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lowpowerZONE Products for the week of April 27, 2009
Linear Technology Says…
LT3092: Programmable 200-mA Two-Terminal Current Source
Offers 10ppm Regulation
An innovative new power IC from Linear Technology Corporation now provides system designers with a two-terminal current source. Until now, current sources were imprecise or had to be designed from other components. The new LT3092 200mA two-terminal current source overcomes the problems associated with previous current sources, with its wide voltage range, high AC and DC impedance, good regulation, low temperature coefficient, and the fact that it requires no capacitors. The device’s two floating terminals make it easy to use.
The LT3092 features 1% initial accuracy and a very low temperature coefficient. Output current is programmable from 0.5mA to 200mA and current regulation over 1.5V to 40V is better than 10ppm/volt. Unlike other analog integrated circuits, the LT3092 leverages special design techniques for high DC and AC impedance, ensuring stable operation without supply bypass capacitors. Since the LT3092 requires no capacitors, it is ideal for “intrinsically safe” applications in which the output may be shorted and no sparking can be tolerated. Reverse supply tolerance and thermal shutdown protect both the IC and the load.
According to Robert Dobkin, Vice President, Engineering and Chief Technical Officer for Linear Technology, “The LT3092 current source adds a significant new component to the designer’s toolbox for future designs. Its ability to easily generate a constant current enables the simplification of many circuits. It can be used in precision or remote current limiting, bias circuits, temperature sensing, active loads, and signaling. Further, the LT3092 can be easily extended to operate at higher voltages or higher currents.”
EN-Genius Says…
Current sources have not been straightforward in our semiconductor world. Throw in a requirement for a two-terminal solution and the choices have been few. The most popular has probably been the two-transistor solution also using two voltage references. But the references and associated pass diodes have different temperature characteristics compared to the transistors and they drift away from one another over temperature. There are also two transistor junctions in series so the headroom needed requires more than 3 V. Current accuracy is also limited to a few percent.
The LT3092 is a very different animal. It has an internal 10 µA current mirror source. The source can be magnified up to 200 mA with two external resistors (SET and OUTPUT), the ratio of which is the multiplication factor. No capacitors are generally required for stability (they or series RC networks may be required for extreme impedance situations) but the noise performance can be improved from the typical 2.7 pA/rtHz of the source – in series with the shot noise of the SET resistor – by bypassing that source noise with a small capacitor: at the cost of start-up time. Both the ac and dc impedances are reportedly high.
The LT3092 offers an initial 1% accuracy and the input voltage range is an astonishing 1.2 V to 40 V. Current regulation over a load range of 1 mA to 200 mA is a typical -100 pA and line regulation is a typical 100 pA/V. Ripple rejection at 120 Hz is a typical 90 dB. The SET pin current varies from 10.050 µA at -50ºC to about the same at +125ºC. From -10ºC to +125ºC the current is essentially flat at just below and just above 10.000 µA.
This new device is based on the (unique itself) LT3080 single resistor voltage LDO, reviewed here in July 2007, a triumph of Robert Dobkin and his team at Linear. The output is modified with a pnp transistor. The design inherently provides reversed battery and reverse current protection.
The data sheet offers a number of applications beyond the single current source: LT3092s can be directly paralleled to either raise the load current or reduce heat dissipation; they can also be paralleled using a ballast resistor; they can be stacked as a high-voltage current source; part of the SET resistor can be a thermistor (paralleled) to act as a remote temperature sensor; SET control could be from a DAC, or it could be pulsed, or switched from an optocoupler (to be fully-floating); it could be connected in a signal path as an ac current limiter.
With the current levels in the SET path and the sizes of resistor being used, some great care needs to be taken in layout and connection cleanliness in your circuits.
It should have been plain to me when I reviewed the LT3080 that this was an obvious direction Linear could develop the part – it wasn’t – and it is a major jump for current sources. It should now become standard textbook material and replace the two-transistor solution in college courses. The number and type of applications for the LT3092 are limitless. They have been priced at we-deserve-it levels and the company will undoubtedly score extremely well in the marketplace, extremely well.
The LT3092 is offered in DFN-8, SOT-23-8, and SOT-223-3. Pricing in 1000-piece lots ranges from $1.65 to $4.73 depending on package and temperature grade.
Data Sheet
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