LM134H

LM134/LM234/LM334 3-Terminal Adjustable Current Sources March 2005 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 64mV at 25˚C 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 LM234-3 and LM234-6 are specified as true temperature sensors with guaranteed initial accuracy of ± 3˚C and ± 6˚C, 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 −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. Features n n n n n n Operates from 1V to 40V 0.02%/V current regulation Programmable from 1µA to 10mA 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 00569725 00569724 Order Number LM334M or LM334MX See NS Package Number M08A TO-46 Metal Can Package Order Number LM334SM or LM334SMX See NS Package Number M08A TO-92 Plastic Package 00569710 00569712 V− Pin is electrically connected to case. Bottom View Order Number LM134H, LM234H or LM334H See NS Package Number H03H Bottom View Order Number LM334Z, LM234Z-3 or LM234Z-6 See NS Package Number Z03A © 2005 National Semiconductor Corporation DS005697 www.national.com LM134/LM234/LM334 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 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 40V 30V 20V 5V 10 mA 400 mW 2000V LM234/LM234-3/LM234-6 LM334 Soldering Information TO-92 Package (10 sec.) TO-46 Package (10 sec.) SO Package Vapor Phase (60 sec.) Infrared (15 sec.) −25˚C to +100˚C 0˚C to +70˚C 260˚C 300˚C 215˚C 220˚C See AN-450 “Surface Mounting Methods and Their Effect on Product Reliability” (Appendix D) for other methods of soldering surface mount devices. −55˚C to +125˚C 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 10µA ≤ ISET ≤ 1mA 1mA < ISET ≤ 5mA 2µA ≤ ISET < 10µA 100µA ≤ ISET ≤ 1mA 1mA ≤ ISET ≤ 5mA 2 µA≤ISET≤100 µA 2µA ≤ ISET ≤ 100µA 100µA < ISET ≤ 1mA 1mA < ISET ≤ 5mA Average Change in Set Current with Input Voltage 2µA ≤ ISET ≤ 1mA 1.5 ≤ V+ ≤ 5V 5V ≤ V+ ≤ 40V 1mA < ISET ≤ 5mA 1.5V ≤ V ≤ 5V 5V ≤ V ≤ 40V Temperature Dependence of Set Current (Note 4) Effective Shunt Capacitance 15 15 pF 25µA ≤ ISET ≤ 1mA 0.96T 0.03 0.02 T 1.04T 0.96T 0.03 0.02 T 1.04T %/V %/V 0.02 0.01 0.05 0.03 0.02 0.01 0.1 0.05 %/V %/V 14 18 14 18 0.8 0.9 1.0 23 LM134/LM234 Typ Max 3 5 8 23 14 18 14 18 0.8 0.9 1.0 26 V V V Min LM334 Typ Max 6 8 12 26 % % % Units Note 1: .“Absolute Maximum Ratings” indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is functional, but do not guarantee specific performance limits. Note 2: Unless otherwise specified, tests are performed at Tj = 25˚C with pulse testing so that junction temperature does not change during test Note 3: Set current is the current flowing into the V+ pin. For the Basic 2-Terminal Current Source circuit shown on the first page of this data sheet. ISET is determined by the following formula: ISET = 67.7 mV/RSET (@ 25˚C). Set current error is expressed as a percent deviation from this amount. ISET increases at 0.336%/˚C @ Tj = 25˚C (227 µV/˚C). www.national.com 2 LM134/LM234/LM334 Electrical Characteristics (Note 2) Note 5: For elevated temperature operation, TJ max is: (Continued) Note 4: ISET is directly proportional to absolute temperature (˚K). ISET at any temperature can be calculated from: ISET = Io (T/To) where Io is ISET measured at To (˚K). LM134 LM234 LM334 Thermal Resistance θja (Junction to Ambient) TO-92 150˚C 125˚C 100˚C TO-46 440˚C/W SO-8 165˚C/W 180˚C/W (0.4" leads) 160˚C/W (0.125" leads) θjc (Junction to Case) Note 6: Human body model, 100pF discharged through a 1.5kΩ resistor. N/A 32˚C/W 80˚C/W 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 100µA ISET ≤ 1mA 100µA ≤ ISET ≤ 1mA 1.5 ≤ V+ ≤ 5V 5V ≤ V+ ≤ 30V 100µA ≤ ISET ≤ 1mA 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 100µA ≤ ISET ≤ 1mA 14 18 Conditions Min 100µA ≤ ISET ≤ 1mA TJ = 25˚ LM234-3 Typ Max Min LM234-6 Typ Max Units ±1 ±3 26 14 18 ±2 ±6 26 % ˚C ±2 15 ±3 % pF 3 www.national.com LM134/LM234/LM334 Typical Performance Characteristics Output Impedance Maximum Slew Rate Linear Operation 00569730 00569731 Start-Up Transient Response 00569732 00569733 Voltage Across RSET (VR) Current Noise 00569734 00569735 www.national.com 4 LM134/LM234/LM334 Typical Performance Characteristics Turn-On Voltage (Continued) Ratio of ISET to IBIAS 00569729 00569703 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 = 10µA, maximum dV/dt is 0.01V/µs; at ISET = 1mA, the limit is 1V/µs. 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 100µA. For example, each 1V increase across the LM134 at ISET = 1 mA will increase junction temperature by ≈0.4˚C 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 100µA. 