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Switch Current calculations for PMIC topologies

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smoothVTer:
Hello All,

Searching on Digikey for DC/DC converters, one often encounters the Current Output parameter listed as (Switch Current) and not just the max current output.

Why is this so?

Also:   for buck, boost, and buck-boost converters, how does one convert from the "Switch Current Limit" to the implemented real-world DC current output capability? 

Is there a resource anywhere that derives this Switch Current Limit for all topologies in one place? It seems that different manufacturers all have a different approach to handling this parameter in their datasheets & the calculations involved, so I am rather confused as to what the base-line theory behind this "switch current" or "peak current" parameter is.


David Hess:

--- Quote from: smoothVTer on March 12, 2019, 07:42:44 pm ---Searching on Digikey for DC/DC converters, one often encounters the Current Output parameter listed as (Switch Current) and not just the max current output.

Why is this so?

Also:   for buck, boost, and buck-boost converters, how does one convert from the "Switch Current Limit" to the implemented real-world DC current output capability?
--- End quote ---

The switching topology has a major effect on output current.  For instance a boosted buck converter using a tapped inductor can multiply the switch current by several times.


--- Quote ---Is there a resource anywhere that derives this Switch Current Limit for all topologies in one place? It seems that different manufacturers all have a different approach to handling this parameter in their datasheets & the calculations involved, so I am rather confused as to what the base-line theory behind this "switch current" or "peak current" parameter is.
--- End quote ---

Linear Technology published application notes which discuss all of the characteristics of different switching topologies.

T3sl4co1l:
Because peak current mode controllers are very popular (and with good reason!), and performance is therefore tied to switch current.

It would be disingenuous to rate these by, say, input or output (DC) current, because that current varies with condition.  For example, a boost/flyback/SEPIC delivers (almost proportionally) more output current at higher input voltage.

Tim

smoothVTer:
I see.

After reading Linear Tech's note and also this series of notes from TI ...

http://www.ti.com/lit/an/slva477b/slva477b.pdf
http://www.ti.com/lit/an/slva372c/slva372c.pdf
http://www.ti.com/lit/an/slva535b/slva535b.pdf


... I think the gist of what I am getting is ( feel free to refute/comment ) ...


>> You can't predict the max DC output current of any PMIC based solely on the switch current limit in the datasheet. You have to take into account your Vin/Vout, min/max duty cycle, and switching frequency and inductor values.

>> You have to run through all the ideal calculations/equations first to determine your ideal duty cycle, fsw, L value, then come up with a ballpark figure of your max switch current.   Then, the values in a parametric Digikey search for "Switch Current" can be useful.

 I am utilizing DC/DC converters in very tight, space-limited systems so I always look for a synchronous rectifier and integrated, internal switch.   Might be why I am seeing such limitations in switch current.   However I found a very nice regulator, the LT1935, which claims an "unprecedented 2A, 40V internal switch"    for one of my applications.     Why is it so difficult to manufacture a 2A/40V switch when there are many many mosfets that can do that easily as discrete transistors?  Also why are NPN switches used in a lot of LT's converters?   Wouldn't it make more sense to use MOSFETs due to the lack of Vcesat? 

Thanks for all the replies, everyone.




T3sl4co1l:
Might've been "unprecedented" at some point... MC34063 is still disturbingly popular, and was probably the only thing that did an ampere (by itself) for a long time?  No idea.

Datasheets rarely carry dates, so it's hard to say what the timeline is when that's what you're looking at most of the day... :-\

At any rate, 2-3A internal switch (regulator) devices are pretty common nowadays, even in very small (SOT-23-6, DFN/QFN..) sizes.

I suspect LT preferred BJTs partly because of their captive foundry capability, and partly for performance, at least at low supply voltages (good luck making a 40V MOS switch that runs on 2 or 3V).

Vce(sat) isn't a constant, it's largely resistive too.  In fact there are low-Vce(sat) and audio muting type BJTs with specs any FET could only dream about.  A jellybean that's 2A, 80V, 20mohm and only 50pF?  You bet! :)

Tim

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