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MAX978EEE Datasheet(PDF) 7 Page - Maxim Integrated Products |
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MAX978EEE Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 11 page Single/Dual/Quad, SOT23, Single-Supply, High-Speed, Low-Power Comparators _______________________________________________________________________________________ 7 2) Choose the hysteresis band required (VHB). For this example, choose 100mV. 3) Calculate R1. R1 = R3 x (VHB / VCC). Plugging in the values for this example, R1 = 1.2M Ω x (100mV / 5.0V) = 24kΩ 4) Choose the trip point for VIN rising. This is the threshold voltage at which the comparator switches from low to high as VIN rises above the trip point. In this example, choose 3.0V. 5) Calculate R2 as follows: Choose a standard value for R2 of 16k Ω. 6) Verify the trip voltage and hysteresis as follows: IR Receiver The Typical Operating Circuit shows an application using the MAX998 as an infrared receiver. The infrared photo- diode creates a current relative to the amount of infrared light present. This current creates a voltage across RD. When this voltage level crosses the voltage applied by the voltage divider to the inverting input, the output transitions. Window Comparator The MAX976 is ideal for making a window detector (undervoltage/overvoltage detector). The schematic shown in Figure 3 uses a MAX6120 reference and com- ponent values selected for a 2.0V undervoltage thresh- old and a 2.5V overvoltage threshold. Choose different thresholds by changing the values of R1, R2, and R3. OUTA provides an active-low undervoltage indication, and OUTB gives an active-low overvoltage indication. ANDing the two outputs provides an active-high, power-good signal. The design procedure is as follows: 1) Select R1. The leakage current into INB- is normally 75nA, so the current through R1 should exceed 1.0µA for the thresholds to be accurate. R1 values in the 50k Ω to 100kΩ range are typical. 2) Choose the overvoltage threshold (VOTH) when VIN is rising, and calculate R2 and R3 with the following formula: RSUM = R2 + R3 = R1 x [VOTH / (VREF + VH) - 1] where VH = 1/2VHYST. 3) Choose the undervoltage threshold (VUTH) when VIN is falling, and calculate R2 with the following formula: R2 = (R1 + RSUM) x [(VREF - VH) / VUTH] - R1 where VH = 1/2VHYST. 4) Calculate R3 with the following formula: R3 = (RSUM) - R2 5) Verify the resistor values. The equations are as follows: VOTH = (VREF + VH) x (R1 + R2 + R3) / R1 VUTH = (VREF - VH) x (R1 + R2 + R3) / (R1 + R2) V rising: V = V x R1 x 1 R1 V falling IN THR REF IN : ++ ⎛ ⎝⎜ ⎞ ⎠⎟ =− ⎛ ⎝⎜ ⎞ ⎠⎟ =− 1 2 1 3 1 3 RR VV Rx V R Hysteresis V V THF THR CC THR THF R2 = 1 V V x R1 1 R1 1 R3 R2 = 1 3.0V 1.2 x 24k 1 24k 1 1.2M 16.2k THR REF ⎛ ⎝⎜ ⎞ ⎠⎟ −− ⎛ ⎝⎜ ⎞ ⎠⎟ −− = ΩΩ Ω VCC MAX976 MAX978 MAX998 OUT 0.1 μF R3 R1 R2 VREF GND VIN VCC Figure 2. Additional Hysteresis 3 1 3 4 R3 82.1k Ω 1% VCC VIN R2 24.9k Ω 1% R1 100k Ω 1% 2 6 OVERVOLTAGE UNDERVOLTAGE POWER GOOD 1/2 MAX976 MAX6120 1 2 VCC 8 7 5 0.1 μF 1/2 MAX976 Figure 3. Window Comparator |
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