Datasheet MAX987, MAX988, MAX991, MAX992, MAX995, MAX996 (Maxim) - 8

link to page 4 MAX987/MAX988/MAX991/ High-Speed, Micropower, Low-Voltage, MAX992/MAX995/MAX996 Rail-to-Rail I/O Comparators
Detailed Description Applications Information
The MAX987/MAX988/MAX991/MAX992/MAX995/
Additional Hysteresis
MAX996 are single/dual/quad low-power, low-voltage comparators. They have an operating supply voltage
MAX987/MAX991/MAX995
range between +2.5V and +5.5V and consume only 48µA The MAX987/MAX991/MAX995 have ±2.5mV internal per comparator, while achieving 120ns propagation delay. hysteresis. Additional hysteresis can be generated Their common-mode input voltage range extends 0.25V with three resistors using positive feedback (Figure 1). beyond each rail. Internal hysteresis ensures clean output Unfortunately, this method also slows hysteresis response switching, even with slow-moving input signals. Large time. Use the following procedure to calculate resistor internal output drivers allow rail-to-rail output swing with values for the MAX987/MAX991/MAX995. up to 8mA loads. 1) Select R3. Leakage current at IN is under 10nA; therefore, The output stage employs a unique design that minimizes the current through R3 should be at least 1µA to supply-current surges while switching, virtually eliminating minimize errors caused by leakage current. The the supply glitches typical of many other comparators. current through R3 at the trip point is (VREF - VOUT) The MAX987/MAX991/MAX995 have a push-pull / R3. Considering the two possible output states and output structure that sinks as well as sources current. solving for R3 yields two formulas: R3 = VREF / 1µA or The MAX988/MAX992/MAX996 have an open-drain output R3 = (VREF - VCC) / 1µA. Use the smaller of the two stage that can be pulled beyond VCC to an absolute maximum resulting resistor values. For example, if VREF = 1.2V of 6V above VEE. and VCC = 5V, then the two R3 resistor values are 1.2MΩ and 3.8MΩ. Choose a 1.2MΩ standard value
Input Stage Circuitry
for R3. The devices’ input common-mode range extends from 2) Choose the hysteresis band required (V -0.25V to (V HB). For this CC + 0.25V). These comparators may operate at example, choose 50mV. any differential input voltage within these limits. Input bias current is typically 1.0pA if the input voltage is between 3) Calculate R1 according to the following equation: the supply rails. Comparator inputs are protected from R1 = R3 x (VHB / VCC) overvoltage by internal body diodes connected to the For this example, insert the values R1 = 1.2MΩ x supply rails. As the input voltage exceeds the supply rails, (50mV / 5V) = 12kΩ. these body diodes become forward biased and begin to conduct. Consequently, bias currents increase exponential y 4) Choose the trip point for VIN rising (VTHR; VTHF is as the input voltage exceeds the supply rails. the trip point for VIN falling). This is the threshold voltage at which the comparator switches its output
Output Stage Circuitry
from low to high as VIN rises above the trip point. For These comparators contain a unique output stage this example, choose 3V. capable of rail-to-rail operation with up to 8mA loads. Many comparators consume orders of magnitude more VCC current during switching than during steady-state operation. R3 However, with this family of comparators, the supply- current change during an output transition is extremely R1 0.1µF small. The Supply Current vs. Output Transition Frequency VIN graph in the Typical Operating Characteristics section VCC OUT shows the minimal supply-current increase as the output R2 VEE switching frequency approaches 1MHz. This characteristic
MAX987
eliminates the need for power-supply filter capacitors to
MAX991
reduce glitches created by comparator switching currents. VREF
MAX995
Battery life increases substantially in high-speed, battery- powered applications. Figure 1. Additional Hysteresis (MAX987/MAX991/MAX995) www.maximintegrated.com Maxim Integrated │ 8