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TPS61099: Output unstable after grounding the FB pin

Nicholas Carley
Genius 10125 points
Part Number: TPS61099


Hello,

My customer was using the TPS610994 with the FB floating on their design, they have had it like this for a couple years. After doing a review it was recommended (based on the datasheet) that they ground the FB pin to reduce noise caused by a floating pin. This E2E post calls out that it will cause additional noise, so the recommendation is to connected it to GND.

After connecting this pin to ground the customer starting seeing instability on the output rail as shown below:

This instability is being passed on to some downstream sensors which is then affecting the sensor output reading. 

Have we seen this type of issue before? Why would grounding the FB pin result in this issue?

Thanks,
Nick

  • 0
    TI__Mastermind 21565 points

    Hi Nick:

    I suspect what customer observed is caused by Enter/Exit Burst mode(PFM), not the traditional loop response.  

    TPS610994 has Burst mode to save the power when the load is light. If the converter enter into Burst Mode, the Vout will be higher than normal operation. If you check the EC table, you can find the Vout during Burst mode is 100mV higher typically.

  • 0 in reply to Minqiu Xie
    TI__Genius 10125 points

    Hi Minqiu,

    Thank you for the response! This aligns with some other scope shots we have. 

    At what point (output current) does the device switch from PFM to PWM mode? Is there an equation to estimate this?

    From the datasheet efficiency curves, there seems to be a consistent change around 0.2mA, is this the change from PFM to PWM modes?

    Thanks,
    Nick

  • 0 in reply to Nicholas Carley
    TI__Mastermind 21565 points

    Hi Nick:

    The enter/exit condition is complex to clarify clearly, as it depends on several factors. I would recommend customer do test on EVM for reference.