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����������������������������������������������������������������� Maxim Integrated Products 9 MAX9647/MAX9648 General-Purpose, Low-Voltage, Tiny Pack Comparators Additional hysteresis can be generated with two resis- tors using positive feedback (Figure 2). Use the following procedure to calculate resistor values: 1) Find output voltage when output is high: VOUT(HIGH) = VDD - ILOAD x RL 2) Find the trip points of the comparator using these formulas: VTH = VREF + ((VOUT(HIGH) - VREF)R2)/(R1 + R2) VTL = VREF(1 - (R2/(R1 + R2))) where VTH is the threshold voltage at which the compara- tor switches its output from high to low as VIN rises above the trip point, and VTL is the threshold voltage at which the comparator switches its output from low to high as VIN drops below the trip point. 3) The hysteresis band is: VHYST = VTH - VTL = VDD(R2/(R1 + R2)) In this example, let VDD = 5V, VREF = 2.5V, ILOAD = 50nA, and RL = 5.1kI. VOUT(HIGH) = 5.0V - (50 x 10-9 x 5.1 x 103I) ≈ 5.0V VTH = 2.5 + 2.5(R2/(R1 + R2)) VTL = 2.5(1 - (R2/(R1 + R2))) Select R2. In this example, choose 1kI. Select VHYST. In this example, choose 50mV. Solve for R1. VHYST = VOUT(HIGH)(R2/(R1 + R2))V 0.050V = 5(1000/(R1 + 1000))V where R1 ≈ 100kI, VTH = 2.525V, and VTL = 2.475V Choose R1 and R2 to be large enough as not to exceed the amount of current the reference can supply. The source current required is VREF/(R1 + R2). The sink current is (VOUT(HIGH) - VREF) x (R1 + R2). Choose RL to be large enough to avoid drawing excess current, yet small enough to supply the necessary cur- rent to drive the load. RL should be between 1kI and 10kI. Choose R1 to be much larger than RL to avoid lowering VOUT(HIGH) ir raising VOUT(LOW). Board Layout and Bypassing Use 0.1FF bypass capacitors from VDD to VSS. To maxi- mize performance, minimize stray inductance by putting this capacitor close to the VDD pin and reducing trace lengths. For slow moving input signals (rise time > 1ms), use a 1nF capacitor between IN+ and IN- to reduce high frequency noise. Figure 1. Threshold Hysteresis Band (Not to Scale) Figure 2. Adding Hysteresis with External Resistors IN+ THRESHOLDS IN- VHYST VTL VTH HYSTERESIS BAND OUT IN+ OUT RL R1 R2 VREF VIN VDD VOUT VDD VSS IN- MAX9647 |
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