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 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
- 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
