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TPS548A20: 5 V operation

VISHNU NAIR
Prodigy 200 points
Part Number: TPS548A20

Hi

I have used TPS548A20 for 1.8V, 10A output in one of my projects. In my initial design with an input voltage of 12V, the circuit works fine, providing continuous current upto 10A. But, while testing the same for a new requirement with an input voltage of 5V, i have observed that the output is oscillating (rather, switching between 0 and 1.8V)when the load current is above 5A and it is repeatable. Though webench suggests to use an inductor of half the value for the new design, I couldnt change it as it not in our inventory. I would like to know the reason for this. I have attached the schematic for reference.1.8V CONVERTER.PDF

  • 0
    TI__Intellectual 1520 points
    Hi Vishnu,
    I believe this is a signal to noise ratio to the FB node that is causing your instability. We typically recommend the inductor ripple current about ~30% of the Icc_max value. In the case of 5VIN at 500kHz, the inductor value can be about 1uH. You can also test this by decreasing the frequency setting to about 300kHz. This should result with a bigger FB ripple.
    Thank you.
    Amnat
  • 0 in reply to Newey Yakamna
    Prodigy 200 points

    Hi Newey

    Thanks for the reply.

    I'll test the circuit again with a lower switching frequency and will post the results.

    In the datasheet page 24, it is mentioned (along with equation 6) that the ripple current, I ind (ripple), should be set at 25-50% of max. output current, so in my design of 10A i should take it as 2.5 - 5A for selecting the inductor, right?

    But, if the design requirement is to keep the ripple voltage less than 5% of the output voltage, (i.e ripple voltage should be less than 90mV), will this be a problem?

    Vishnu

  • 0 in reply to VISHNU NAIR
    TI__Intellectual 1520 points
    Hi Vishnu,
    Yes you are correct. I typically set the inductor ripple to about 1/3 of the max output current. To keep the output ripple less than 5%, will require certain amount of output capacitance which can be calculated based on your output ripple and transient requirement. Please see the equations 8, 9, and 10. The inductor ripple is included in the calculation.
    Thank you.
    Amnat
  • 0 in reply to Newey Yakamna
    Prodigy 200 points

    Hi

    I tried reducing the switching frequency to around 300KHz. But still the output voltage drops at higher load currents. The voltage is designed for 1.7 V, when the load current is increased above 4 Amperes, the voltage drops to 0.7 V.

    Thanks

    Vishnu

  • 0 in reply to VISHNU NAIR
    TI__Intellectual 1520 points

    Hi Vishnu,

    Would you please let me know what you've changed in the schematic for the 1.7VOUT setup? 

    Thank you.

    Amnat

  • 0 in reply to Newey Yakamna
    Prodigy 200 points

    Hi

    I changed R75 to 30K. This was done to reduce the switching frequency.

    Input voltage is 5V, L2 is 2.2uH. (Please refer the schematics attached with my initial query).

    Thanks

    Vishnu

  • 0 in reply to VISHNU NAIR
    TI__Intellectual 1520 points
    Hi Vishnu,
    Please do this if you have not done this yet.
    1. Please change R77 to 18.2K and keep R82 at 10K for 1.7VOUT.
    2. Set the frequency to 500kHz. R76=240K and R75=76.5K. Please see the link below for the frequency selection table.
    www.ti.com/.../sluubc0.pdf
    3. You need to change L2 to at least 1uH. This will result with the inductor ripple of about 25% of Iccmax (10A). With the 2.2uH, the switching frequency cannot go low enough to increase the inductor ripple of at least 25% of Iccmax.
    4. Are you testing this setup on the TI evaluation board?

    Thanks.

    Amnat
  • 0 in reply to Newey Yakamna
    Prodigy 200 points

    Hi

    Thanks for the suggestions. I will change the values and post the result.

    4. No, I'm testing the circuit which i have implemented in one of our projects (having 12V input) for the new requirement of 5V input.

    Thanks

    Vishnu

  • 0 in reply to VISHNU NAIR
    TI__Intellectual 1520 points
    Hi Vishnu,
    Ok. Sounds good.
    Thanks.
    Amnat