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

Since the 7805 will continue to provide a 5V output 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

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

temperature of the NPN transistor reaches the level

selected for the high-temperature alarm. R2 can be

NPN transistor’s temperature reaches the low-tempera-

ture limit.