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From: Winfield Hill
Subject: Re: Constant current source with low voltage drop
Date: 2 Oct 2002 05:18:05 -0700
Organization: Rowland Institute
X-Newsreader: Direct Read News 2.98
>>> 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
. | | |
. \_|__ \__|__ 1.00k |
. / \ \ / \---+-/\/\--+ |
. /___\ /___\ | | |
. LM385- | | | | V
. 2.5 | 1.21k | | | 150mA
. +--/\/\-- | ---' |
. | | E
. \ +--|<|--- B
. 1k / | 1n4148 C
. \ \ |
. | 390 / '------O
. | \ 2n4403
. 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.
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