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| LM134/LM234/LM334 3-Terminal Adjustable Current Sources May 1998 LM134/LM234/LM334 3-Terminal Adjustable Current Sources General Description The LM134/LM234/LM334 are 3-terminal adjustable current sources featuring 10,000:1 range in operating current, excellent current regulation and a wide dynamic voltage range of 1V to 40V. Current is established with one external resistor and no other parts are required. Initial current accuracy is 3%. The LM134/LM234/LM334 are 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 sense voltage used to establish operating current in the LM134 is 64 mV at 25C and is directly proportional to absolute temperature (K). The simplest one external resistor connection, then, generates a current with +0.33%/C temperature dependence. Zero drift operation can be obtained by adding one extra resistor and a diode. Applications for the current sources include bias networks, surge protection, low power reference, ramp generation, LED driver, and temperature sensing. The LM134-3/ LM234-3 and LM134-6/LM234-6 are specified as true temperature sensors with guaranteed initial accuracy of 3C and 6C, respectively. These devices are ideal in remote sense applications because series resistance in long wire runs does not affect accuracy. In addition, only 2 wires are required. The LM134 is guaranteed over a temperature range of -55C to +125C, the LM234 from -25C to +100C and the LM334 from 0C to +70C. These devices are available in TO-46 hermetic, TO-92 and SO-8 plastic packages. Features n n n n n n Operates from 1V to 40V 0.02%/V current regulation Programmable from 1 A to 10 mA True 2-terminal operation Available as fully specified temperature sensor 3% initial accuracy Connection Diagrams SO-8 Surface Mount Package SO-8 Alternative Pinout Surface Mount Package TO-46 Metal Can Package DS005697-12 DS005697-25 DS005697-24 V- Pin is electrically connected to case. Order Number LM334M See NS Package Number M08A Order Number LM334SM See NS Package Number M08A Bottom View Order Number LM134H, LM134H-3, LM134H-6, LM234H or LM334H See NS Package Number H03H TO-92 Plastic Package DS005697-10 Bottom View Order Number LM334Z, LM234Z-3 or LM234Z-6 See NS Package Number Z03A (c) 1999 National Semiconductor Corporation DS005697 www.national.com Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. V+ to V- Forward Voltage LM134/LM234/LM334 LM134-3/LM134-6/LM234-3/LM234-6 V+ to V- Reverse Voltage R Pin to V- Voltage Set Current Power Dissipation ESD Susceptibility (Note 6) Operating Temperature Range (Note 5) LM134/LM134-3/LM134-6 40V 30V 20V 5V 10 mA 400 mW 2000V -55C to +125C LM234/LM234-3/LM234-6 -25C to +100C LM334 0C to +70C Soldering Information TO-92 Package (10 sec.) 260C TO-46 Package (10 sec.) 300C SO Package Vapor Phase (60 sec.) 215C Infrared (15 sec.) 220C See AN-450 "Surface Mounting Methods and Their Effect on Product Reliability" (Appendix D) for other methods of soldering surface mount devices. Electrical Characteristics (Note 2) Parameter Set Current Error, V+ = 2.5V, (Note 3) Ratio of Set Current to Bias Current Minimum Operating Voltage Conditions Min 10 AISET1 mA 1 mA LM134/LM234 Typ Max 3 5 8 14 18 14 18 0.8 0.9 1.0 0.02 0.01 0.03 0.02 0.96T T 1.04T 0.96T 0.05 0.03 23 23 14 Min LM334 Typ Max 6 8 12 18 14 18 0.8 0.9 1.0 0.02 0.01 0.03 0.02 T 1.04T 0.1 0.05 26 26 Units % % % V V V %/V %/V %/V %/V 25 AISET1 mA LM134 LM234 LM334 Thermal Resistance ja (Junction to Ambient) jc (Junction to Case) 150C 125C 100C TO-92 180C/W (0.4" leads) 160C/W (0.125" leads) N/A 32C/W 80C/W TO-46 440C/W SO-8 165C/W Note 6: Human body model, 100 pF discharged through a 1.5 k resistor. www.national.com 2 Electrical Characteristics (Note 2) Parameter Set Current Error, V+ = 2.5V, (Note 3) Equivalent Temperature Error Ratio of Set Current to Bias Current Minimum Operating Voltage Average Change in Set Current with Input Voltage Temperature Dependence of Set Current (Note 4) and Equivalent Slope Error Effective Shunt Capacitance 15 100 A ISET1 mA 100 AISET1 mA 1.5V+5V 5VV+30V 