Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
Subject: Re: Constant current source with low voltage drop
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Date: Sun, 29 Sep 2002 09:09:26 +0100
NNTP-Posting-Date: Sun, 29 Sep 2002 09:09:29 BST
"Bob Wilson" wrote in message
> In article ,
> firstname.lastname@example.org says...
> >"Jim Thompson" wrote in message
> >> On Sat, 28 Sep 2002 23:18:35 +0100,
> >> "markp" ,
> >> In Newsgroup: sci.electronics.design,
> >> Article: ,
> >> Entitled: "Constant current source with low voltage drop",
> >> Wrote the following:
> >> |Hi All,
> >> |
> >> |I need a constant current circuit adjustable from 0 to 150mA
> >> |5V supply. This is to drive LEDs of an LCD backlight. The spec
> >> |4.4V drop @ 150mA (2 LEDs in series?) so I've got to do this with
> >> |of 0.6V voltage drop. Any suggestions?
> >> |
> >> |Thanks!
> >> |
> >> |Mark.
> >> |
> >> P-Channel power MOSFET controlled by OpAmp with common-mode range
> >> includes positive rail; small sense resistor in source of
> >> ...Jim Thompson
> >Yes, that's my favourite so far. However I was considering a PNP
> >as a switch followed by a P channel FET. The PNP would turn on at
> >0.6V or so and shut off the FET. Current is then controlled by a
> >from +ve to base of PNP. Your solution is better though as the PNP
> >is on the edge!
> Forget the FET; it is irrelivant. Using a PNP pass transistor, you can
> very low dropout voltage. You are incorrect that the PNP will have
> across it. When fully saturated, you it will get downt to mere tens of
> Now a word of caution with ANY low dropout regulator (Built from
> OR integrated): The PNP (or P-channel FET) pass element has gain (i.e.
> not an emitter or source follower). Thus you may run into self
> unless you use the correct value of output capacitance. This is unlike
> normal regulators using NPN or N-FETS what are stable with no output
It is correct that the collector output has gain, but it is debatable
that this is a good description why the topology is prone to
oscillation. Arguable, a better description might be this. In a voltage
output regulator the collector output is high resistance. This
resistance drives the output capacitor, forming an inherent 1st order
roll off. Since there are other poles in the loop, phase shift in the
loop can be enough to cause instability. ESR in the capacitor usually
needs to be a minium to compansate for this.
In a current regulator like this, the current feedback sense is at the
pnp emitter, taking the capacitor load, essentially, out of the feedback
loop altogether. So, in this arrangement, there is no loop gain
associated with the capacitor, and hence it will not affect stability.
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