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MAX991ESA Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX991ESA Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 12 page _______________Detailed Description The MAX987/MAX988/MAX991/MAX992/MAX995/ MAX996 are single/dual/quad low-power, low-voltage comparators. They have an operating supply voltage range between +2.5V and +5.5V and consume only 48µA per comparator, while achieving 120ns propaga- tion delay. Their common-mode input voltage range extends 0.25V beyond each rail. Internal hysteresis ensures clean output switching, even with slow-moving input signals. Large internal output drivers allow rail-to- rail output swing with up to 8mA loads. The output stage employs a unique design that mini- mizes supply-current surges while switching, virtually eliminating the supply glitches typical of many other comparators. The MAX987/MAX991/MAX995 have a push/pull output structure that sinks as well as sources current. The MAX988/MAX992/MAX996 have an open- drain output stage that can be pulled beyond VCC to an absolute maximum of 6V above VEE. Input Stage Circuitry The devices’ input common-mode range extends from -0.25V to (VCC + 0.25V). These comparators may oper- ate at any differential input voltage within these limits. Input bias current is typically 1.0pA if the input voltage is between the supply rails. Comparator inputs are pro- tected from overvoltage by internal body diodes con- nected to the supply rails. As the input voltage exceeds the supply rails, these body diodes become forward biased and begin to conduct. Consequently, bias cur- rents increase exponentially as the input voltage exceeds the supply rails. Output Stage Circuitry These comparators contain a unique output stage capable of rail-to-rail operation with up to 8mA loads. Many comparators consume orders of magnitude more current during switching than during steady-state oper- ation. However, with this family of comparators, the supply-current change during an output transition is extremely small. The Typical Operating Characteristics graph Supply Current vs. Output Transition Frequency shows the minimal supply-current increase as the out- put switching frequency approaches 1MHz. This char- acteristic eliminates the need for power-supply filter capacitors to reduce glitches created by comparator switching currents. Battery life increases substantially in high-speed, battery-powered applications. __________Applications Information Additional Hysteresis MAX987/MAX991/MAX995 The MAX987/MAX991/MAX995 have ±2.5mV internal hysteresis. Additional hysteresis can be generated with three resistors using positive feedback (Figure 1). Unfortunately, this method also slows hysteresis response time. Use the following procedure to calcu- late resistor values for the MAX987/MAX991/MAX995. 1) Select R3. Leakage current at IN is under 10nA; therefore, the current through R3 should be at least 1µA to minimize errors caused by leakage current. The current through R3 at the trip point is (VREF - VOUT) / R3. Considering the two possible output states and solving for R3 yields two formulas: R3 = VREF / 1µA or R3 = (VREF - VCC) / 1µA. Use the smaller of the two resulting resistor values. For example, if VREF = 1.2V and VCC = 5V, then the two R3 resistor values are 1.2M Ω and 3.8MΩ. Choose a 1.2M Ω standard value for R3. 2) Choose the hysteresis band required (VHB). For this example, choose 50mV. 3) Calculate R1 according to the following equation: R1 = R3 x (VHB / VCC) For this example, insert the values R1 = 1.2M Ω x (50mV / 5V) = 12k Ω. 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. For this example, choose 3V. High-Speed, Micropower, Low-Voltage, SOT23, Rail-to-Rail I/O Comparators VCC MAX987 MAX991 MAX995 OUT 0.1 µF R3 R1 R2 VREF VEE VIN VCC Figure 1. Additional Hysteresis (MAX987/MAX991/MAX995) 8 _______________________________________________________________________________________ |
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