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MAX922EPA Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX922EPA Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 17 page comparator is used with the reference, the combined peak-to-peak noise is about 1mV. This, of course, is much higher than the RMS noise of the individual components. Care should be taken in the layout to avoid capacitive coupling from any output to the reference pin. Crosstalk can significantly increase the actual noise of the reference. __________Applications Information Hysteresis Hysteresis increases the comparators’ noise margin by increasing the upper threshold and decreasing the lower threshold (see Figure 2). Hysteresis (MAX921/MAX923) To add hysteresis to the MAX921 or MAX923, 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 MAX923, the same hysteresis applies to both comparators. Hysteresis (MAX922/MAX924) Hysteresis can be set with two resistors using positive feedback, as shown in Figure 4. This circuit generally draws more current than the circuits using the HYST pin on the MAX921 and MAX923, and the high feedback impedance slows hysteresis. The design procedure is as follows: 1. Choose R3. The leakage current of IN+ is under 1nA (up to +85°C), so the current through R3 can be around 100nA and still maintain good accuracy. The current through R3 at the trip point is VREF/R3, or 100nA for R3 = 11.8M Ω. 10MΩ is a good practical value. 2. Choose the hysteresis voltage (VHB), the voltage between the upper and lower thresholds. In this example, choose VHB = 50mV. 3. Calculate R1. 4. Choose the threshold voltage for VIN rising (VTHR). In this example, choose VTHR = 3V. 5. Calculate R2. A 1% preferred value is 64.9k Ω. R2 = V (V 1 R1 1 R3 3 (1.182 100k) 1 100k 1 10M 65.44k THR REF R1) 1 1 × ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ −− ⎡ ⎣ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ = × ⎛ ⎝⎜ ⎞ ⎠⎟ −− ⎡ ⎣ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ = Ω R1 = R3 V V 10M 0.05 5 100k HB × + =× =Ω R1 (k ) = V (mV) HB Ω R1 = V 2 I R2 = 1.182 – V 2 I HB REF HB REF × () ⎛ ⎝⎜ ⎞ ⎠⎟ Ultra Low-Power, Single/Dual-Supply Comparators 10 ______________________________________________________________________________________ GND V- V+ MAX924 OUT R3 R1 R2 VREF VIN V+ Figure 4. External Hysteresis |
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