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Temperature-compensated current source

Temperature-compensated current source: National Semiconductors’ LM334Z is a three-pin presettable current source, whose output may be set between 1 pA and 10 mA. It may also be used as ‘floating’.

In principle, just one resistor is needed for setting the current. However, the current is then strongly dependent on temperature: about +0.33% °C-1. (This would enable the device to be used as a temperature sensor). Therefore, to obtain a stable current source, an additional resistor and a diode are needed.

For good stability. the diode must be coupled thermally to the IC (the self-heating o the source is then compensated). This is best done by squeezing the IC and the diode, separated by heat paste, into a piece of insulating sleeving as shown in Fig. 2. Although the current source may be set between 1 pA and 10 mA with the aid of it is most accurate between 10 pA and 1 mA. The current provided by the source may calculate from
I= 2/15Rs

Resistor Rk should have a value of 1ORs. Set as described and with good thermal coupling between and 1C1, the prototype showed a temperature drift of not greater than 0.02% C-1, occurring at 5 Is= mA. All measurements were Carried out with a supply voltage Of 9 V.

The LM134/LM234/LM334 is a true floating current. sources with no separate power supply connections. In addition, reverse applied voltages of up to 20V will draw only a few dozen microamperes of current, allowing the devices to act as both a rectifier and current source in AC applications.
The LM134 is guaranteed over a temperature range of −55˚C to +125˚C, the LM234 from −25˚C to +100˚C, and the LM334 from 0˚C to +70˚C. These devices are available in TO-46 hermetic, TO-92, and SO-8 plastic packages.

The LM134/LM234/LM334 are 3-terminal adjustable current sources characterized by:

• an operating current range of 10000: 1an excellent current regulation
• a wide dynamic voltage range of 1V t 10V
Features
• Operates from 1V to 40V
• 0.02%/V current regulation
• Programmable from 1µA to 10mA
• True 2-terminal operation
• Available as the fully specified temperature sensor
• ±3% initial accuracy

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