Three terminal adjustable current sources
Document overview
- Manufacturer or author: STMICROELECTRONICS
- PDF pages: 16
Technical content
Features
■ Operates from 1V to 40V ■ 0.02%/V current regulation ■ Programmable from 1µA to 10mA ■ ±3% initial accuracy
Description
The LM134/LM234/LM334 are 3-terminal adjustable current sources characterized by: ■ an operating current range of 10000: 1 ■ an excellent current regulation ■ a wide dynamic voltage range of 1V t 10V The current is determined by an external resistor without requiring other external components. Reverse voltages of up to 20V will only draw a current of several microamperes. This enables the circuit to operate as a rectifier and as a source of current in a.c. applications. For the LM134/LM234/LM334, the voltage on the control pin is 64mV at +25°C and is directly proportional to the absolute temperature (°K). The simplest external resistor connection generates a current with approximately 0.33%/°C temperature dependence. Zero drift can be obtained by adding an additional resistor and a diode to the external circuit. Z TO-92 (Plastic package) D SO-8 (Plastic micropackage) ADJv+ v- 21 3 TO-92 (Bottom view) 8765 1234 NC NC V- NC ADJ NC NC V+ SO-8 (Top view) Pin connections
1 Schematic diagram
Figure 1. Schematic diagram
2 Absolute maximum ratings
Table 1. Absolute maximum ratings
3 Electrical characteristics
Table 2. Electrical characteristics
- The set current is the current flowing into th e V+ pin. It is determined by the following formula:
The set current error is expressed as a percent deviation from this amount.
- I set is directly proportional to absolute temperature (°K). Iset at any temperature can be calculated from
where Io is Iset measured at To (°K).
Figure 8. Turn-on voltage Figure 9. Ratio of I set to V- current
4 Application information
4.1 Slew rate
At slew rates above a threshold (see Figure 4 and Figure 5), the LM134, LM234, LM334 can have a non-linear current characteristic. The slew rate at which this takes place is directly proportional to I Slew rates of more than 1V/µs do not damage the circuit nor do they produce high currents.
4.2 Thermal effects
Internal heating can have a significant effect on current regulation for an Iset above 100µA. For example, each increase of 1V in the voltage across the LM134 at Iset = 1mA will increase the junction temperature by ≈ 0.4°C (in still air). The output current (Iset) has a temperature coefficient of about 0.33%/°C. Thus the change in current due to the increase in temperature will be (0.4) (0.33) = 0.132%. This is a degradation of 10 : 1 in regulation versus the true electrical effects. Thermal effects should be taken into account when d.c. regulation is critical and I set is higher than 100µA.
4.3 Shunt capacitance
In certain applications, the 15pF value for the shunt capacitance should be reduced:
- because of loading problems,
- because of limitation of output impedance of the current source in a.c. applications. Y ou can easily reduce the capacitance by adding a FET as shown in Typical applications on page 8. The value of this capacitance can be reduced by at least 3pF and regulation can be improved by an order of magnitude without any modifications of the d.c. characteristics (except for the minimum input voltage).
4.4 Noise
The current noise produced by LM134, LM234, and LM334 is about 4 times that of a transistor. If the LM134, LM234, LM334 is used as an active load for a transistor amplifier, the noise at the input will increase by about 12dB. In most cases this is acceptable, and a single amplifier can be built with a voltage gain higher than 2000.
4.5 Lead resistance
The sense voltage which determines the current of the LM134, LM234, LM334 is less than 100mV. At this level, the thermocouple effects and the connection resistance should be reduced by locating the current setting resistor close to the device. Do not use sockets for the ICs. A contact resistance of 0.7Ω is sufficient to decrease the output current by 1% at the 1mA level.
4.6 Sensing temperature
Kelvin degrees according to the following equation. point trim because the output of the device extrapolates to zero at 0°K. Figure 10. Device calibration available. Wirewound resistors can be used when very high stability is required.
Figure 19. In-line current limiter Figure 20. Fet cascading for low capacitance
5 Package information
In order to meet environmental requirements, STMicroelectronics offers these devices in ECOPACK® packages. These packages have a lead-free second level interconnect. The category of second level interconnect is marked on the package and on the inner box label, in compliance with JEDEC Standard JESD97. The maximum ratings related to soldering conditions are also marked on the inner box label. ECOPACK is an STMicroelectronics trademark. ECOPACK specifications are available at: www.st.com
5.1 SO-8 package mechanical data
Ref. Dimensions Millimeters Inches A1 . 7 5 0 . 0 6 9 A1 0.10 0.25 0.004 0.010 A2 1.25 0.049 b 0.28 0.48 0.011 0.019 c 0.17 0.23 0.007 0.010 e 1.27 0.050 h 0.25 0.50 0.010 0.020 L 0.40 1.27 0.016 0.050 k 1 °8 ° 1 °8 ° ccc 0.10 0.004
5.2 TO-92 ammopack and tape & reel package mechanical data
Dim. Millimeters Inches AL 5.0 0.197 A 5.0 0.197 T 4.0 0.157 d 0.45 0.018 I1 2.5 0.098 Δh -1 0 1 -0.039 0 0.039 ΔP -1 0 1 -0.039 0 0.039 W2 0.5 0.020 H2 0 0 . 7 8 7 H1 25 0.984 L1 11 0.433 P P A T H H1 H d I1 H0 W2 W0 W1 W F1 F2
5.3 TO-92 bulk package mechanical data
Dim. Millimeters Inches L 1.27 0.05 K1 2 . 7 0 . 5 a 0.35 0.0138
6 Ordering information
7 Revision history
Table 3. Order codes 2-May-2003 1 Initial release. 28-Oct-2005 2 Internal revision. (it is a bottom view, not a top view).