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LM27CIM5-2HJ Datasheet(PDF) 5 Page - National Semiconductor (TI) |
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LM27CIM5-2HJ Datasheet(HTML) 5 Page - National Semiconductor (TI) |
5 / 10 page Functional Description LM27 OPTIONS The LM27 can be factory programmed to have a trip point anywhere in-between 120˚C to 150˚C. Applications Hints AFTER-ASSEMBLY PCB TESTING The LM27’s V TEMP output allows after-assembly PCB testing by following a simple test procedure. Simply measuring the V TEMP output voltage will verify that the LM27 has been assembled properly and that its temperature sensing cir- cuitry is functional. The V TEMP output has very weak drive capability that can be overdriven by 1.5mA. Therefore, one can simply force the V TEMP voltage to cause the digital output to change state, thereby verifying that the comparator and output circuitry function after assembly. Here is a sample test procedure that can be used to test the LM27CIM5X-2HJ which has a 140˚C trip point. 1. Turn on V + and measure V TEMP. Then calculate the temperature reading of the LM27 using the equation: V O = (−3.552x10 −6x(T−30)2)+ (−10.69576x10 −3x(T−30)) + 1.8386V (1) or (2) 2. Verify that the temperature measured in step one is within (±3˚C + error of reference temperature sensor) of the ambient/board temperature. The ambient/board tem- perature (reference temperature) should be measured using an extremely accurate calibrated temperature sen- sor, which is in close proximity to and mounted on the same PCB as the LM27 perhaps even touching the GND lead of the LM27 if possible. The LM27 will sence the board temperature not the ambient temperature (see Section Titled Mounting Considerations) 3. A. Observe that OS is high. B. Drive V TEMP to ground. C. Observe that OS is now low. D. Release the V TEMP pin. E. Observe that OS is now high. 4. A. Observe that OS is high. B. Drive V TEMP voltage down gradually. C. When OS goes low, note the V TEMP voltage. D. V TEMPTrig=VTEMP at OS trigger (HIGH->LOW) E. Calculate Ttrig using Equation (2). 5. A. Gradually raise V TEMP until OS goes HIGH. Note V TEMP. B. Calculate T HYST using Equation (2). V TEMP LOADING The V TEMP output has very weak drive capability (1 µA source, 40 µA sink). So care should be taken when attaching circuitry to this pin. Capacitive loading may cause the V TEMP output to oscillate. Simply adding a resistor in series as shown in Figure 2 will prevent oscillations from occurring. To determine the value of the resistor follow the guidelines given in Table 1. The same value resistor will work for either placement of the resistor. If an additional capacitive load is placed directly on the LM27 output, rather than across C LOAD, it should be at least a factor of 10 smaller than C LOAD. 20030712 LM27-_ _J 20030713 LM27-_ _K 20030714 LM27-_ _L 20030715 LM27-_ _N FIGURE 1. Output Pin Options Block Diagrams www.national.com 5 |
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