100 AISET1 mA 0.98T 0.02 0.01 T 0.05 0.03 1.02T 0.97T 0.02 0.01 T 0.01 0.05 1.03T %/V %/V 0.9 0.9 V 100 AISET1 mA 14 18 Conditions 100 AISET1 mA Tj = 25 LM134-3, LM234-3 Min Typ Max LM134-6, LM234-6 Min Typ Max Units % C 1 3 26 14 18 2 6 26 2 15 3 % pF Typical Performance Characteristics Output Impedance Maximum Slew Rate Linear Operation Start-Up DS005697-30 DS005697-31 DS005697-32 Transient Response Voltage Across RSET (VR) Current Noise DS005697-33 DS005697-34 DS005697-35 3 www.national.com Typical Performance Characteristics Turn-On Voltage (Continued) Ratio of ISET to IBIAS DS005697-29 DS005697-3 Application Hints The LM134 has been designed for ease of application, but a general discussion of design features is presented here to familiarize the designer with device characteristics which may not be immediately obvious. These include the effects of slewing, power dissipation, capacitance, noise, and contact resistance. CALCULATING RSET The total current through the LM134 (ISET) is the sum of the current going through the SET resistor (IR) and the LM134's bias current (IBIAS), as shown in Figure 1. for most set currents. SLEW RATE At slew rates above a given threshold (see curve), the LM134 may exhibit non-linear current shifts. The slewing rate at which this occurs is directly proportional to ISET. At ISET = 10 A, maximum dV/dt is 0.01V/s; at ISET = 1 mA, the limit is 1V/s. Slew rates above the limit do not harm the LM134, or cause large currents to flow. THERMAL EFFECTS Internal heating can have a significant effect on current regulation for ISET greater than 100 A. For example, each 1V increase across the LM134 at ISET = 1 mA will increase junction temperature by 0.4C in still air. Output current (ISET) has a temperature coefficient of 0.33%/C, so the change in current due to temperature rise will be (0.4) (0.33) = 0.132%. This is a 10:1 degradation in regulation compared to true electrical effects. Thermal effects, therefore, must be taken into account when DC regulation is critical and ISET exceeds 100 A. Heat sinking of the TO-46 package or the TO-92 leads can reduce this effect by more than 3:1. SHUNT CAPACITANCE In certain applications, the 15 pF shunt capacitance of the LM134 may have to be reduced, either because of loading problems or because it limits the AC output impedance of the current source. This can be easily accomplished by buffering the LM134 with an FET as shown in the applications. This can reduce capacitance to less than 3 pF and improve regulation by at least an order of magnitude. DC characteristics (with the exception of minimum input voltage), are not affected. where n is the ratio of ISET to IBIAS as specified in the Electrical Characteristics Section and shown in the graph. Since n is typically 18 for 2 A ISET 1 mA, the equation can be further simplified to DS005697-27 FIGURE 1. Basic Current Source A graph showing the ratio of these two currents is supplied under Ratio of ISET to IBIAS in the Typical Performance Characteristics section. The current flowing through RSET is determined by VR, which is approximately 214 V/K (64 mV/ 298K z 214 V/K). Since (for a given set current) IBIAS is simply a percentage of ISET, the equation can be rewritten www.national.com 4 Application Hints NOISE (Continued) APPLICATION AS A ZERO TEMPERATURE COEFFICENT CURRENT SOURCE Adding a diode and a resistor to the standard LM134 configuration can cancel the temperature-dependent characteristic of the LM134. The circuit shown in Figure 3 balances the positive tempco of the LM134 (about +0.23 mV/C) with the negative tempco of a forward-biased silicon diode (about -2.5 mV/C). Current noise generated by the LM134 is approximately 4 times the shot noise of a transistor. If the LM134 is used as an active load for a transistor amplifier, input referred noise will be increased by about 12 dB. In many cases, this is acceptable and a single stage amplifier can be built with a voltage gain exceeding 2000. LEAD RESISTANCE The sense voltage which determines operating current of the LM134 is less than 100 mV. At this