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Current source operates without capacitors

Posted: 30 Apr 2009 ?? ?Print Version ?Bookmark and Share

Keywords:current source? capacitor? resistor?

Sometimes, the most "innovative" product is not one that packs more functions into the package, has increased speed or is cheaper, but instead is one that solves a conceptually small but chronic problem for the circuit designer by doing one thing, doing it well, and not getting in the way of the user's objectives. The LT3092 current source from Linear Technology Corp. is an example.

Designed by Linear's VP of engineering and chief technology officer Robert Dobkin, the LT3092 is a two-terminal current source that provides resistor-programmable current between 0.5mA and 200mA and can be paralleled (using a 40m? resistance, usually just a PCB trace) for greater total current. It is an easy-to-use, higher-performance alternative to conventional, finicky current-source designs based on depletion-mode FETs or two-transistor, two-Zener diode approaches.

The core of the design is the innovative three-terminal LT3080 low-dropout regulator, also designed by Dobkin, and introduced about a year ago. The current source does not require any input or output capacitors and thus can be used in "intrinsically safe" applications (no stored charge or energy) as well as floating applications such as the ubiquitous current loop. The absence of any capacitors also means that both AC and DC impedances are high and maintained, even when connected is series with unknown reactance; impedance is 100M? at 1mA and 1M? at 100mA.

Regulation over the 1.2V to 40V supply range is better than 10ppm/V, and the IC includes reverse-battery and reverse-current protection. Initial set-pin current accuracy is 1 percent. Additional key typical specs include load regulation of better than 1mV and line regulation of better than 0.001 percent/V.

The LT3092 is available in 3mm x 3mm DFN-8, 8-lead TSOT-23, and 3-lead SOT-223 packages. Operating ranges include -40C to +125C and -55C to +125C junction temperatures. Pricing begins at $1.65.

- Bill Schweber
Planet Analog

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