Datasheet LTC1841, LTC1842, LTC1843 (Analog Devices) - 9

LTC1841/LTC1842/LTC1843
U U W U APPLICATIONS INFORMATION
1. Choose the required hysteresis voltage band and Choose R2 = 61.9k (1% standard value) calculate values for R4 and R5 according to the formu- 5. Calculate R3: las in the hysteresis section. In this example, ±5mV of hysteresis has been added at the comparator input R3 = (R2 + R3) – R2 (VH = VHB/2). Note that the hysteresis apparent at VIN = 1.068M – 61.9k will be larger because of the input resistor divider. = 1.006M 2. Select R1. The leakage current into IN B – is under 1nA so the current through R1 should exceed 100nA, to Choose R3 = 1M (1% standard value) ensure threshold accuracy. R1 values up to about 10M 6. Verify the resistor values. The equations are as can be used, but values in the 100k to 1M range are follows, evaluated for the above example: usually easier to deal with. In this example choose R1 = 294k. Overvoltage threshold: 3. Calculate R2 + R3. The overvoltage threshold should be R1 R2 R3 set at 5.5V. The design equation is as follows: V = V ( +V ) + + OTH REF H R1 =  5 V 474 . V  R2 + R3 = R UTH 1 – 1  V + V Undervoltage threshold: REF H   5.5  = k 294 – 1 R1 R2 R3  = ( – ) + + 1.182 + 0.005  V V V UTH REF H R1+ R2 = 1. M 068 = . 4 V 484 4. Calculate R2. The undervoltage threshold should be set R5 at 4.5V. The design equation is as follows: where the hysteresis voltage V = V ( ) H REF R4 V – V R2 = R1 ( +R2+R REF H 3 – R1 ) VLTH 1.182 – 0 005 = . k 294 ( +1. M 068 – k 294 ) 4.5 = 62. k 2 9