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LM234 Datasheet(PDF) 5 Page - STMicroelectronics |
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LM234 Datasheet(HTML) 5 Page - STMicroelectronics |
5 / 11 page LM134 - LM234 - LM334 5/11 APPLICATION HINTS SLEW RATE At slew rates above a threshold (see curve) the LM134, LM234, LM334 can have a non-linear cur- rent characteristic. The slew rate at which this takes place is directly proportional to Iset. At Iset = 10µA, dv/dt max. = 0.01V/µs ; at Iset = 1mA, dv/dt max. = 1V/µs. Slew rates of more than 1V/µs do not damage the circuit nor do they produce high currents. THERMAL EFFECTS Internal heating can have a significant effect on current regulation for an Iset above 100µA. For ex- ample, 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 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 Iset is higher than 100µA. SHUNT CAPACITANCE In certain applications, the 15pF value for the shunt capacitance should be reduced : u because of loading problems, u because of limitation of output impedance of the current source in a.c. applications. This reduction of capacitance can be easily carried out by adding a FET as indicated in the typical applications. 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 in- put voltage). NOISE The current noise produced by LM134, LM234, 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 ac- ceptable, and a single amplifier can be built with a voltage gain higher than 2000. 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 locat- ing the current setting resistor close to the device. Do not use sockets for the ICs. A contact resis- tance of 0.7 Ω is sufficient to decrease the output current by 1% at the 1mA level. SENSING TEMPERATURE The LM134, LM234, LM334 are excellent remote controlled temperature sensors because their op- eration as current sources preserves their accura- cy even in the case of long connecting wires. The output current is directly proportional to the abso- lute temperature in Kelvin degrees according to the following equation. The calibration of the LM134, LM234, LM334 is simplified by the fact that most of the initial accura- cy is due to gain limitation (slope error) and not an offset. Gain adjustment is a one point trim be- cause the output of the device extrapolates to zero at 0°K. This particularity of the LM134, LM234, LM334 is illustrated in the above diagram. Line abc repre- sents the sensor current before adjustment and line a’b’c’ represents the desired output. A gain adjustment provided at T2 will move the output from b to b’ and will correct the slope at the same time so that the output at T1 and T3 will be correct. This gain adjustment can be carried out by means of Rset or the load resistor used in the circuit. After adjustment, the slope error should be less than 1%. A low temperature coefficient for Rset is nec- essary to keep this accuracy. A 33ppm/°C temper- ature drift of Rset will give an error of 1% on the slope because the resistance follows the same temperature variations as the LM134, LM234, LM334. Three wires are required to isolate Rset from the LM134, LM234, LM334. Since this solu- tion is not recommended. Metal-film resistors with a drift less than 20ppm/°C are now available. Wirewound resistors can be used when very high stability is required. Iset = 227µV/°K) (T () Rset ----------------------------------------- |
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