Datasheet MAX931, MAX932, MAX933, MAX934 (Analog Devices) - 10

Ultra Low-Power, Low-Cost Comparators with 2% Reference
maximum difference between REF and HYST (50mV) 2. Choose the hysteresis voltage (VHB), the voltage will therefore produce a 100mV max hysteresis band. between the upper and lower thresholds. In this Use the following equations to determine R1 and R2: example, choose VHB = 50mV. V 3. Calculate R1. R1 = HB V 2 ( × I ) REF R1 = R3 HB × V+  V  1.182 – HB 0.05  2  = 10M × R2 = 5 IREF = 100kΩ Where IREF (the current sourced by the reference) 4. Choose the threshold voltage for VIN rising (VTHR). In should not exceed the REF source capability, and this example, choose VTHR = 3V. should be significantly larger than the HYST input 5. Calculate R2. current. IREF values between 0.1µA and 4µA are usually appropriate. If 2.4MΩ is chosen for 1 R2 =
MAX931-MAX934
R2 (I   REF = 0.5µA), the equation for R1 and VHB can be  V  1 1 THR approximated as:    − −   (V R1 R3  REF R1) ×  R1 (k ) Ω = V (mV)   HB 1 When hysteresis is obtained in this manner for = the MAX932/MAX933, the same hysteresis applies to   3  1 1  both comparators.    (1.182 100k)  ×  − − 100k 10M  
Hysteresis (MAX934)
= 65.44kΩ Hysteresis can be set with two resistors using positive A 1% preferred value is 64.9kΩ. feedback, as shown in Figure 4. This circuit generally draws more current than the circuits using the HYST 6. Verify the threshold voltages with these formulas: pin on the MAX931/MAX932/MAX933, and the high V rising : IN feedback impedance slows hysteresis. The design procedure is as follows:  1 1 1  V = V × R1 × + + THR REF   1. Choose R3. The leakage current of IN+ is under 1nA R1 R2 R3 (up to +85°C), so the current through R3 can be around 100nA and still maintain good accuracy. V falling : IN The current through R3 at the trip point is VREF/R3, or 100nA for R3 = 11.8MΩ. 10MΩ is a good R1 ( × V+) V = V − practical value. THF THR R3 2.5V TO 11V V+ 7 R3 IREF 6 V+ REF R1 VIN R1 MAX931 V+ OUT MAX932 R2 MAX934 V- MAX933 5 GND HYST V- R2 VREF 2 Figure 3. Programming the HYST Pin Figure 4. External Hysteresis
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