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TPS22975: Proportion of input cap and output cap

Part Number: TPS22975

Hi Team,

According to the datasheet, following input cap vs output cap proportion is recommended but my customer want to put much amount of cap at CPU side(TPS22975's output side) as close as possible in order to minimize voltage noise due to load transient. Current their design has 88uF as TPS22975's output side and almost same amount at input as well, 1:1 proportion now.

"it is recommended to have an input capacitor about 10 times higher than the output capacitor (CL ) to avoid excessive voltage drop."

I could not understand where's excessive voltage drop will be concerned not to keep this recommended cap proportion, so could you add some more details?
I just thought more output cap should be needed to prevent voltage drop at TPS22975's load, but datasheet says input cap is important. It doesn't make sense for me.
It's nice if you can share any app note describing that. 

Regards,

Takashi Onawa

  • Hello Onawa-san,

    The 10:1 ratio is stated primarily to protect the supply. If your supply can handle the amount of inrush current your load will draw without the supply voltage dipping then there is no need to follow the 10:1 ratio. 

    Regards,

    Kalin Burnside

  • Hi Burnside-san,

    OK, I understood that the intention of the sentence was to avoid UVLO due to the voltage drop on the input side by inrush current during start up timing.

    Sorry, I have one more question regarding Output voltage abs max spec. According to 10.1.3 Output Capacitor (CL) (Optional), it looks TPS22975 might get damage when Vout > Vin condition but "7.1 Absolute Maximum Ratings" indicates TPS22975 can withstand 0.5 to 6V absolute voltage, not 0.5 to Vin + 0.5V. Is this typo?

    "Because of the integrated body diode in the NMOS switch, a CIN greater than CL is highly recommended. A CL greater than CIN can cause VOUT to exceed VIN when the system supply is removed."

    Regards,

    Takashi Onawa

  • Hello Onawa-san,

    If you mean Vout: 0.5 to Vin + 0.5V then no. The diode forward bias voltage only occurs when Vout > Vin which isn't a recommended operating condition.

    Regards,

    Kalin Burnside