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About application's circuit of TPS40200

Tsuguhiko Asai
Expert 1260 points

1.
What is the R1 resistance (10 ohms) placed in Vdd-input-pin in data sheet Figure47 used for?

Is R1 a current limiting resistor to protect Vdd-input-pin?

In addition, please teach us the recommended range of the resistance value.

min?<10ohms<max?

2.
In data sheet Figure27, filter constitution using CF, RF1 and RF2 is introduced,

but, in the case of constitution without RF2, is there the calculating formula to decide a value of CF and RF1?

In addition, please teach us the ranges of the recommendation value of CF and RF1.

Using TI's tool "Switcher Pro", the recommended value of CF1 and RF1 was as follows.

We need to introduce a calculating formula and a recommendation value and a recommended range.

Best Regards,

Tsuguhiko Asai

  • 0
    TI__Expert 4350 points

     Hi Tsuguhiko,

    Thank you for your interest in TI parts. To answer your questions:

    1. What is the R1 resistance (10 ohms) placed in Vdd-input-pin in data sheet Figure47 used for?

    I just looked at the most recent DS on the web, and I think you are referring to R9. If so, this resistor forms a filter with C9. The switch current in Q3 can cause a fair amount of noise on the input bus across C2 and C3, so in order to clean up the voltage at the Vdd pin, this filter is added. The higher the resistor value, the better the filtering, but it must be small enough to avoid dropping too much voltage due to Vdd bias current. 10 ohms is recommended, but you can experiment with values from 5 to 15 ohms.

    2. In data sheet Figure27, filter constitution using CF, RF1 and RF2 is introduced, but, in the case of constitution without RF2, is there the calculating formula to decide a value of CF and RF1?

    The combination of Rf1, Rf2 and Cf form a filter and voltage divider. The source of the voltage is the voltage across Rilim, and this voltage can contain noise and/or ringing due to non-ideal layout and parasitic impedances. In order to clean up this voltage (to avoid OCP tripped by the unwanted noise), the R-C filter in introduced. The value of R is the parallel combination of Rf1 and Rf2, and the C is Cf. The voltage divider is introduced only if the voltage across Rilim is larger than the trip voltage at the desired trip current (depending on the value of Rilim and desired trip current). Normally you would want to reduce the value of Rilim in this case, but if that is not possible due to available values, you can use this divider. The values of Rf1 and Rf2 will depend on how much of a reduction in the voltage you need. A good starting point would be to try with Rf1 value of about 1k, and then adjust Rf2 as needed. A good starting point for the filter would be to set the filter corner frequency at about 10X the Fsw.

    MC.