level, thermocouple or lead resistance effects should be minimized by locating the current setting resistor physically close to the device. Sockets should be avoided if possible. It takes only 0.7 contact resistance to reduce output current by 1% at the 1 mA level. SENSING TEMPERATURE The LM134 makes an ideal remote temperature sensor because its current mode operation does not lose accuracy over long wire runs. Output current is directly proportional to absolute temperature in degrees Kelvin, according to the following formula: Calibration of the LM134 is greatly simplified because of the fact that most of the initial inaccuracy is due to a gain term (slope error) and not an offset. This means that a calibration consisting of a gain adjustment only will trim both slope and zero at the same time. In addition, gain adjustment is a one point trim because the output of the LM134 extrapolates to zero at 0K, independent of RSET or any initial inaccuracy. DS005697-28 FIGURE 3. Zero Tempco Current Source The set current (ISET) is the sum of I1 and I2, each contributing approximately 50% of the set current, and IBIAS. IBIAS is usually included in the I1 term by increasing the VR value used for calculations by 5.9%. (See CALCULATING RSET .) The first step is to minimize the tempco of the circuit, using the following equations. An example is given using a value of +227 V/C as the tempco of the LM134 (which includes the IBIAS component), and -2.5 mV/C as the tempco of the diode (for best results, this value should be directly measured or obtained from the manufacturer of the diode). DS005697-4 FIGURE 2. Gain Adjustment This property of the LM134 is illustrated in the accompanying graph. Line abc is the sensor current before trimming. Line a'b'c' is the desired output. A gain trim done at T2 will move the output from b to b' and will simultaneously correct the slope so that the output at T1 and T3 will be correct. This gain trim can be done on RSET or on the load resistor used to terminate the LM134. Slope error after trim will normally be less than 1%. To maintain this accuracy, however, a low temperature coefficient resistor must be used for RSET. A 33 ppm/C drift of RSET will give a 1% slope error because the resistor will normally see about the same temperature variations as the LM134. Separating RSET from the LM134 requires 3 wires and has lead resistance problems, so is not normally recommended. Metal film resistors with less than 20 ppm/C drift are readily available. Wire wound resistors may also be used where best stability is required. 5 With the R1 to R2 ratio determined, values for R1 and R2 should be determined to give the desired set current. The formula for calculating the set current at T = 25C is shown below, followed by an example that assumes the forward voltage drop across the diode (VD) is 0.6V, the voltage across R1 is 67.7 mV (64 mV + 5.9% to account for IBIAS), and R2/R1 = 10 (from the previous calculations). www.national.com Application Hints (Continued) If the estimate for the tempco of the diode's forward voltage drop was off, the tempco cancellation is still reasonably effective. Assume the tempco of the diode is 2.6 mV/C instead of 2.5 mV/C (an error of 4%). The tempco of the circuit is now: This circuit will eliminate most of the LM134's temperature coefficient, and it does a good job even if the estimates of the diode's characteristics are not accurate (as the following example will show). For lowest tempco with a specific diode at the desired ISET, however, the circuit should be built and tested over temperature. If the measured tempco of ISET is positive, R2 should be reduced. If the resulting tempco is negative, R2 should be increased. The recommended diode for use in this circuit is the 1N457 because its tempco is centered at 11 times the tempco of the LM134, allowing R2 = 10 R1. You can also use this circuit to create a current source with non-zero tempcos by setting the tempco component of the tempco equation to the desired value instead of 0. EXAMPLE: A 1 mA, Zero-Tempco Current Source First, solve