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TPS40305EVM-488: Modifying the EVM for different Vin and Vout

Part Number: TPS40305EVM-488
Other Parts Discussed in Thread: TPS40305,

I need to step down a 2S IMR battery to 5V, with a max load of ~2.5A. Battery life and PCB area requirements are both challenging. Webench suggested the TPS40305 and I’d like to evaluate it using the TPS40305EVM-488. However,

1. The EVM’s data sheet says repeatedly that the input voltage range is 8V – 14V. Why, when the input voltage range for the TPS40305 is 3V – 20V? What am I missing?

2. Changing the feedback resistors to give 5V output is trivial, but do you have a utility for calculating compensation values?

3. Any other suggestions?



    The EVM's User's Guide repeatedly references the designed input range of 8V to 14V because that is the input supply voltage that the design was made to support.  The recommended minimum input voltage of 8V limits the inductor ripple current and input capacitor RMS ripple current as the input voltage decreases and the duty cycle increases.

    In addition to changing the feedback divider and compensation, you may want to consider:

    1) Changing the inductor value to better accommodate the updated duty cycle and load current.  The 320nH inductor has 3.6A pk-pk ripple current at 8V to 1.8V @ 1.2MHz.   For 8V to 5V @ 1.2MHz, the peak to peak ripple current will be about 4.9A, extremely high for a 2.5A load current.  Typically designs limit peak to peak ripple current to about 30% of full load, or 0.75A for a 2.5A load.  That would be a 2uH inductor.

    2) Changing output capacitors. While the 6.3V rated 22uF ceramic output capacitors will work at 5V output, the DC bias effect on their output capacitance will be sever, and they will likely have less than 10uF effective capacitance at 5V output.  I would recommend selecting 10V or 16v rated output capacitors instead.  WIth a 2uH inductor, 2x 10uF or 2x 22uF capacitors should meet ripple expectations with the 1.2MHz switching frequency, but additional output capacitance may be needed for transient response, depending on the drop-out and overshoot requirements.

    3) The TPS40305 has a maximum duty cycle of 85%, when the 2S IMR battery discharges below 5.9V, the output voltage is likely to being dropping due to the maximum current.  If VIN drops too low to support VOUT > UVP (4.6V), the TPS40305 will declare output under voltage and shut-down.  If the TPS40305 remains enabled, the output will repeatedly hiccup, attempting to restart periodically and then shutting down again when VIN can not support the 5V output.

    There is likely no need to change the input capacitors.  WIth the lower current load of 2.5V, the input RMS current will be lower even with the increased duty-cycle.

    For compensation design, there is a compensation design Excel tool in the TPS40305's product folder under the Design and Development target -