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TPSM84205: Capacitor

Ray Vincent Aranzaso
Genius 15215 points
Part Number: TPSM84205

Hi,

Good Day. I have a customer who is working with TPSM84205EAB. Please see below his query for your reference. Thank you very much.

I am trying to change out my DC DC switching converter to this TI part TPSM84205EAB

Unfortunately, I have not changed the capacitors, so I would like to know what would happen if my input capacitors do not meet the minimum requirement of 10uF and I would like to know what would happen if its too low?

I have a switching regulator that's out of stock - that's been built to a specific configurations. The output caps are 47uF, and the input cap is 1uF + 1uF and a .1uF and the TPSM84205EAB has a requirement of 10uF on the input.

I would like to know what would be some issues we get from this switching regulator if we do not meet those Required specs. Would it overheat? Fail? Lose stability on the output?

Best Regards,

Ray Vincent

  • 0
    TI__Mastermind 47020 points

    Hi Ray,

    The min 10uF ceramic input cap is required for TPSM84205 proper operation. The input cap buffers the input voltage for transient events and decouples the converter from supply. With lower input cap, it may cause unexpected shutdown, higher input voltage ripple, lower efficiency and bad EMI( interference with other circuits). Please change 2x1uF to at least 2x4.7uF with higher voltage rating. 

    Thanks,

    Nancy

  • 0 in reply to Nancy Zhang
    TI__Genius 15215 points

    Hi Nancy,

    Good Day. Please see below the response of our expert to your query. Thank you very much.

    So what’s the nominal Voltage of this Switching Regulator? Could that have some affect?

    I understand that this Regulator has range of 7-28V. We are running a 12 volt supply through it with these filter caps.

    I guess you will probably have the same response for this, but is there any information for the effects of the longevity of the component? Will it likely fail in a few months? Days?

    You are saying “May Cause” so its giving me some hope that it won’t fail. I know I would have to change those caps now to meet that spec.

    So far based on our measurements, the component is still outputting around 4.91 V which is nice, but the voltage ripple is greater than our previous regulator, actually even with the appropriate filter capacitors, it is still a great deal of unstable ripple. We tried to use 94uF on the output by stacking caps and replacing one input cap with 10uF before and so no change to the components output other than the Voltage RMs being 5.01V.

    I might be giving you too much information, but I wanted to know what could cause that instability and how can I make it more stable?

    Best Regards,

    Ray Vincent

  • 0 in reply to Ray Vincent Aranzaso
    TI__Mastermind 47020 points

    Hi Ray Vincent,

    Less input cap may not cause the output loop instability and output accuracy. But it does affect input side. For buck converter, the current delivered to the DC/DC converter is an average(DC) current and the current in the switching MOSFET is pulsating. A large capacitor should be placed at the input of converter to average the input current. This input capacitor provides a low-impedance voltage source of the converter and helps to filter the pulsating current. If you use a small input cap, the peak-peak ripple will be larger and may cause the UVLO circuit to inadvertently shudown the converter. Also the large noise spike may cause intermittent operation of converter. The EMI will be bad too. 1uF cap will be even lower capacitance after derating and it is not enough. 1uF cap will be stressful with higher input ripple current and hurt its long term reliability. For a reliable design, the input cap must be at least 10uF.

    Hope this helps,

    Thanks,

    Nancy