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TPS7A47-Q1: TPS7A47-Q1 output failure issue

Yang Wu
Expert 3020 points
Part Number: TPS7A47-Q1

Hi Team,

My customer is using TPS7A47-Q1 and recently reports 5 failure cases (in total 30 boards). 

Background:

1) Input voltage: 24.5~28V (Input comes from the multi-windings Flyback, it's the aux-winding output and varies due to cross regulation).

2) Output voltage: 24V

3) Output load current：0.02A~0.33A

4) As descried above, the input varies due to cross regulation. If the main output of the flyback is in full load, then this aux-winding can output 28V/0.02A. If the main output of the Flyback is in light load, then this aux-winding can output 24.5V/0.33A.

 

Failure Description:

1) The failure shows TPS7A47-Q1 has output when load > 0.02A

2) If disconnecting the load, the failure shows lower output (output @21.5~23V, but can't achieve 24V)

3) Test if NG samples has short circuit between pins (Input & GND, Output & GND, Input + & Output), the answer is no short circuit.

4) A-B-A test shows the failure follows the NC samples.

Here is the schematic. Have you heard similar failure from other customers? Do you know what can cause this kind of failure? I am not sure if we need to submit a FA request. 

best regards,

Yang

  • TI__Expert 4540 points

    Hi Yang,

    There are two potential issues that I am seeing in the schematic which might trigger some problems. First, it is recommended to connect both output pins (1 and 20) together for better dropout performance and for current limit problems during start-up. 

    The other problem is at the enable pin (13) which has a minimum voltage requirement of 2V to enable as per the recommended conditions:

    The resistor divider network used at the enable pin is making it so that VEN < 2V in some cases where VIN < 28V which might be why the output isn't reaching the desired value in some of the boards. Would it be possible to swap R3 and R7 and run the test again? 

    Regards,

    Andres

  • in reply to Andres Sagredo
    TI__Expert 3020 points

    Hi Andres,

    The test shows swapping R3 and R7 doesn't solve this issue.

    About the enable setup, even we don't change swap R3 & R7, customer found that TPS7A47-Q1 could startup at Vin = 22V Iout = 20mA (use external DC power source). But the output is ~21.9V. the LDO works for seconds and then stop working. 

    If Vin = 22V, voltage @EN = 1.69V, then why TPS7A47-Q1 can startup? because EN needs at least 2V. Are you also able to test again in the EVM if you have it?

    thanks a lot.

    Best regards,

    Yang

  • in reply to Yang Wu
    TI__Expert 4540 points

    Hi Yang,

    Thank you for testing by swapping the resistors. The VEN minimum value in the recommended operating conditions table is the voltage in which the LDO will start up in normal conditions.

    I currently do not have this EVM, however I can try to get one ASAP to test this LDO and conditions. 

    Also, in the test did the customer tie the two output pins together?

    Regards,

    Andres

  • in reply to Andres Sagredo
    TI__Expert 3020 points

    Hi Andres,

    thank your for the reply. The test tied the two output pins and and found no difference.

    Customer did another test (power on and off cycling) at Vin = 22V Iout = 20mA (use external DC power source). waveform as below.

    please let us know if there having more points of view. thanks.

    Best regards,

    Yang

  • in reply to Yang Wu
    TI__Expert 4540 points

    Hi Yang,

    I got some questions regarding both the test and the schematic:

    What is the purpose of R4? Have you tried testing without it?

    For what purpose is there a resistor divider on EN pin? Have you tried to simply connect Vin directly to EN without any resistance? There is an internal pull-down resistor on the enable circuit.

    During testing are you using a resistive load? 

    Could you also show how the waveforms look on the EN pin? Also, during this test of 2nd power on, have you tried waiting for Vin to fully power off before powering the LDO back on? Is the ramp-up rate of Vin the same in the 1st power on and the 2nd power on?

    Regards,

    Andres