Constant current problem on discreet regulator

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As part of a larger project, I have a MAX1701 switching booster in which I want to use a spare opamp to provide a regulated charge to a lipo.

I want the output to be 4.2V and limited to 500mA.

The voltage regulator itself works fine, but I can't get the current limit to work... thing is I have no clue why it doesn't work.

I attached a simplified shematic (there are also a 100uF tant cap on output I forgot in the image)

Anyone can see why it doesn't work ?

Thanks !

MAX1701 http://datasheets.maxim-ic.com/en/ds/MAX1700-MAX1701.pdf
XP162A12A6PR http://www.torex.co.jp/chinese/products/pro07/pdf072/10-XP162A12A6PR.pdf

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Last Edited: Sat. Apr 9, 2011 - 07:19 AM
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R1 R2 and Q1 provide for positive feedback.

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That's the idea no ? So more current in R1 will force the P fet to close more, or am I missing the obvious ?

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supply voltage 5v
R1= 2.2 ohm
current limit = 500ma
drop across R1 = 1.1V
5 - 1.1 =3.9V just right for a current limited 4.2V output
Seriously though.. what is not hapening.. or more to the point what is hapening?

Consider decoupling current limiting and voltage control signals ( from each other) at the gate of the PFET and connect the collector of the transistor to the voltage divider ( junction R5 and R6 )

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Control current to a maximum cell voltage; unregulated DC power source followed by a 2 terminal 500ma to 0.01C current regulator (between source and the cell).
An off the shelf regulator (voltage or current) is a Linear Technology LT3080; else, transistor, sense resistor, and a variable voltage reference.
Any boost current sources (boost LED driver with variable current control)? If yes, control its current to limit cell voltage to 4.2v.
Edit: correct.

"Dare to be naïve." - Buckminster Fuller

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I think that the obvious is that the circuit mainly is voltage controlled.For current control also need a second indipendent circuit in negative line for current monitoring controlling the biasing of output mosfet.

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Thanks all for your insights, it turned out to simply be a bad pinout on the transistor with collector/emitter swapped :-(
The kind of thing you look at 20 times and still don't see it... just in front of you.
Still, I followed the advice of ignoramus and biased the voltage divider which works nicely.
Didn't have much space left, so extra parts was hard to justify when you have a spare opamp in there ;-)

Thanks again.