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TPS 2557 Output Capacitor

JoFu10
Intellectual 2385 points
Other Parts Discussed in Thread: TPS2557

Hi,

I am looking at the TPS2557 datasheet for a customer and he had some questions regarding this paragraph regarding input and output capacitance on the datasheet: 

Input and output capacitance improves the performance of the device; the actual capacitance should be optimized for the particular application. For all applications, a 0.1μF or greater ceramic bypass capacitor between IN and GND is recommended as close to the device as possible for local noise decoupling. This precaution reduces ringing on the input due to power-supply transients. Additional input capacitance may be needed on the input to reduce voltage overshoot from exceeding the absolute-maximum voltage of the device during heavy transient conditions. This is especially important during bench testing when long, inductive cables are used to connect the evaluation board to the bench power supply. Output capacitance is not required, but placing a high-value electrolytic capacitor on the output pin is recommended when large transient currents are expected on the output.

Specifically, his questions are: 

  1. What is a high-value capacitor? (please specify Micro-Farads)

I looked on EVM and saw 150uF as the output cap value, can you confirm?

  1. Why do we need an Electrolytic type Capacitor here?
  2. What are the mechanics behind using a Capacitor at the output (besides the “when large transient currents are expected on the output”)
  3. Does this Capacitor(s) need to be physically close to the switch? or
  4. Can this Capacitor be on the other side of 16”-18” cable where the input to the Tablet is located? 

Thanks for your feedback,

Josh

  • 0
    TI__Mastermind 42795 points

    Here is another post for your reference:

    https://e2e.ti.com/support/power_management/power_interface/f/204/t/439243

    Most is general good EE practice (decoupling and such).

    You can also find a lot of the "why" in the USB.ORG documentation especially the why about the output capacitors.

    I would suggest getting an EVM and then experimenting with various values of capacitor and also type. For example, you could experiment with placement (where the cap is placed with respect to the load or the switch) and view the voltage droop waveforms under transient conditions. This should help shed light on some of your questions.

    You will find that much of the system level performance is dependent on the system parasitics including the capacitor parasitics such as ESL, ESR, cable resistance, etc. In part, that is why no definitive answer can be provided about which capacitor type and value (input and output) should be used and why only general guidelines are provided in the datasheet.