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TPS62172: Output capacitance limit

Chris Kimble
Intellectual 270 points
Part Number: TPS62172
Other Parts Discussed in Thread: TPS62130, TPS62147

I’m planning to use TPS62172 to supply 3.3 V to a load that will vary between ~100 mA and ~350 mA.  Referring to Table 1 in the app note, Optimizing the TPS62130/40/50/60/70 Output Filter, I see that the converter should be stable with a 10-uH inductor and an output capacitor no larger than 22 uF.  I’d like to use 10 uH to keep the converter in continuous conduction mode.  

With respect to that 22-uF limit, to what extent do I need to consider the distributed capacitance of my board—all of the decoupling caps, as well as the capacitance between power and ground planes?

Also, Table 1 indicates that “effective” capacitance is one-half of the nominal capacitance.  Is that a reference to the voltage coefficient of capacitance of (non-COG) ceramic caps?  

Thanks for any insights you can give me...

  • 0
    TI__Guru**** 150820 points
    Hi Chris,

    To achieve what you want, a CCM buck, have a look at the TPS62147/8. This is a newer buck with a forced PWM mode feature that achieves exactly that.

    Yes, you need to consider all the capacitance on the output. The electrons don't care which page of the schematic it is on--it is all connected to the output ;)

    Yes, effective capacitance for power supplies almost always means the loss from the DC bias effect.