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ICL7665AEPA Datasheet(PDF) 9 Page - Maxim Integrated Products |
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ICL7665AEPA Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 12 page Microprocessor Voltage Monitor with Dual Over/Undervoltage Detection _______________________________________________________________________________________ 9 Combination Low-Battery Warning and Low-Battery Disconnect Nickel cadmium (NiCd) batteries are excellent recharge- able power sources for portable equipment, but care must be taken to ensure that NiCd batteries are not damaged by overdischarge. Specifically, a NiCd battery should not be discharged to the point where the polarity of the lowest-capacity cell is reversed, and that cell is reverse charged by the higher-capacity cells. This reverse charging will dramatically reduce the life of a NiCd battery. The Figure 8 circuit both prevents reverse charging and gives a low-battery warning. A typical low-battery warning voltage is 1V per cell. Since a NiCd “9V” battery is ordi- narily made up of six cells with a nominal voltage of 7.2V, a low-battery warning of 6V is appropriate, with a small hysteresis of 100mV. To prevent overdischarge of a bat- tery, the load should be disconnected when the battery voltage is 1V x (N – 1), where N = number of cells. In this case, the low-battery load disconnect should occur at 5V. Since the battery voltage will rise when the load is disconnected, 800mV of hysteresis is used to prevent repeated on/off cycling. Power-Fail Warning and Power-Up/Power-Down Reset Figure 9 illustrates a power-fail warning circuit that monitors raw DC input voltage to the 7805 three-termi- nal 5V regulator. The power-fail warning signal goes high when the unregulated DC input falls below 8.0V. When the raw DC power source is disconnected or the AC power fails, the voltage on the input of the 7805 decays at a rate of IOUT / C (in this case, 200mV/ms). Since the 7805 will continue to provide a 5V output at 1A until VIN is less than 7.3V, this circuit will give at least 3.5ms of warning before the 5V output begins to drop. If additional warning time is needed, either the trip voltage or filter capacitance should be increased, or the output current should be decreased. The ICL7665 OUT2 is set to trip when the 5V output has decayed to 3.9V. This output can be used to prevent the microprocessor from writing spurious data to a CMOS battery-backup memory, or can be used to acti- vate a battery-backup system. AC Power-Fail and Brownout Detector By monitoring the secondary of the transformer, the cir- cuit in Figure 10 performs the same power-failure warn- ing function as Figure 9. With a normal 110V AC input to the transformer, OUT1 will discharge C1 every 16.7ms when the peak transformer secondary voltage exceeds 10.2V. When the 110V AC power-line voltage is either interrupted or reduced so that the peak voltage is less than 10.2V, C1 will be charged through R1. OUT2, the power-fail warning output, goes high when the voltage on C1 reaches 1.3V. The time constant R1 x C1 determines the delay time before the power-fail warning signal is activated, in this case 42ms or 21⁄2 line cycles. Optional components R2, R3 and Q1 add hysteresis by increasing the peak secondary voltage required to dis- charge C1 once the power-fail warning is active. Battery Switchover Circuit The circuit in Figure 11 performs two functions: switch- ing the power supply of a CMOS memory to a backup battery when the line-powered supply is turned off, and lighting a low-battery-warning LED when the backup battery is nearly discharged. The PNP transistor, Q1, connects the line-powered +5V to the CMOS memory whenever the line-powered +5V supply voltage is greater than 3.5V. The voltage drop across Q1 will only be a couple of hundred millivolts, since it will be satu- rated. Whenever the input voltage falls below 3.5V, OUT1 goes high, turns off Q1, and connects the 3V lithium cell to the CMOS memory. The second voltage detector of the ICL7665 monitors the voltage of the lithium cell. If the battery voltage falls below 2.6V, OUT2 goes low and the low-battery-warning LED turns on (assuming that the +5V is present, of course). Another possible use for the second section of the ICL7665 is the detection of the input voltage falling below 4.5V. This signal could then be used to prevent the microprocessor from writing spurious data to the CMOS memory while its power-supply voltage is out- side its guaranteed operating range. Simple High/Low Temperature Alarm The circuit in Figure 12 is a simple high/low tempera- ture alarm, which uses a low-cost NPN transistor as the sensor and an ICL7665 as the high/low detector. The NPN transistor and potentiometer R1 form a Vbe multi- plier whose output voltage is determined by the Vbe of the transistor and the position of R1’s wiper arm. The voltage at the top of R1 will have a temperature coeffi- cient of approximately -5mV/°C. R1 is set so that the voltage at VSET2 equals the VSET2 trip voltage when the temperature of the NPN transistor reaches the level selected for the high-temperature alarm. R2 can be adjusted so that the voltage at VSET1 is 1.3V when the NPN transistor’s temperature reaches the low-tempera- ture limit. |
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