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LM334 Datasheet(PDF) 5 Page  National Semiconductor (TI) 


5 page Typical Performance Characteristics (Continued) 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 con tact resistance. CALCULATING R SET The total current through the LM134 (I SET) is the sum of the current going through the SET resistor (I R) and the LM134â€™s bias current (I BIAS), as shown in Figure 1. A graph showing the ratio of these two currents is supplied under Ratio of I SET to IBIAS in the Typical Performance Characteristics section. The current flowing through R SET is determined by V R, which is approximately 214ÂµV/ËšK (64 mV/ 298ËšK âˆ¼ 214ÂµV/ËšK). Since (for a given set current) I BIAS is simply a percentage of I SET, the equation can be rewritten where n is the ratio of I SET to IBIAS as specified in the Elec trical Characteristics Section and shown in the graph. Since n is typically 18 for 2ÂµA â‰¤ I SET â‰¤ 1mA, the equation can be further simplified to for most set currents. SLEW RATE At slew rates above a given threshold (see curve), the LM134 may exhibit nonlinear current shifts. The slewing rate at which this occurs is directly proportional to I SET.At I SET = 10ÂµA, maximum dV/dt is 0.01V/Âµs; at ISET = 1mA, the limit is 1V/Âµs. 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 regu lation for I SET greater than 100ÂµA. For example, each 1V in crease across the LM134 at I SET = 1 mA will increase junc tion temperature by â‰ˆ0.4ËšC in still air. Output current (I SET) has a temperature coefficient of â‰ˆ0.33%/ËšC, 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 I SET exceeds 100ÂµA. Heat sinking of the TO46 package or the TO92 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 regu lation by at least an order of magnitude. DC characteristics (with the exception of minimum input voltage), are not af fected. TurnOn Voltage DS00569729 Ratio of I SET to IBIAS DS0056973 DS00569727 FIGURE 1. Basic Current Source www.national.com 5 
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