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LM334SM Datasheet(PDF) 6 Page  Texas Instruments 


6 page LM134, LM234, LM334 SNVS746E â€“ MARCH 2000 â€“ REVISED MAY 2013 www.ti.com 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 13. Figure 13. Basic Current Source A graph showing the ratio of these two currents is supplied under Ratio of ISET to IBIAS in Typical Performance Characteristics. The current flowing through RSET is determined by VR, which is approximately 214Î¼V/Â°K (64 mV/298Â°K âˆ¼ 214Î¼V/Â°K). (1) Since (for a given set current) IBIAS is simply a percentage of ISET, the equation can be rewritten where â€¢ n is the ratio of ISET to IBIAS as specified in Electrical Characteristics and shown in the graph (2) Since n is typically 18 for 2 Î¼A â‰¤ ISET â‰¤ 1mA, the equation can be further simplified to (3) 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 ISET. At ISET = 10Î¼A, maximum dV/dt is 0.01V/Î¼s; at ISET = 1mA, the limit is 1V/ Î¼s. 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 100Î¼A. For example, each 1V increase across the LM134 at ISET = 1 mA will increase junction temperature by â‰ˆ0.4Â°C in still air. Output current (ISET) has a temperature coefficient of â‰ˆ0.33%/Â°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 100Î¼A. Heat sinking of the TO package or the TO92 leads can reduce this effect by more than 3:1. 6 Submit Documentation Feedback Copyright Â© 2000â€“2013, Texas Instruments Incorporated Product Folder Links: LM134 LM234 LM334 
