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Original question:

TPS74801: TPS74801DRCR equivalent part with low Vdo

TPS74801: TPS74801DRCR equivalent part with low Vdo

Nanjunda M
Expert 2485 points
Part Number: TPS74801

Hi,

W.r.t. my previous post: 

We are using this regulator in one of our board.

We have provided a supply of 5V to VIN and VBIAS pins and the output is 3.3V @ 1.5A

At one point, we are observing a voltage of about 4.5V on the input.

1) whether this fall in voltage (to 4.5V) would affect the IC functionality?

2) In the datasheet, I see that there is recommendation to have a 4.7uF to Vbias pin. In our case, we have 1uF capacitor mounted at that point. Whether this will effect the IC functionality?

We are facing issue with inconsistency in the output voltage(3.3V).

Could you please help.

Thanks & Regards,

Nanjunda M

  • 0
    TI__Guru 55940 points
    Hi Nanjunda,

    Are you using the same supply for both Vin and Vbias?

    If Vbias is dropping below 4.5 V, it is possible that you could be going into dropout on some devices due to Vbias dropout. As you observed in your previous thread, Vbias dropout maximum is 1.6 V.

    If Vin and Vbias are tied together locally at the LDO, you need a 4.7 uF capacitor local to the LDO in order to achieve the performance depicted in the datasheet.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Expert 2485 points

    Hi Ryan,

    Yes, I have shorted the VIN and Vbias pins on the board.

    Whether I can do anything to get a stable output?

    Please let me know.

    Below is the schematics of the regulator design.

    Thanks & Regards,

    Nanjunda M

  • 0 in reply to Nanjunda M
    TI__Guru 55940 points
    Hi Nanjunda,

    In order to meet the minimum requirements specified in the datasheet, C354 and C356 need to sum to be at least 4.7 uF. Furthermore, C338 (Cout) needs to be at least 2.2 uF. Increasing C356 to 4.7 uF and C338 to 2.2 uF will meet the minimum requirements for a stable application; however, your application may require even more capacitance if large transients are expected.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Expert 2485 points
    Hi Ryan,

    Thanks for the above inputs. We will try the suggestions.

    One more doubt, If we leave the BIAS pin floating(keeping all other connections same), will that will solve the issue we are facing?

    Thanks & Regards,
    Nanjunda M
  • 0 in reply to Nanjunda M
    TI__Guru 55940 points
    Hi Nanjunda,

    BIAS should not be floated. In order for TPS748 to operate as designed, BIAS must be tied to a voltage equal or higher to Vout + Vdo(bias). We highly recommend that Vbias be tied to a voltage of Vout + 1.6 V or higher in order to ensure regulation. If BIAS is floated, then your application will be in dropout and other functions such as PG will not operate as designed as they are powered from the BIAS rail.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Expert 2485 points

    Hi Ryan,

    In our case, it will be difficult to provide the input to BIAS pin = Vout+1.6 (i.e. 3.3V+1.6V=4.9V), as we have the voltage Vbias = 4.5V only.

    So, if I float the BIAS pin, the device will go to dropout mode, in there the typical dropout voltage is 1.31 as per the datasheet. But even in that case my device may have issues, as the required Vin = Vout+Vdrop (i.e. 3.3V+1.31V=4.61V).

    Is there any other option to resolve the issue?

    Thanks & Regards,

    Nanjunda M

  • 0 in reply to Nanjunda M
    TI__Guru 55940 points
    Hi Nanjunda,

    If you are not able to supply adequate BIAS voltage to remain out of dropout, unfortunately TPS748 is not a good LDO for your application. Unfortunately we do not have a pin to pin LDO to suggest as a replacement that does not have similar Vbias requirements. As such the only options that I see are to find a way to increase Vbias in the application or choose a different LDO that will require a board spin.

    Very Respectfully,
    Ryan