Author Topic: Choose an inductor and capacitor for AOz3103di buck converter  (Read 654 times)

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nour

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Choose an inductor and capacitor for AOz3103di buck converter
« on: July 27, 2017, 03:25:19 pm »
I want to build a Fixed 5v buck converter with 2.5 Amps output from a 12v input, I will be using AOZ3103DI.
I want to select the inductor to be able to handle the output current with minimal output ripple, I have tried to read the datasheet but I am a little bit lost.

http://www.aosmd.com/res/data_sheets/aoz3103di.pdf
if what I have wrote doesn't make sense for you or you think there is something wrong, please correct me, I am still beginner and what I know probably less than what you know

blueskull

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Re: Choose an inductor and capacitor for AOz3103di buck converter
« Reply #1 on: July 27, 2017, 03:56:59 pm »
Let Iout=2.5A, let ripple percentage be 30%, Iripple=0.75A.
For Vin=12V, Vout=5V, CCM mode, Vs product=(12V-5V)*5V/12V*1/500kHz=5.83usV, Iripple=Vs/2/L=0.75A-->L=3.89uH.
I would use a 4.7uH or 6.8uH inductor.

Keep in mind that Iripple percentage will increase as output current decreases, but it will be okay. I don't want to dive too much into DCM, simulated diode mode and light load mode, but basically in full load CCM you get the highest ripple current, and that dictates output ripple voltage.

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nour

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Re: Choose an inductor and capacitor for AOz3103di buck converter
« Reply #2 on: July 27, 2017, 05:59:29 pm »
I am expecting the load to range from 1-2.5 amp
if what I have wrote doesn't make sense for you or you think there is something wrong, please correct me, I am still beginner and what I know probably less than what you know

blueskull

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Re: Choose an inductor and capacitor for AOz3103di buck converter
« Reply #3 on: July 27, 2017, 06:32:44 pm »
I am expecting the load to range from 1-2.5 amp

Ripple current percentage is never a goal. The goal is ripple voltage and loss. You can design for 2.5A even if you only run it at open circuit.

Beware that your inductor must be able to handle 2.5A+30%, which is 3.25A. I would choose an inductor with saturation current higher than 4A.

nour

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Re: Choose an inductor and capacitor for AOz3103di buck converter
« Reply #4 on: July 27, 2017, 07:10:42 pm »
Ok that is very good, what about the capacitor, the CC and the CBST

I don't know why don't they give values for the typical application, at least recommend one or two general outputs like 5v or 3.3v?
Thanks
if what I have wrote doesn't make sense for you or you think there is something wrong, please correct me, I am still beginner and what I know probably less than what you know

blueskull

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Re: Choose an inductor and capacitor for AOz3103di buck converter
« Reply #5 on: July 27, 2017, 07:21:44 pm »
Ok that is very good, what about the capacitor, the CC and the CBST

I don't know why don't they give values for the typical application, at least recommend one or two general outputs like 5v or 3.3v?
Thanks

Cbst provides energy for bootstrapped high side gate driver, in this case, pick 0.1uF~1uF, and Rbst can be 1Ohm~22Ohms. The lower the Rbst, the higher the efficiency (not sensitive, maybe practically no difference), but the more EMI it generates. When EMI doesn't matter,Rbst can be zero.

Cc and Rc are calculated based on control loop characteristics. Here, assuming minimum load is 5V 1A, output ceramic capacitor is 22uF plus any sensible capacitance of electrolytic capacitor (due to the high ESR, it doesn't affect control loop that much, can be ignored), Cc can be 10nF, and Rc can be 10k.

At 3.3V, the Rc should be increased to 15k, while Cc can be decreased to 6.8nF.

You can always increase Cc to "tame" the control loop to gain better stability at the cost of slower response time.

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nour

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Re: Choose an inductor and capacitor for AOz3103di buck converter
« Reply #6 on: July 27, 2017, 07:38:25 pm »
Thanks a lot, I think for the Cout I will go for low ESR 100 or 220uF tantalum cap for 2.5A
if what I have wrote doesn't make sense for you or you think there is something wrong, please correct me, I am still beginner and what I know probably less than what you know

Smf