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High side current sense signal referred to low side.

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treez

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Hello,

In the attached 6W Buckboost LED driver schematic, the high-side current sense is referred down to ground via the common base connected PNP (FMMT560Q).

Can you confirm, in this schematic, that collector and emitter currents are always likely to remain close in magnitude? (-even at high temperature and low Ic, where the datasheet does not discuss)

FMMT560Q datasheet
https://www.diodes.com/assets/Datasheets/FMMT560Q.pdf

LTspice simulation also attached
 

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  • 6W Buckboost LED driver.pdf
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  • 6W Buckboost LED driver.txt
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Collector current should always be less than emitter
current (99% of, at beta(OP)=100).

There are ICs for this now that may do a better job
accuracy-wise and common-mode-range-wise. But
LED lighting seems to be in the "OK, now start shaving
femtopennies and don't stop" phase of product
maturity so maybe this isn't on the table.

Curious why the high side sits on top of an 18V source
rather than making the string ground referred and losing
all the high side current sense complexity (or much of it).
Seems like a properly scaled sense resistor and right into
COMP (why not FB?) would get it done in that case.
 
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Thanks again, and I mean, the PNP in the schematic of the top post cannot saturate because its Base-Collector junction cannot get forward biased in that schematic and setup.
But we were worried about leakage current from base to collector in the PNP making Ic not very equal to Ic.

It seems weird though that such a simple method of referring a sense signal down from the high side is not more widely used? I mean, its just basically a cheap PNP transistor as shown in the top post. It makes you search for some hidden gremlin in this PNP method
 

There will be Icb but this should be spec'd well enough to
analyze, and margin against in the current feedback design.
Like, if you expect nA of Icbo then make sure that 1uA
delivered, is still interpreted as "zero" by the PWM's read
of the current to voltage converter at the low side. You
have to be able to soak up some slop, right?

Some of the integrated solutions I mention, appear to just
be this scheme "bundled up". Maybe with MOSFETs instead
of BJTs (get rid of base current errors and variability).
 
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It seems weird though that such a simple method of referring a sense signal down from the high side is not more widely used?
Probably because the opamp requires a supply rail above the sense node, which won't usually be available.
 
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Thanks Mtwieg,
Good point, ..in our actual schematic, we provide the high side 5v supply for the opamp with an r/zener/BJT regulator.....feeding down from the LED rail
 

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