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TPS2331: About input capacitor

Part Number: TPS2231

Hi,

Let me talk about the TPS2231 recommended input capacitor.
The application notes have the following description.
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''The ExpressCard Standard limits the maximum capacitance allowed by the host at each of the output
voltage rails to 20 μF. The amount of output capacitance on the host is implementation specific;
however,it is recommended that a 0.1-μF capacitor in parallel with at least a 4.7-μF capacitor be placed on each of
the voltage rails The input voltage rails to the TPS2231 ExpressCard power switch/controller should also have some type
of filtering close to the input voltage pins of the TPS2231 ExpressCard power switch/controller. Again, a
0.1-μF capacitor in parallel with at least a 4.7-μF capacitor is recommended at the input of each voltage
rail. This is especially important if long trace runs and/or long wiring exists between the bulk capacitance
at the output of the voltage source and the input to the TPS2231 ExpressCard power switch/controller.''
(http://www.ti.com/lit/an/slva193/slva193.pdf)
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In this document, capacity limiting of the output capacitor is described.
However,There was no description about the input capacitor.
Customer wants advice on the capacity limit value of the input capacitor.
Please tell me the recommended maximum input capacitor.

Best Regards,
Yusuke/Japan Disty

  • Yusuke,

    We recommend the  0.1uF combined with 4.7uF minimum on the input.  The 0.1uF is for HF transients (high SRF) and 4.7uF for medium to high freq portion of the transient that would be caused by high input inductance during a current interruption.  If you can, increase Cin above 4.7uF with low esr capacitance.  I think the only limitation of Cin is what the downstream system can see through the internal switches, but it would generally see the large bulk capacitance (much larger than we are talking about here) regardless so it should not be a concern.  The more capacitance the better the input voltage suppression due to input inductance.

    Brian