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From: "Helmut Sennewald"
Subject: Re: op-amp for a regulated power supply(linear,series pass)...
Date: Sat, 16 Nov 2002 00:02:17 +0100
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
"Helmut Sennewald" schrieb im Newsbeitrag
> "KILOWATT" schrieb im Newsbeitrag
> > Hi everyones...thanks to read!
> > I'm designing a power suppy/battery charger for
> > a friend's camcorder. He then will be able to
> > power is camcorder at home as a webcam or wathever
> > when charging a battery for it at the same time, with the
> > same unit. I have an op-amp remaining from an LM324 IC
> > that i want to use for the camcorder's 6.5V regulated power
> > supply section. Since i did a lot of power supplies for my
> > projects,but like many ones i've used a regulator IC,
> > especially the 78XX series,(the easy way!) i did not really
> > gained experience and basics. I breadboarded the thing
> > and tested it, but as i expected i had some stability problems.
> > (see the link lower on this message)
> Hello Alain,
> your requirements are so easy, that the most simple circuit
> with an adjustable LT1085 regulator will do the job.
> This IC is a widely used 3A regulator.
> > I've checked on Google with tens of keywords combinations
> > for "power supply" "regulated" "op-amp"...ect...
> > i've found a lot of infos,but not exactly what i wanted. Found more
> > about Low Dropout regulators(LDO) ICs, and switching power
> > supplies.
> As I stated before, you don't need a circuit with an opamp and
> a discrete power stage.
> > But with the few infos i've found...i think i did something
> > that is performing. I've also added a capacitor in the feedback
> > loop and things are more stable than ever.
> > However since i'm
> > not 100% shure about the reliability of what i've done...i need
> > (please?!!) some comments about my schematic and if possible
> > a link where i can find infos about regulators made of op-amps.
> > http://www3.sympatico.ca/kilo.watt/images/ps_schematic.bmp
> This circuit has some serious design flaws especially in the output stage.
> * Overall to much unpredictable gain.
> * The 100kOhm at the +/- terminal of the opamp are one order
> of magnitude to high, use 10kOhm.
> * Unwanted low pass action due to the high resistor feeding the
> base of Q1.
> * Uncontrolled gain of Q1
> * The same for Q2
> * No short circuit protection
> * No thermal limiter for long lasting shorts.
> How could it be improved:
> Simply add no extra voltage gain in the output stage Q1, Q2.
> You only need one NPN darlington Q2 connected to the output
> of the opamp directly.
Sorry, put a 220 Ohm resistor between opamp output and the
base of the darlington transistor. The opamp could become
unstable if the input capacitance of the transistor is too high.
> But still you have no current limit
> and also no thermal limiter for short circuit protection.
> That all is integrated in the mentioned LT1085 voltage regulator.
> Take care when you order the LT1085 that you get the adjustable
> version and not any fixed version(LT1085-3,4,5).
> Best Regards
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