Datasheet LM334S (Analog Devices) - 5

LM134 Series
U U W U APPLICATIO S I FOR ATIO Basic Theory of Operation
the device is ±2% when the room temperature current is set to the exact desired value. The equivalent circuit of the LM134 is shown in Figure 1. A reference voltage of 64mV is applied to the minus input
Supply Voltage Slew Rate
of A1 with respect to the V– pin. A1 serves the drive to Q2 to keep the R pin at 64mV, independent of the value of At slew rates above a given threshold (see curve), the R LM134 may exhibit nonlinear current shifts. The slewing SET. Transistor Q1 is matched to Q2 at a 17:1 ratio so that the current flowing out of the V– pin is always 1/18 of the rate at which this occurs is directly proportional to ISET. At total current into the V+ pin. This total current is called I I SET SET = 10µA, maximum dv/dt is 0.01V/µs; at ISET = 1mA, and is equal to: the limits is 1V/µs. Slew rates above the limit do not harm the LM134, or cause large currents to flow.  64mV 18 67 7mV
Thermal Effects
 R 17 = . R SET SET Internal heating can have a significant effect on current V+ regulation for I I SET greater than 100µA. For example, each SET 1V increase across the LM134 at ISET = 1mA will increase Q1 Q2 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) R A1 = 0.132%. This is a 10:1 degradation in regulation com- – pared to true electrical effects. Thermal effects, therefore, + RSET must be taken into account when DC regulation is critical 64mV and ISET exceeds 100µA. Heat sinking of the TO-46 pack- – age or the TO-92 leads can reduce this effect by more than V– 3:1. 134 F01
Figure 1. Shunt Capacitance
The 67.7mV equivalent reference voltage is directly pro- In certain applications, the 15pF shunt capacitance of the portional to absolute temperature in degrees Kelvin (see LM134 may have to be reduced, either because of loading curve, “Operating Current vs Temperature”). This means problems or because it limits the AC output impedance of that the reference voltage can be plotted as a straight line the current source. This can be easily accomplished by going from 0mV at absolute zero temperature to 67.7mV buffering the LM134 with a FET, as shown in the applica- at 298°K (25°C). The slope of this line is 67.7mV/298 = tions. This can reduce capacitance to less than 3pF and 227µV/°C. improve regulation by at least an order of magnitude. DC The accuracy of the device is specified as a percent error characteristics (with the exception of minimum input at room temperature, or in the case of the -3 and -6 voltage) are not affected. devices, as both a percent error and an equivalent tem- perature error. The LM134 operating current changes at a
Noise
percent rate equal to (100)(227µV/°C)/(67.7mV) = 0.336%/ Current noise generated by the LM134 is approximately 4 °C at 25°C, so each 1% operating current error is equiva- times the shot noise of a transistor. If the LM134 is used lent to ≈3°C temperature error when the device is used as as an active load for a transistor amplifier, input referred a temperature sensor. The slope accuracy (temperature noise will be increased by about 12dB. In many cases, this coefficient) of the LM134 is expressed as a ratio com- is acceptable and a single stage amplifier can be built with pared to unity. The LM134-3, for instance, is specified at a voltage gain exceeding 2000. 0.98 to 1.02, indicating that the maximum slope error of 5