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TPS43061 / WEBENCH RECOMMENDATIONS FOR OUTPUT CAPS

Ran Feldman
Intellectual 780 points

Hi,

Using the Webench to do the following design using the TPS43601:

1. Vin = 6-9V

2. Vout = 12V

3. Output Current = 3A.

The design comes up with a 47A of MOSFETs and 13*10uF of output capacitors. Is that correct. from the data sheet the output caps are determined by the maximum transient response needed, how does the webench calculate the output caps? what transient response does he determine? Why such heavy current transistors are selected?

 

For another thing, there are 2 reference designs in the data sheet, if the current sense resistor in the input would of changed to a limit of 3.5A, could it be used as a reference design for a 3A load? I am asking this as the FETs in the 1st reference design is a NexFET capable of delivering 20A...of couse transients should be taken into considerations and BW, Loop response etc. but if the current limit would of change can this be optimized to a 3A design?

Both of the reference designs in the data sheet use a Pi-Filter to filter out unwanted noise on the current sense pins, but the webench doesn't support the Inductor feature, it recommends a resistor "star" formation, is it better to go with the data sheet on this case?

Thanks,

Ran

 

  • 0
    TI__Mastermind 35185 points

    Hi Ran,

    The amount of output capacitors was correct for the method WEBENCH used for calculating. Currently it is more conservative than the datasheet but we are going to adjust it to match closer. In the datasheet Equation 23, the dItran was 1A or 1/2 the maximum output current. In the current WEBENCH release the full output current or 2A would be used which doubles the required output capacitance.

    As for the MOSFETs, typically they are not chosen for the current rating but many other factors. The main one is the RDSon to limit the conduction losses to what can be handled by the package. There are also switching losses which are considered. Lastly WEBENCH will also go through the library of MOSFETs to select one to try to optimize size and cost.

    For you question on the reference designs, yes the first design can have the current limit resistors adjusted for a 3.5A load. The compensation may need to be recalculated because when the current sense resistor is changes, this changes the gain.

    I'm not sure what you are referring to for the pi filter but will take a guess. Resistors R15 and R12 are only there for place holders in order to allow the EVMs these designs are based on to support DCR sensing. The components used in WEBENCH are what is needed in reality when using sense resistors.

    Hope this clears it up for you.

    Best Regards,
    Anthony