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TPS7A19: Input / Output Capacitor

Part Number: TPS7A19

Hi there,

My customer is evaluating TPS7A19 for 40V/24V => 13V 20mA(RMS) power rail.

How should he calculate the input and output capacitance?

The load device has a 80MHz operation frequency, should this be considered when choosing output capacitor?

Best,

Zeming

  • Hi Zeming,

    I am a little confused by the specifications for the use of TPS7A19. It says that the output for TPS7A19 is expected to be 13V, 20mA(RMS). Why is it specified as RMS current? Is TPS7A19 being used in a floating-ground configuration?

    The datasheet does not offer a way to calculate the input or output capacitors, but it does provide some information to help select the values. The device characterization was performed using Cin = 22uF:

    Some of the other tests were performed using Cin = 10uF:

    So for Cin, using a value in the range 10uF - 22uF will ensure the performance meets the datasheet specifications and will ensure stability.

    For Cout, section 8.2.2 details the required range:

    Zeming Kong said:
    The load device has a 80MHz operation frequency, should this be considered when choosing output capacitor?

    The load device must be a digital device operating at 80MHz, am I correct? If this is the case, this is not a factor in choosing Cout. Using a larger Cout will improve transient responses and keep the rail more stable, which is normally desirable for digital components in general. 

    Best regards,

    Nick

  • Hey Zeming,

    The TPS7A19 is stable with anoutput capacitor of 22uF and 500uF capacitors which have an ESR of 1 mOhm to 20 Ohm.

    As for the input capacitor, while an input cap isn't specifically required it is good analog design practice to have an input capacitor to help filter out excessive noise or transient events which may occur on the input supply. The exact needs will depend on the application since the voltage source and transients on Vin can vary from one application to another.

    As for the load device which is operating at 80MHz, that is much beyond the bandwidth of pretty much all LDOs so it would be good to have a capacitor at the input of the load which can help filter out noise generated at that frequency. For this type of filtering they will need the impedance of their capacitor to be low at this frequency, which would typically mean using ~10nF capacitor but they will need to evaluate what capacitors they have access to and what the impedance looks like at 80MHz.

  • Hi Nick and Kyle,

    Thanks a lot for your reply.

    I have suggested the customer to choose capacitors according to datasheet.

    Best,

    Zeming