for R1 and R2: A 1 mA LM134 current source with no temperature compensation would have a set resistor of 68 and a resulting tempco of So even if the diode's tempco varies as much as 4% from its estimated value, the circuit still eliminates 98% of the LM134's inherent tempco. Typical Applications Ground Referred Fahrenheit Thermometer The values of R1 and R2 can be changed to standard 1% resistor values (R1 = 133 and R2 = 1.33 k) with less than a 0.75% error. If the forward voltage drop of the diode was 0.65V instead of the estimate of 0.6V (an error of 8%), the actual set current will be DS005697-15 *Select R3 = VREF/583 A. VREF may be any stable positive voltage 2V Trim R3 to calibrate an error of less than 5%. www.national.com 6 Typical Applications (Continued) Low Output Impedance Thermometer Terminating Remote Sensor for Voltage Output DS005697-6 *Output impedance of the LM134 at the "R" pin is approximately DS005697-14 where R2 is the equivalent external resistance connected from the V- pin to ground. This negative resistance can be reduced by a factor of 5 or more by inserting an equivalent resistor R3 = (R2/16) in series with the output. Low Output Impedance Thermometer Higher Output Current DS005697-5 DS005697-16 *Select R1 and C1 for optimum stability Basic 2-Terminal Current Source DS005697-1 7 www.national.com Typical Applications (Continued) Low Input Voltage Reference Driver Micropower Bias DS005697-17 DS005697-18 Ramp Generator DS005697-19 www.national.com 8 Typical Applications (Continued) 1.2V Regulator with 1.8V Minimum Input 1.2V Reference Operates on 10 A and 2V DS005697-20 *Select ratio of R1 to R2 to obtain zero temperature drift DS005697-7 *Select ratio of R1 to R2 for zero temperature drift Zener Biasing Alternate Trimming Technique Buffer for Photoconductive Cell DS005697-51 DS005697-49 DS005697-50 *For 10% adjustment, select RSET 10% high, and make R1 3 RSET FET Cascoding for Low Capacitance and/or Ultra High Output Impedance DS005697-21 *Select Q1 or Q2 to ensure at least 1V across the LM134. Vp (1 - ISET/IDSS) 1.2V. DS005697-22 9 www.national.com Typical Applications (Continued) In-Line Current Limiter Generating Negative Output Impedance DS005697-23 *ZOUT -16 * R1 (R1/VIN must not exceed ISET) DS005697-9 *Use minimum value required to ensure stability of protected device. This minimizes inrush current to a direct short. Schematic Diagram DS005697-11 www.national.com 10 11 Physical Dimensions inches (millimeters) unless otherwise noted Order Number LM134H, LM134H-3, LM134H-6, LM234H or LM334H NS Package Number H03H www.national.com 12 Physical Dimensions inches (millimeters) unless otherwise noted (Continued) SO Package (M) Order Number LM334M or LM334SM NS Package Number M08A Order Number LM334Z, LM234Z-3 or LM234Z-6 NS Package Number Z03A 13 www.national.com LM134/LM234/LM334 3-Terminal Adjustable Current Sources LIFE SUPPORT POLICY NATIONAL'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 2. A critical component is any component of a life support 1. Life support devices or systems are devices or sysdevice or system whose failure to perform can be reatems which, (a) are intended for surgical implant into sonably expected to cause the failure of the life support the body, or (b) support or sustain life, and whose faildevice or system, or to affect its safety or effectiveness. ure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. National Semiconductor Corporation Americas Tel: 1-800-272-9959 Fax: 1-800-737-7018 Email: support@nsc.com National Semiconductor Europe Fax: +49 (0) 1 80-530 85 86 Email: europe.support@nsc.com Deutsch Tel: +49 (0) 1 80-530 85 85 English Tel: +49 (0) 1 80-532 78 32 Francais Tel: +49 (0) 1 80-532 93 58 Italiano Tel: +49 (0) 1 80-534 16 80 National Semiconductor Asia Pacific Customer Response Group Tel: 65-2544466 Fax: 65-2504466 Email: sea.support@nsc.com National Semiconductor Japan Ltd. Tel: 81-3-5639-7560 Fax: 81-3-5639-7507 www.national.com National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. |
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