MAX9647_12

 General-Purpose, Low-Voltage, Tiny Pack Comparators ( 11 Page)

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 9 page ����������������������������������������������������������������� Maxim Integrated Products 9MAX9647/MAX9648General-Purpose, Low-Voltage,Tiny Pack ComparatorsAdditional hysteresis can be generated with two resis-tors using positive feedback (Figure 2). Use the followingprocedure to calculate resistor values:1) Find output voltage when output is high:VOUT(HIGH) = VDD - ILOAD x RL2) Find the trip points of the comparator using theseformulas: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 abovethe trip point, and VTL is the threshold voltage at whichthe comparator switches its output from low to high asVIN 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) ≈ 5.0VVTH = 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))V0.050V = 5(1000/(R1 + 1000))Vwhere R1≈ 100kI, VTH = 2.525V, and VTL = 2.475VChoose R1 and R2 to be large enough as not to exceedthe 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 excesscurrent, yet small enough to supply the necessary cur-rent to drive the load. RL should be between 1kI and10kI. Choose R1 to be much larger than RL to avoidlowering VOUT(HIGH) ir raising VOUT(LOW).Board Layout and BypassingUse 0.1FF bypass capacitors from VDD to VSS. To maxi-mize performance, minimize stray inductance by puttingthis capacitor close to the VDD pin and reducing tracelengths. For slow moving input signals (rise time > 1ms),use a 1nF capacitor between IN+ and IN- to reduce highfrequency noise.Figure 1. Threshold Hysteresis Band (Not to Scale)Figure 2. Adding Hysteresis with External ResistorsIN+THRESHOLDSIN-VHYSTVTLVTHHYSTERESISBANDOUTIN+OUTRLR1R2VREFVINVDDVOUTVDDVSSIN-MAX9647