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TPS61090: Regarding the input inductor and output capacitor values and configurations.

Daniel Graham69
Prodigy 105 points
Part Number: TPS61090


I had a couple questions regarding a TI buck/boost converter. I recently had a couple PCB panels made and I’ve run into a bit of an issue with sourcing the right sized parts. I made the mistake of assuming that I would be able to find all of my components in a 0201 (imperial) package size and I’m unable to find two inductors and a capacitor. The design I’ve implemented uses a TI TPS61090 “Synchronous Boost Converter” to regulate my voltage for my system. Here are my questions:

1. When I do the calculations for the L1 inductor (assuming Vout is 3.3V, Vbat is 2.75V, and a maximum output current of 500mA), I get a value of ~5uH. The largest inductance I can find in a 0201 package size is ~270nH. My understanding is that the input inductor helps minimize “ringing.” What would be the ramification of using an inductor that much lower than the calculated value? Why does the required inductance increase as the maximum potential current decreases? Would it be more ideal to go this route, or to solder super small wires onto the board and have a larger inductor (that won’t fit on my pads) floating in free space?

2. My second question is essentially the same but for the output capacitor. With my calculations (same parameters and ripple of 10mV) I get a value of ~14uF. The largest capacitor I can find in a 0201 package size is ~10uF. However, the note suggests that the capacitor I use should be almost double the calculation due to load transients. Also, it appears the capacitor should be polarized. How necessary is it for the capacitor to match? What will the effect be if it is off by a bit? Does it have to be polarized or can it be unpolarized (unpolarized would be ideal)?

Thanks.

  • 0
    TI__Mastermind 46905 points
    hello
    it seem the solution size is critical for your application,because you want to use 0201 components. the TPS61090 is not good solution because of its package and frequency.
    please share the input/output condition, I'll suggest other small size solution.
  • 0 in reply to Jasper Li
    Prodigy 105 points

    Hello Jasper, I appreciate the response.  At this point it's not really a matter of whether it's the best package or not.  The PCBs have already been produced and the landing pad/circuit was designed around the TPS61090, so it's more about getting what I have to work. Even if it's crude I can use slightly larger components and connect them slightly above the board with wires to the 0201 pads.

    I'm just trying to figure out the effects of adjusting the inductor / capacitor as stated in my original question. I've designed the circuit to allow output upwards of 1000mA at 3.3V. Realistically I can tone that back to 500mA at 3.3V. The input is a LiPo with it's own charging circuit.

  • 0 in reply to Daniel Graham69
    TI__Mastermind 46905 points
    1. "My understanding is that the input inductor helps minimize ringing." not, the inductor is a critical components for a PWM switching boost converter to implement voltage step up function. the value of the inductor should be within the recommended range. otherwise we can't guarantee the performance. too small inductance could even cause unexpected problem.
    2. the COUT mainly has two function, smooth the switching ripple and stabilize the converter. small COUT result in large ripple and device unstable.
    3. the voltage of a lipo battery normally range from 3V to 4.2V. the output voltage is 3.3V in your application. you should select a buck converter or a buck-boost converter. the TPS61090 doesn't support VIN > VOUT operation.