Alt-BEAM Archive

Message #05551



To: beam@sgiblab.sgi.com
From: Sean Rigter rigter@cafe.net
Date: Sat, 07 Aug 1999 10:12:37 -0700
Subject: [alt-beam] Re: LDO regulator was h-bridge problems


Bug report!

Asleep at the wheel, I made a major goof up! When I said 2N2222, I meant
2N2907! The PNP series pass transistor is a 2N3906 or 2N2907. The circuit
design and discussion are fine just substitute 2N2907 for 2N2222 and all will be
well again.

appologies for any inconvenience!

wilf

Wilf Rigter wrote:

> Hello Scott,
>
> Thanks for your question! After browsing through my email directory, I
> discovered that so far I have published a few articles that use the LDO
> voltage regulator as part of a more complex circuit ie embedded applications
> of the LDO regulator. Since this is such a versatile, low cost and
> accessible design which can be used for a number of other applications,
> attached is a general purpose LDO regulator circuit. The adjustable LDO
> regulator output voltage ranges from 2.4V to Vin-400mV and Vdropout is
> generally about 400mV. The Vreg will also vary about 200mV over the full Vin
> and Iout current range but quiescent supply current is (approximately) only
> 200uA + Iout/50.
>
> By comparison, a 7805/317 regulator has a Vdropout of about 3V and draws
> about minimum 10mA quiescent current. Some single chip LDO regulators I
> have used are from National (100mA/LM2931 or 1A/LM2940) but a host of other
> more exotic LDO regulators are available from Maxim and others.
>
> Unlike the 7805 or LM317 and similar (more or less indestructible)
> regulators, you do have to think about the application of this LDO
> regulator. For example, the maximum power dissipation for a 2N2222 is
> should be limited to about 500mW without heat sink. For example, if a 9V
> battery is used for Vin with Vreg set to 5V, then the voltage drop (Vdrop)
> across the 2N2222 is 9V-5V=4V and the maximum allowable continuous output
> current (Iout) is 500mW/4V=125ma. The maximum available short circuit
> current of the adjustable LDO regulator is about 250mA with Vdrop=Vin,
> meaning that the circuit will self destruct in less than 10 seconds. So a
> little finger trouble shorting the output is OK but use precautions when
> connecting the LDO to a new circuit since there may be a permanent short or
> solderbridge which will result in melt down.
>
> The LDO regulator in the D2 circuit is more forgiving since it has inherent
> foldback current limiting which drops the Iout current to a few mA under
> full short circuit conditions. In the D2 application there is also a
> shutdown mode available which turns off the LDO and reduces quiescent
> current to about 10 uA when Vin falls below a preset value and turns the LDO
> regulator back on when the Vin is above a preset value. With a low current
> source (ie solar cell) used for Vin (and with LDR removed), these same D2
> features cause the characteristic SE behaviour of charging a battery or
> supercap up to a voltage trigger level, turning on and latching the SE
> output, and discharge stored energy into a load down to a lower preset
> voltage level where the SE turns off and the cycle repeats. With the
> addition of a LDR the D2, like the D1 SE, holds off the discharge cycle
> until after dark and then discharges only once for maybe an hour, making
> pretty lights for the garden etc, until recharging the next morning.
>
> While embedded regulators have to be simple affairs, a full featured (but
> simple to build) adjustable LDO regulator used as a rugged rechargeable
> portable bench supply with a meaningful bargraph LED display to indicate
> voltage and error conditions could be specifically designed for BEAM landers
> (ie with features to fast charge smaller bot NiCad batteries) if there is
> any interest.
>
> regards
>
> Wilf Rigter mailto:wilf.rigter@powertech.bc.ca
> tel: (604)590-7493
> fax: (604)590-3411
>
> <> <>
>
> > -----Original Message-----
> > From: Scott Burns [SMTP:s-burns@uiuc.edu]
> > Sent: Wednesday, August 04, 1999 8:13 AM
> > To: Wilf.Rigter@powertech.bc.ca
> > Subject: Re: h-bridge problems
> >
> > At 01:33 PM 7/25/99 -0700, you wrote:
> >
> > >I recently posted a 100ma Low Drop-Out voltage regulator design that
> > >exploits this low Vcc concept to the max by using an actual LED as the
> > >regulator voltage reference. That way the LDO regulator holds the 74HC
> > >Vcc supply voltage at the optimum level for HC outputs to drive LEDs
> > >directly. In addition, the LED Vf referenced regulator tracks Vcc with
> > >temperature for constant LED brightness over a wide input voltage range
> > >of 2.1V to 9V.
> >
> > Dear Wilf:
> >
> > I've been searching for the description of the LDO regulator in my
> > archived
> > emails, but can't find it. Can you direct me to it on the web or
> > elsewhere?
> > BTW, I've really enjoyed reading your BEAM postings! My son and I are in
> > the process of building a power smart head.
> >
> > Regards, Scott
> > ------------------------------------------
> > Prof. Scott A. Burns
> > Dept of General Engineering and Civil Engineering
> > University of Illinois at Urbana-Champaign
> > 104 S. Mathews, Urbana, IL 61801
> > Phone 217/333-1618, FAX 217/244-5705
>
> ------------------------------------------------------------------------
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>
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