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Small Inductor in Buck DC/DC Converter

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pancho_hideboo

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See attached figure.
This is a excerption from some reference circuit.

Here series connection of large inductor(10uH) and small inductor(15nH) is used for Buck DC/DC Converter.

I can't understand a purpose of this series connection.

What do you think about this series connection ?
 

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KlausST

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Hi,

I assume this rather shows the unwanted stray or wire/trace inductivity.

Klaus
 

schmitt trigger

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Very likely it is to suppress very high frequency noise.
The larger inductor may have a significantly low SRF which makes it unsuitable to filter that HF noise. Thus a smaller inductor with a much higher SRF takes care of that..

This technique is very similar to the one where a 100uF electrolytic capacitor is in parallel with a 0.1uF ceramic capacitor.
 
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KlausST

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Hi,

maybe.
But I assume the reference circuit and/or it´s description should give an answer.

Klaus
 

pancho_hideboo

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Very likely it is to suppress very high frequency noise.
I don't think so.
15nH is too small.

And if it is for HF noise reduction, small shunt capacitor should be added parallel to 1uF in addition to 15nH.

But I assume the reference circuit and/or it´s description should give an answer.
There is no mention about it.

See Figure 170 at page-546 of the following.
https://infocenter.nordicsemi.com/pdf/nRF52832_PS_v1.4.pdf
 

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You should have mentioned that it's for an internal boost converter of a Bluetooth transceiver. The 10 uH boost inductor will effectively short GHz noise by it's parallel capacitance. Depending on the noise level present at the boost converter DCC node, one may want to use additional suppression means. Personally, I would prefer a GHz ferrite bead in this place, but the 15 nH inductor is at least an option.

At the end of the day, a designer has to choose bypass capacitors and filter inductors on his own responsibility and based on EMC measurements.
 

pancho_hideboo

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You should have mentioned that it's for an internal boost converter of a Bluetooth transceiver.
No, this is buck.

There is no such small inductor in reference circuit of similar product of other vendor, e.g. dialog semiconductor.

So I don't think purpose of this small inductor is for noise reduction.

Swicthing rate is variable in this DC/DC converter.
I suspect that this small inductor is for an operation at high switching rate(64MHz ?) with small load current.
High speed startup of DC/DC ?
 
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FvM

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I suspect that this small inductor is for an operation at high switching rate(64MHz ?) with small load current.
High speed startup of DC/DC ?
Surely not. Not possible in a series circuit.

It would be an error of reasoning to conclude from the application circuit that it's a necessary part.
 

asdf44

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The BOM in your link calls it “High frequency chip inductor”. Seems likely that it’s answered: high frequency noise suppression.
 

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