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From: Fred Bloggs
User-Agent: Mozilla/5.0 (Windows; U; Win 9x 4.90; en-US; rv:1.0.1) Gecko/20020823 Netscape/7.0
X-Accept-Language: en-us, en
Subject: Re: Constant current source with low voltage drop
Date: Wed, 02 Oct 2002 13:35:15 GMT
NNTP-Posting-Date: Wed, 02 Oct 2002 06:35:15 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
Winfield Hill wrote:
> Winfield wrote...
>>>>I need a constant current circuit adjustable from 0 to 150mA powered
>>>>from a 5V supply. This is to drive LEDs of an LCD backlight. The spec
>>>>says a max of 4.4V drop @ 150mA (2 LEDs in series?) so I've got to do
>>>>this with a maximum of 0.6V voltage drop.
>>The LP2952 chip's separate 1.24V reference is a very nice feature.
>>If Vfb was lower than Vref, it would let you obtain a setpoint
>>voltage of less than 1.24V on the 0.5-ohm current-sense resistor.
> Here's a simple current source employing a favorite overlooked IC,
> NSC's elegant LM385-ADJ, which is a little three-pin TO-92 amplifier
> with a 1.24V sense pin. In this circuit, a 2.5V LM385 is added so
> we can regulate the 2-ohm current-sense resistor voltage at 250mV.
> . LM385-ADJ 2.0 ohms
> . o----+---------+----/\/\----,
> . | | |
> . \_|__ \__|__ 1.00k |
> . / \ \ / \---+-/\/\--+ |
> . /___\ /___\ | | |
> . LM385- | | | | V
> . 2.5 | 1.21k | | | 150mA
> . +--/\/\-- | ---' |
> . | | E
> . \ +--|<|--- B
> . 1k / | 1n4148 C
> . \ \ |
> . | 390 / '------O
> . | \ 2n4403
> . gnd |
> . gnd
> The diode is needed, because without it the LM385-ADJ would have
> well under 1.25V of operating voltage.
> With a 4.4V required minimum output compliance range, the supply-
> voltage range is about 4.8 to 16V.
> - Win
This is the ultimate in low-cost precision LDO current regulation, and
maybe more than the LED array in backlight deserves:-)
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