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TPS61025: Vin noise

SteveG49822
Prodigy 70 points
Part Number: TPS61025
Other Parts Discussed in Thread: TPS61020EVM-025

I found that the part is causing substantial Vin noise on my implementation at most input voltages. I performed the same set of tests with the TPS61020EVM-025 and found mostly the same results, my implementation is actually a little better, at a Vin of 3.05V, the noise is gone.

This is not really an acceptable condition, any suggestions on how to remove the Vin noise?

Test conditions:
 * Vin as shown with scope pictures.
* Vout is always 3.3V +-1%
* Load is 73.5 ohms, drawing a constant 45mA which simulates actual use load.
* The Vin source is a bench supply. I did try 3 different bench supplies as well as 2 Alkaline batteries. All of these sources showed the same results.

Except for the first picture, all others are using the EVM so the schematic and layout are TI's...

Yellow trace is Vout, Blue is Vin, both are channels are set to 100mV per div ,AC coupled.

My design @ Vin = 3.05

EVM @ Vin = 3.05

EVM @ Vin = 3.22

EVM @ Vin = 2.88

EVM @ Vin = 2.00

Regards

Steve

  • 0
    TI__Mastermind 21565 points

    Hi Steve:

    I think it's caused by PFM mode. Power save mode is used to improve efficiency at light load. In power save mode the converter only operates when the

    output voltage trips below a set threshold voltage. But it will bring the lower frequency ripple. 

    May I know if it's harmful to your system? And have you tried set PS pin to HIGH to disable the power save mode?

  • 0 in reply to Minqiu Xie
    Prodigy 70 points

    Hi Minqiu,

    Vin is also my Vbat used to power an RTC and a small amount of NV RAM. At ~175mv the noise increases minimum battery voltage that can be used by at least 20% which translates to ~35% reduction in battery life. And I'll have to replace our simple battery monitor algorithm with something much more complex.

    I have not recently tried setting PS high, I will try this tomorrow.

    However, if I recall correctly from our initial part evaluation, setting PS high increases our idle power consumption to unacceptable levels. Time to dig out the lab notebook and look for the test results.

    Regards

    Steve

  • 0 in reply to SteveG49822
    TI__Mastermind 21565 points

    Hi Steve:

    If the issue is gone when setting PS high, than we can believe it caused by power save mode. 

    Enlarge the input capacitor with low ESR,ESL is an effective way to reduce the input ripple. And add a resistance or inductance in series with power supply is another way, but it will bring additional power consumption.

  • 0 in reply to Minqiu Xie
    Prodigy 70 points

    Minqiu,

    With PS high, the noise goes away. Efficiency also goes away, dropping below 50% at output current < 10 mA.

    A high value, low ESR cap reduces the Vin ripple by less than 20%.

    Looks like it's a do-over on our power supply. :-(

    Steve