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MAX987EUK-T Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX987EUK-T Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 13 page High-Speed, Micropower, Low-Voltage, SOT23, Rail-to-Rail I/O Comparators _______________________________________________________________________________________ 9 5) Calculate R2 as shown. For this example, choose an 8.2kΩ standard value: 6) Verify trip voltages and hysteresis as follows: MAX988/MAX992/MAX996 The MAX988/MAX992/MAX996 have ±2.5mV internal hysteresis. They have open-drain outputs and require an external pullup resistor (Figure 2). Additional hys- teresis can be generated using positive feedback, but the formulas differ slightly from those of the MAX987/MAX991/MAX995. Use the following procedure to calculate resistor values: 1) Select R3 according to the formulas R3 = VREF / 1µA or R3 = (VREF - VCC) / 1µA - R4. Use the smaller of the two resulting resistor values. 2) Choose the hysteresis band required (VHB). For this example, choose 50mV. 3) Calculate R1 according to the following equation: R1 = (R3 + R4) x (VHB / VCC) 4) Choose the trip point for VIN rising (VTHR; VTHF is the trip point for VIN falling). This is the threshold voltage at which the comparator switches its output from low to high as VIN rises above the trip point. 5) Calculate R2 as follows: 6) Verify trip voltages and hysteresis as follows: Circuit Layout and Bypassing These comparators’ high-gain bandwidth requires design precautions to maximize their high-speed capa- bility. The recommended precautions are: 1) Use a PCB with an unbroken, low-inductance ground plane. 2) Place a decoupling capacitor (a 0.1µF ceramic capacitor is a good choice) as close to VCC as possible. 3) On the inputs and outputs, keep lead lengths short to avoid unwanted parasitic feedback around the comparators. 4) Solder the devices directly to the PCB instead of using a socket. V rising: V = V x R1 x 1 R1 V falling IN THR REF IN : ++ + ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ = − + ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ = − 1 2 1 34 1 34 RR R VV Rx V RR Hysteresis V V THF THR CC THR THF R2 = 1 V V THR REF xR R R R 1 1 1 1 34 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ −− + V rising: V = V x R1 x 1 R1 V falling IN THR REF IN : ++ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ = − ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ = − 1 2 1 3 1 3 RR VV Rx V R Hysteresis V V THF THR CC THR THF R2 = 1 V V R2 = 1 3.0V 1.2 x 12k THR REF . . xR R R kM k 1 1 1 1 3 1 12 1 22 803 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ −− ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ −− = ΩΩ Ω Ω VEE VCC OUT R3 R2 R1 R4 0.1µF VREF VIN VCC MAX988 MAX992 MAX996 Figure 2. Additional Hysteresis (MAX988/MAX992/MAX996) |
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