EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: newtekuser on November 02, 2024, 06:33:32 am
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I'm using the MP8765 switching IC to down step 12V to 5V. According to Ti documentation for calculating the power stage for a buck converter, the formulae for calculating R1 and R2 are:
R2 = VFB / IR1/2, where IR1/2 needs to be 100 times as big as IFB
R1 = R2 ((Vout-VFB) -1)
In the MP8765 data sheet, I have selected VFB to be 600mV (0.6V) and IFB for the same is 10nA (0.00000001A).
If I plug these values into the formula for R1 and R2 I get:
R1 = 440Mohm and R2 = 60Mohm.
Something does not look right to me, the values are huge! What I'm I missing?
https://www.monolithicpower.com/en/documentview/productdocument/index/version/2/document_type/Datasheet/lang/en/sku/MP8765/document_id/777 (https://www.monolithicpower.com/en/documentview/productdocument/index/version/2/document_type/Datasheet/lang/en/sku/MP8765/document_id/777)
https://www.ti.com/lit/an/slva477b/slva477b.pdf?ts=1730500800156 (https://www.ti.com/lit/an/slva477b/slva477b.pdf?ts=1730500800156)
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I_R12 should be at least 100 times bigger than I_FB.
1uA in your case but I would put, if requirements allow, something like 100uA.
For example: 47K + 6K4.
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1) Pick worst case scenario for IFB. For example 50nA for MP8765.
2) That will be your upper limit for RFB2 selection. Using the 100x rule of thumb that will be RFB2(MAX) = 600mV/5uA = 120k
3) Of course you may pick lower resistance than the upper limit (duh)...
4) You may even deliberately pick significantly lower resistance if you have other consideration. e.g. leakage, noise immunity, BOM consolidation, etc. or even just because you don't particularly mind the increased quiescent draw.
5) Through the magic of my proprietary calculation, 220k : 30k sounds about right for 5VOUT
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Thank you all, it's clear now!