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TPS2041C, TPS2051C - output capacitance

Hirotaka Matsumoto
Guru 27585 points
Other Parts Discussed in Thread: TPS2051C, TPS2041C

Hi all,

Would you mind if we ask TPS2041C, TPS2051C?

<Q1>
The datasheet shows "Additionally, bypassing the output with a 0.01-μF to 0.1-μF ceramic capacitor improves the immunity of the device to short-circuit transients."
What does "improves the immunity of the device to short-circuit transients" mean?
Does it mean a measure of transients using capacitance?

<Q2>
When our customer use this device as USB standard applications, is it required to definitely use 150uF for output capacitance?
Our customer wonder whether it is possible to use 10uF or 47uF capacitance for output capacitance or not.
So in this case, the condition of the Cout  is 150µF about  several test result on the datasheet.
If it used 10uF or 47uF, will the undershoot be bigger?
And  the following  is not out of range, is it?
"The voltage undershoot should be controlled to less than 1.5 V for 10 µs."

Kind regards,

Hirotaka Matsumoto

  • 0
    Guru 27585 points

    Hi team

    We guess that your team is so busy, but could youlet us know it?

    Kind regards,

    Hirotaka Matsumoto

  • 0
    TI__Mastermind 42795 points

    Hello Hirotaka-san

    For Q1, I am looking in the TPS20xxC, TPS20xxC-2 datasheet (SLVSAU6G –JUNE 2011–REVISED JULY 2013) and cannot find the wording "Additionally, bypassing the output with a 0.01-μF to 0.1-μF ceramic capacitor improves the immunity of the device to short-circuit transients." This wording might be from another datasheet, but it basically means that this output capacitor can provide a short burst of current to a shorted load (or noisy load with short bursts of high current) and help prevent the power switch from reacting too quickly. For longer load current bursts, the larger value of capacitance can help sustain the longer pulse (the 0.1uF can only handle shorter duration bursts) and help prevent the power switch from reacting too quickly and shutting off.

    For Q2, the USB standard does specifiy a 120uF for the downstream facing port of a hub.

    The hub downstream facing port VBUS power lines must be bypassed (CHPB) with no less than 120 uF of low-ESR capacitance per hub. Standard bypass methods should be used to minimize inductance and resistance between the bypass capacitors and the connectors to reduce droop. The bypass capacitors themselves should have a low dissipation factor to allow decoupling at higher frequencies.

    It is really about keeping the port voltage from drooping too much (< 330mV) during USB device start up and loading and the hub capacitance helps meet this requirement.

    Regarding the datasheet test waveforms, many of the charts used 150uF as the output capacitor. Generally speaking, the waveforms will change if using a smaller value of capacitor. If a specific waveform/figure is needed, it can be evaluated on the EVM or by using the PSPICE model.

    And  the following  is not out of range, is it? "The voltage undershoot should be controlled to less than 1.5 V for 10 µs." According to the datasheet:

    However, if the application does not require 120 μF of capacitance, and there is potential to drive the output negative, a minimum of 10 μF ceramic capacitance on the output is recommended. The voltage undershoot should be controlled to less than 1.5 V for 10 μs.

    So, 10uF will work in many situations but the undershoot should be evaluated with the expected USB cable lengths to ensure that they are controlled or clamped.