Alt-BEAM Archive
Message #05546
To: "'Scott Burns'" s-burns@uiuc.edu
From: Wilf Rigter Wilf.Rigter@powertech.bc.ca
Date: Fri, 6 Aug 1999 12:29:14 -0700
Subject: [alt-beam] Re: LDO regulator was h-bridge problems
content-type: text/plain;
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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