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MAX931CPA Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX931CPA Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 16 page 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 MAX932/MAX933, the same hysteresis applies to both comparators. Hysteresis (MAX934) 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 MAX931/MAX932/MAX933, 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. 6. Verify the threshold voltages with these formulas: V rising : V V R1 1 R1 1 R2 1 R3 V falling : V V R1 V R3 IN THR REF IN THF THR =× × + + =− ×+ () 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, Low-Cost Comparators with 2% Reference 10 ______________________________________________________________________________________ GND V- V+ MAX934 OUT R3 R1 R2 VREF VIN V+ Figure 4. External Hysteresis 7 2 5 6 HYST REF V- V+ R1 R2 MAX931 MAX932 MAX933 2.5V TO 11V IREF Figure 3. Programming the HYST Pin |
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