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 5 www.national.com 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 2µA ≤ ISET ≤ 1mA, the equation can be further simplified to 00569727 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 214µV/˚K (64 mV/298˚K ∼ 214µV/˚K). Since (for a given set current) IBIAS is simply a percentage of ISET, the equation can be rewritten LM134/LM234/LM334 Application Hints (Continued) regulation by at least an order of magnitude. DC characteristics (with the exception of minimum input voltage), are not affected. NOISE 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 12dB. 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 100mV. 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 0.7Ω 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 0˚K, independent of RSET or any initial inaccuracy. 00569704 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. 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). www.national.com 6 LM134/LM234/LM334 Application Hints (Continued) 00569728 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 +227µV/˚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). 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 1mA, Zero-Tempco Current Source First, solve for R1 and R2: 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 = 25˚C 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 67.7mV (64 mV + 5.9% to account for IBIAS), and R2/R1 = 10 (from the previous calculations). The values of R1 and R2 can be changed to standard 1% resistor values (R1 = 133Ω and R2 = 1.33kΩ) with less than a 0.75% error. 7 www.national.com LM134/LM234/LM334 Application Hints (Continued) Terminating Remote Sensor for Voltage Output 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 an error of less than 5%. 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 2.6mV/˚C instead of 2.5mV/˚C (an error of 4%). The tempco of the circuit is now: 00569714 Low Output Impedance Thermometer A 1mA LM134 current source with no temperature compensation would have a set resistor of 68Ω and a resulting tempco of 00569706 *Output impedance of the LM134 at the “R” pin is approximately 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 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 00569715 *Select R3 = VREF/583µA. VREF may be any stable positive voltage ≥ 2V Trim R3 to calibrate 00569716 www.national.com 8 LM134/LM234/LM334 Typical Applications (Continued) Higher Output Current 00569705 *Select R1 and C1 for optimum stability Basic 2-Terminal Current Source 00569701 Micropower Bias Low Input Voltage Reference Driver 00569718 00569717 9 www.national.com LM134/LM234/LM334 Typical Applications (Continued) Ramp Generator 00569719 1.2V Reference Operates on 10 µA and 2V Zener Biasing 00569749 00569720 Alternate Trimming Technique *Select ratio of R1 to R2 to obtain zero temperature drift 1.2V Regulator with 1.8V Minimum Input 00569750 *For ± 10% adjustment, select RSET10% high, and make R1 ≈ 3 RSET 00569707 *Select ratio of R1 to R2 for zero temperature drift www.national.com 10 LM134/LM234/LM334 Typical Applications (Continued) Buffer for Photoconductive Cell 00569751 FET Cascoding for Low Capacitance and/or Ultra High Output Impedance 00569721 *Select Q1 or Q2 to ensure at least 1V across the LM134. Vp (1 − ISET/IDSS) ≥ 1.2V. 00569722 FIGURE 4. Generating Negative Output Impedance In-Line Current Limiter 00569723 *ZOUT ≈ −16 • R1 (R1/VIN must not exceed ISET) 00569709 *Use minimum value required to ensure stability of protected device. This minimizes inrush current to a direct short. Schematic Diagram 11 www.national.com LM134/LM234/LM334 Schematic Diagram (Continued) 00569711 www.national.com 12 LM134/LM234/LM334 Physical Dimensions unless otherwise noted inches (millimeters) Order Number LM134H, LM234H or LM334H NS Package Number H03H SO Package (M) Order Number LM334M, LM334MX, LM334SM or LM334SMX NS Package Number M08A 13 www.national.com LM134/LM234/LM334 3-Terminal Adjustable Current Sources Physical Dimensions inches (millimeters) unless otherwise noted (Continued) Order Number LM334Z, LM234Z-3 or LM234Z-6 NS Package Number Z03A 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. For the most current product information visit us at www.national.com. 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