Datasheet MAX987, MAX988, MAX991, MAX992, MAX995, MAX996 (Maxim) - 9

MAX987/MAX988/MAX991/ High-Speed, Micropower, Low-Voltage, MAX992/MAX995/MAX996 Rail-to-Rail I/O Comparators 5) Calculate R2 as shown. For this example, choose an Use the following procedure to calculate resistor 8.2kΩ standard value: values: 1 1) Select R3 according to the formulas R3 = V R2 = REF / 1µA  V or R3 = (V THR  1 1 REF - VCC) / 1µA - R4. Use the smaller of   − − the two resulting resistor values.  RE V F x R1 R1 R3 2) Choose the hysteresis band required (V 1 HB). For this R2 = 8. = 03kΩ example, choose 50mV.  3.0V  1 1 −  −  3) Calculate R1 according to the following equation: 1.2 x 12kΩ 12kΩ 2.2MΩ R1 = (R3 + R4) x (VHB / VCC) 6) Verify trip voltages and hysteresis as follows: 4) Choose the trip point for VIN rising (VTHR; VTHF is the trip point for V  1 1 1  IN falling). This is the threshold voltage I V N rising: VTHR = RE V F x R1 x + +  at which the comparator switches its output from low R1 R2 R3   to high as VIN rises above the trip point. R1 x VCC  5) Calculate R2 as follows: I V N falling: VTHF V = THR −  R3    1 Hysteresis V = THR V − THF R2 =  VTHR  1 1   − −
MAX988/MAX992/MAX996
RE V F x R1 R1 R3 R +   4 The MAX988/MAX992/MAX996 have ±2.5mV internal 6) Verify trip voltages and hysteresis as follows: hysteresis. They have open-drain outputs and require an external pullup resistor (Figure 2). Additional hysteresis can be generated using positive feedback, but the formulas I V N rising: VTHR = RE V F x R1 x differ slightly from those of the MAX987/MAX991/MAX995.  1 1 1   + + R1 R2 R3 R4 +  R1 x VCC  I V N falling: VTHF V = THR −  VCC  R3 R4  +  Hysteresis V = R3 THR V − THF 0.1µF R1 R4
Circuit Layout and Bypassing
VIN These comparators’ high-gain bandwidth requires design VCC OUT precautions to maximize their high-speed capability. The R2 recommended precautions are: VEE 1) Use a PCB with an unbroken, low-inductance ground
MAX988
plane. V
MAX992
REF
MAX996
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 Figure 2. Additional Hysteresis (MAX988/MAX992/MAX996) to avoid unwanted parasitic feedback around the comparators. 4) Solder the devices directly to the PCB instead of using a socket. www.maximintegrated.com Maxim Integrated │ 9