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ICL7665EPA Datasheet(PDF) 8 Page - Maxim Integrated Products |
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ICL7665EPA Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 12 page Microprocessor Voltage Monitor with Dual Over/Undervoltage Detection 8 _______________________________________________________________________________________ Figure 4 1) Choose a resistor value for R11. Typical values are in the 10k Ω to 10MΩ range. 2) Calculate R21 for the desired upper trip point, VU, using the formula: VU - VSET VU – 1.3V R21 = R11 (——————)= R11 (—————) VSET 1.3V 3) Calculate R31 for the desired amount of hysteresis: (R21) (V+ – VSET) (R21) (V+ – 1.3V) R31 = ————————— = ————————— VU – VL VU – VL or, if V+ = VIN: (R21) (VL – VSET) (R21) (VL – 1.3V) R31 = ————————— = ————————— VU – VL VU – VL 4) The trip voltages are not affected by the absolute value of the resistors, as long as the impedances are high enough that the resistance of R31 is much greater than the HYST output’s resistance, and the current through R31 is much higher than the HYST output’s leakage current. Normally, R31 will be in the 100k Ω to 22MΩ range. Multiplying or dividing all three resistors by the same factor will not affect the trip voltages. Figure 5 1) Select a value for R11, usually between 10k Ω and 10M Ω. 2) Calculate R21: VL – VSET VL – 1.3V R21 = R11 (——————) = R11 (—————) VSET 1.3 3) Calculate R31: VU – VL R31 = R11 (—————) VSET 4) As in the other circuits, all three resistor values may be scaled up or down in value without changing VU and VL. VU and VL depend only on the ratio of the three resistors, if the absolute values are such that the hysteresis output resistance and the leakage currents of the VSET input and hysteresis output can be ignored. __________Applications Information Fault Monitor for a Single Supply Figure 6 shows a typical over/undervoltage fault monitor for a single supply. In this case, the upper trip points (con- trolling OUT1) are centered on 5.5V, with 100mV of hys- teresis (VU = 5.55V, VL = 5.45V); and the lower trip points (controlling OUT2) are centered on 4.5V, also with 100mV of hysteresis. OUT1 and OUT2 are connected together in a wire-OR configuration to generate a power-OK signal. Multiple-Supply Fault Monitor The ICL7665 can simultaneously monitor several power supplies, as shown in Figure 7. The easiest way to calculate the resistor values is to note that when the VSET input is at the trip point (1.3V), the current through R11 is 1.3V / R11. The sum of the currents through R21A, R21B and R31 must equal this current when the two input voltages are at the desired low-voltage detection point. Ordinarily, R21A and R21B are chosen so that the current through the two resis- tors is equal. Note that, since the voltage at the ICL7665 VSET input depends on the voltage of both supplies being monitored, there will be some interaction between the low- voltage trip points for the two supplies. In this example, OUT1 will go low when either supply is 10% below nominal (assuming the other supply is at the nominal voltage), or when both supplies are 5% or more below their nominal voltage. R31 sets the hysteresis, in this case, to about 43mV at the 5V supply or 170mV at the 15V supply. The second section of ICL7665 can be used to detect overvoltage or, as shown in Figure 7, can be used to detect the absence of negative supplies. Note that the trip points for OUT2 depend on both the voltages of the negative power supplies and the actual voltage of the +5V supply. Figure 5. Threshold Detector, VIN = V+ VL2 VU2 ICL7665 OUT1 OUT2 SET2 SET1 R21 R11 VIN HYST1 HYST2 OUT1 OUT2 VIN V+ GND OVERVOLTAGE UNDERVOLTAGE R31 R32 R22 R12 VL1 VU1 |
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