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MAX972EBL-T Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX972EBL-T Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 17 page Ultra-Low-Power, Open-Drain, Single/Dual-Supply Comparators ______________________________________________________________________________________ 11 __________Applications Information Hysteresis Hysteresis increases the comparators’ noise margin by increasing the upper threshold and decreasing the lower threshold (Figure 2). Hysteresis (MAX9_1/MAX982/MAX9_3) To add hysteresis to the MAX9_1, MAX982, or MAX9_3, connect resistor R1 between REF and HYST, and connect resistor R2 between HYST and V- (Figure 3). If no hysteresis is required, connect HYST to REF. When hysteresis is added, the upper threshold increases by the same amount that the lower threshold decreases. The hysteresis band (the difference between the upper and lower thresholds, VHB) is approximately equal to twice the voltage between REF and HYST. The HYST input can be adjusted to a maximum voltage of REF and to a minimum voltage of (REF - 50mV). The maximum difference between REF and HYST (50mV) will therefore produce a 100mV (max) hysteresis band. Use the following equations to determine R1 and R2: where IREF (the current sourced by the reference) should not exceed the REF source capability, and should be significantly larger than the HYST input current. IREF values between 0.1µA and 4µA are usually appropriate. If 2.4M Ω is chosen for R2 (IREF = 0.5µA), the equation for R1 and VHB can be approximated as: When hysteresis is obtained in this manner for the MAX982/MAX9_3, the same hysteresis applies to both comparators. Hysteresis (MAX972/MAX9_4) Hysteresis can be implemented with any comparator using positive feedback, as shown in Figure 4. This approach generally draws more current than circuits using the HYST pin on the MAX9_1/MAX982/MAX9_3, and the high feedback impedance slows hysteresis. In addition, because the output does not source current, any increase in the upper threshold is dependent on the load or pullup resistor on the output. Board Layout and Bypassing Power-supply bypass capacitors are not needed if the supply impedance is low, but 100nF bypass capacitors should be used when the supply impedance is high or when the supply leads are long. Minimize signal lead lengths to reduce stray capacitance between the input and output that might cause instability. Do not bypass the reference output. Window Detector The MAX9_3 is ideal for making window detectors (undervoltage/overvoltage detectors). The schematic is shown in Figure 5, with component values selected for a 4.5V undervoltage threshold and a 5.5V overvoltage threshold. Choose different thresholds by changing the values of R1, R2, and R3. To prevent chatter at the output when the supply voltage is close to a threshold, hysteresis has been added using R4 and R5. Taken alone, OUTA would provide an active-low undervoltage indication, and OUTB would give an active-low overvoltage indication. Wired-ORing the two outputs provides an active-high, power-good signal. The design procedure is as follows: 1) Choose the required hysteresis level and calculate values for R4 and R5 according to the formulas in the Hysteresis (MAX9_1/MAX982/MAX9_3) section. In this example, ±5mV of hysteresis has been added at the comparator input (VH = VHB/2). This means that the hysteresis apparent at VIN will be larger because of the input resistor divider. R1 (k ) = V (mV) HB Ω R1 = V 2 I R2 = 1.182 – V 2 I HB REF HB REF × () GND V- V+ MAX9_4 OUT RH RPULLUP VREF VIN V+ Figure 4. External Hysteresis |
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