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TPS7A47: TPS7A4701RGWR reliability problem on a production batch

Simon Tinguely
Prodigy 230 points
Part Number: TPS7A47

We have a product in production using TPS7A4701RGWR with 24V input, and configured with feedback resistors to have a 24V output. The device is mainly used as a protection in case of short circuit in the output. The enable pin is used to activate/deactivate the output.

On our first production batch, we have several boards that are not working as expected: when the enable pin is high, the output voltage is 24V as expected, but as soon as a load is connected (even low, for exemple 100 Ohm), the output voltage drops to almost 0V. When the load is removed, the voltage goes back to 24V.

The reference design has been implemented. The layout guidelines have been followed. Input capacitor is a ceramic 50V 10uF X7R. Output capacitors are a ceramic 50V 10uF X7R + an electrolytic 10uF 50V capacitor.

I suspect that the TPS7A4701RGWR has been damaged by ESD at some point during production/assembly/packing/unpacking. Would you expect ESD to produce such kind of issues?

Would you see any other reasons that could cause damage on the device and such behaviour?

Simon

  • 0
    TI__Genius 12060 points

    Hello Simon,

                        When the part (TPS7A47) is loaded, a certain amount of dropout (Vin - Vout differential) is required for regulation to be maintained. The dropout required depends on the load current and the on-resistance of the pass FET, RDS ON. For example, the datasheet states that the maximum dropout at Iout of 1A is 450mV. This implies that the maximum value of RDS ON is 0.45Ω. As a result, for a 100Ω load (Iout = 240mA, as Vout = 24V), the minimum required Vin-Vout differential is ~110mV (0.24A*0.45Ω). With Vin = 24V and Vout = 24V, you are not operating with the necessary dropout to support a load. With Vin = 24.5V, you should the device functioning as expected (if Iout < 1A), providing Vout = 24V. 

    Regards,

    Srikanth  

  • 0
    Prodigy 230 points

    Hello Srikanth,

    Thanks for the reply, but I'm aware of the way a LDO works and I know I cannot have 24V output with 24V input when I load the device. But that's not the problem I have. I expect that the output will be around 23.5V instead of 24V with a load, but here the output is dropping to 0V.

    Best regards,

    Simon

  • 0 in reply to Simon Tinguely
    TI__Genius 12060 points

    Hi Simon, 

                   I agree that it is a bit strange for the output to drop to 0V, when Vin = 24V. Could you provide a scope shot of the input and enable pin when the load is applied? Thanks!

    Regards,

    Srikanth

  • 0
    Prodigy 230 points

    Hi Srikanth,

    Please see the attached scope shots, where:

    yellow line is input

    blue line is output

    red line is enable pin

    I saved:

    1) start of the LDO when enable pin goes high

    2) behaviour when the load is connected

    3) zoom on enable pin when the load is connected

    Hope it helps,

    Simon

  • 0 in reply to Simon Tinguely
    TI__Genius 12060 points

    Hi Simon,

                   Is the load being connected an E-load or is it a resistive load? Is it possible to increase the Enable Pin voltage by another Volt (to ~4.5V) and verify if the issue still occurs? Its a long shot, but the spike in the enable pin could be turning off the device and a current source load is perhaps preventing recovery of the LDO. Could you share the schematic showing the LDO connections? Thanks!

    Regards,

    Srikanth

  • 0 in reply to Srikanth Lolla Venkata
    Prodigy 230 points

    Hi, it's a resistive load. When the output drops, the current goes down to zero. Very probably the LDO has been damaged. I'm suspecting ESD.

  • 0 in reply to Simon Tinguely
    TI__Genius 12060 points

    Hi Simon,

                   I would agree that the device is not exhibiting expected behavior. Whether or not it is an ESD damage, would require an FA analysis. Could you provide me a schematic showing the LDO connections? Is it possible to verify if the Vout goes to zero when loaded and with Vin = 24.5V? Thanks! 

    Regards,

    Srikanth