Microprocessor supervisor and regulator form in-range voltage monitor
Edited by Bill Travis
Ilie Poenaru and Sabin Eftimie, Catalyst Semiconductor, Bucharest, Romania -- EDN, June 10, 2004
Low-cost microprocessor supervisors reset controllers when power-supply voltages fall below given levels. As added protection, you can also reset the microcontroller when the power supply is too high by combining a low-cost shunt-voltage regulator with a supervisor that has a manual reset input. A simple overvoltage/undervoltage-protection circuit is easy to make (Figure 1). The circuit's output is active (low) when the monitored supply voltage, VCC, is outside a predefined range. After the supply voltage returns to within the functioning limits, the reset output, 
remains active for a minimum of 140 msec. This interval gives the system time to stabilize before you remove the reset. The threshold voltage of the CAT811 microprocessor reset circuit, IC2, sets the lower limit of the voltage range, VLOW. This threshold can be 2.32 to 4.63V using standard products. Custom threshold devices with thresholds as low as 1.8V are also available. The 1.24V CAT431L shunt regulator, IC1, and two resistors, R1 and R2, set the upper limit, VHIGH: VHIGH=VREF(1+R1/R2), where VREF is the internal reference voltage of IC1 (VREF=1.24V). The maximum VHIGH that you can set is 5.5V, and the maximum supply voltage is 9V.
This design uses the CAT811 supervisor with a threshold voltage of 4.63V for tests. The upper limit of the voltage range is 5.5V using a 10-kΩ potentiometer with R1=7.75 kΩ and R2=2.25 kΩ. As the supply voltage rises into range, the reset pin becomes active (low) for a minimum of 140 msec (Figure 2). When the supply voltage falls into range, the reset pin also becomes active (low) for a minimum of 140 msec (Figure 3). A reset signal asserts when the supply voltage increases out of range (Figure 4). A reset signal also asserts when the supply voltage falls out of range (Figure 5).
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