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TPS65270 Inductor Sizing

Charles Poulin
Prodigy 20 points
Other Parts Discussed in Thread: TPS65270

I am using the TPS65270 in my Senior Capstone project to provide 5V and 3.3V from a 12V supply. I am new to SMPS design and the synchronous buck converter. Using an equation I found in a SPMS training module online:

L  = (Vin-Vout)(Vout/Vin)/(Fsw*Iripple)

Where

Vin  = 12V

Vout = 3.3V

Fsw = 400kHz (set with a 680kOhm resistor)

Iripple = 10% (1A Iload) = 0.1A

I calculate a minimum inductor size of approximately L = 60uH. (I am assuming I can use the larger 3.3V output inductor for the 5V output as well to keep part count down)

This is significantly larger than the 4.7uH used in the TPS65270 reference design for the evaluation module (http://www.ti.com/lit/ug/slvu474b/slvu474b.pdf). The datasheet seems to be lacking any kind of range or method of selection for the inductor.

After a bit of research on the TI forums, I found nothing for the TPS65270, but posts for other DC/DC SPMS ICs suggested that the IC would not operate over a large range of inductor values and would be configured for something closer to the 4.7uH in the reference design. Which leaves me with the question: How do I size and spec an inductor for my application? If my 60uH calculation is correct, I have an 82uH Wurth 74435588200 selected. However, if a 4.7uH or similar sized inductor is needed, I will select one with similar specs to the Wurth 744311470 suggested in the reference design.

Any help would be greatly appreciated.

-Charlie

  • 0
    TI__Guru**** 191445 points

    Typically the ripple percentage is 20-30%.  Also your load current is low compared to the typical rating for TPS65270 and you have a relatively low switching frequency.  All those combined will tend to drive your inductor size to a high value.  For your case there are some things you will want to consider.  For  current mode control, the inductor (switch) ripple current is essentially acting as the PWM ramp.  I would probably want to maintain some ripple current.  Are you going to use the pulse skipping function to increase light load efficiency?  The ripple current will set the pulse skipping threshold.  Also for the same package size, a high value inductor will have higher DCR and lower efficiency.   Are you going to use ceramic output capacitors?  In general, they can tolerate higher ripple currents.  For your design , I might look at a 15 uH inductor, but it depends on the above criteria.

  • 0 in reply to JohnTucker
    Prodigy 20 points

    John,

    Thanks for the response. I was not sure how to gauge the ripple current, I assumed a lower value would reduce the voltage ripple as well, hence the 10% assumption. I am very much out of my element, so I will defer to you and the community for an appropriate Iripple percentage.

    I do have the the LOW_P tied to the VCC pin to enable the automatic pulse skipping mode. And as far as average load, the converters primary load is a dsPIC33EP512MU810 on the 3.3V side. I should say this is providing system side power for and isolated DC/DC converter with output voltage and current measurement that is spec'd for "high" efficiency. Essentially it is a 12V to 5V and 3.3V kill-a-watt. The TPS65270 is not providing a ton of power, but I needed any system side power conversion to be more efficient than an LDO. I will say it again for emphasis, I am very much out of my element, so I am not sure about my design decisions.

    I am using 1210 47uF ceramic output capacitors rated to 16V.

    Based on this additional information, does your recommendation of 15uH stand? I assume I should also select a minimal DCR to further reduce losses.

    Thank you for your time,

    -Charlie