    Ishaan