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TPS61021EVM-723: Jittering and instability when output is plugged in MCU (SimpleLink™ Ethernet MSP432E401Y MCU)

Christopher Bouillon
Prodigy 40 points
Part Number: TPS61021EVM-723

Hi everyone,

I'm currently working on a design with this chip. The input is 3.3V and the output is typically 50mA (current driven in MCU). SuperCap is also plugged in the input so when the power is shut down it survives for an additional 20s. 

I am having issue with the fact that when said output of the TPS61021 is going in the MCU there is an instability and jittering, while when I'm plugging a passive LOAD (65 ohms so the same 50mA is driven by the chip) there is absolutely no jittering and everything is working fine.

Actually wondering if there is any known solution for this issue

First picture : output on channel 2 with passive Load (65 ohms)

2nd picture : output on channel 2 with MCU as output

  • 0
    TI__Mastermind 40080 points

    Dear Sir,

    The key reason I think is because : the load current is a constant value with  65 ohm load while the current always changes with a MCU load.

    With the MCU load, how much capacitors used at the output side?

  • 0 in reply to Helen chen
    Prodigy 40 points

    Hello, thanks for your quick answer.

    Well the EVM schematic says that there is 2x22uF capacitors on the output so for now I'm only using those 2 on the output side :

  • 0 in reply to Christopher Bouillon
    TI__Mastermind 40080 points

    Dear Sir,

    Since the MCU current is dynamically changed, you need to ad more caps at the output side. besides the 2X22uF ceramic cap, you can add one 100-220uF EL cap at the output side to check whether it could be helpful.

  • 0 in reply to Helen chen
    Prodigy 40 points

    Thanks again for your time,

    I've plugged both a 100uF and 220uF EL at the output. It filtered a bit of the jitter, but not all of it. I thought of adding a voltage divider at the enable pin, as you can see on the joined picture below it seems to be in the voltage range between the logic HIGH and LOW indicated in the specsheet (0.36V-0.84V) when Vin is over 1.2V Only problem with this voltage divider on EN pin is that it will reduce the lifetime of the output HIGH, henceforth removing the reason why I need a SUPERCAP to increase the chip output HIGH time when Vin is turned off.

    Do you have any solutions?

    Ch1 = source

    Ch2 = TPS61021 evm output

    Ch3 = SUPERCAP voltage 

    Thanks again,

    Chris

  • 0 in reply to Christopher Bouillon
    TI__Mastermind 40080 points

    Dear Sir,

    After you add a bigger cap at the output side, if the load is a changed dynamic load, then the extra big cap can help to make the output voltage waveform change smoothly, or the Vout waveform will ringing seriously .  So a little  bit jitter is normal.

    For the super cap, I don't know what your mean? do u want to remove it from the input side?

  • 0 in reply to Helen chen
    Prodigy 40 points

    Hello,

    No, the SuperCap on the input side needs to stay there, to feed the TPS61021 for an aditionnal time after Vin is turned off. I was planning on putting a voltage divider on the enable pin, because I thought that the unstability of the TPS61021 output was in part due to the fact that when you look at above picture, channel 3 seems to be in the range shown below, hence inducing a sort of uncertainty on which state to output : 

    Thank you very much once again for your time, it's really appreciated

  • 0 in reply to Christopher Bouillon
    TI__Mastermind 40080 points

    Dear Sir,

    When the Vin is low, the output capability is very weak. and it is more likely to lead to jittering. So turn-off the IC in advance is a good idea.