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LM56BIMMX Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LM56BIMMX Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 13 page Application Hints 1.0 LM56 TRIP POINT ACCURACY SPECIFICATION For simplicity the following is an analysis of the trip point accuracy using the single output configuration show in Fig- ure 2 with a set point of 82˚C. Trip Point Error Voltage = V TPE, Comparator Offset Error for V T1E Temperature Sensor Error = V TSE Reference Output Error = V RE 1. V TPE = ±VT1E −VTSE +VRE Where: 2. V T1E = ±8 mV (max) 3. V TSE = (6.20 mV/˚C) x (±3˚C) = ±18.6 mV 4. V RE = 1.250V x (±0.01) R2/(R1 + R2) Using Equations from page 1 of the datasheet. V T1=1.25VxR2/(R1+R2)=(6.20 mV/˚C)(82˚C) +395 mV Solving for R2/(R1 + R2) = 0.7227 then, 5. V RE = 1.250V x (±0.01) R2/(R1 + R2) = (0.0125) x (0.7227) = ±9.03 mV The individual errors do not add algebraically because, the odds of all the errors being at their extremes are rare. This is proven by the fact the specification for the trip point accuracy stated in the Electrical Characteristic for the temperature range of −40˚C to +125˚C, for example, is specified at ±3˚C for the LM56BIM. Note this trip point error specification does not include any error introduced by the tolerance of the actual resistors used, nor any error introduced by power supply variation. If the resistors have a ±0.5% tolerance, an additional error of ±0.4˚C will be introduced. This error will increase to ±0.8˚C when both external resistors have a ±1% tolerance. 2.0 BIAS CURRENT EFFECT ON TRIP POINT ACCURACY Bias current for the comparator inputs is 300 nA (max) each, over the specified temperature range and will not introduce considerable error if the sum of the resistor values are kept to about 27 k Ω as shown in the typical application of Figure 1 . This bias current of one comparator input will not flow if the temperature is well below the trip point level. As the temperature approaches trip point level the bias current will start to flow into the resistor network. When the temperature sensor output is equal to the trip point level the bias current will be 150 nA (max). Once the temperature is well above the trip point level the bias current will be 300 nA (max). There- fore, the first trip point will be affected by 150 nA of bias current. The leakage current is very small when the com- parator input transistor of the different pair is off (see Figure 3). The effect of the bias current on the first trip point can be defined by the following equations: where I B = 300 nA (the maximum specified error). The effect of the bias current on the second trip point can be defined by the following equations: where I B = 300 nA (the maximum specified error). The closer the two trip points are to each other the more significant the error is. Worst case would be when V T1 =VT2 =V REF/2. 01289317 FIGURE 2. Single Output Configuration www.national.com 8 |
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