ICL7665AESA Maxim Integrated Products, ICL7665AESA Datasheet - Page 9

• Manufacturer Part Number: ICL7665AESA
• Description: Supervisory Circuits Integrated Circuits (ICs)
• Manufacturer: Maxim Integrated Products

Specifications of ICL7665AESA

• Number Of Voltages Monitored: 2
• Undervoltage Threshold: 1.275 V, 1.225 V
• Overvoltage Threshold: 1.325 V, 1.375 V
• Manual Reset: Not Resettable
• Watchdog: No Watchdog
• Supply Voltage (max): 16 V
• Supply Voltage (min): 2 V
• Supply Current (typ): 15 uA
• Maximum Power Dissipation: 471 mW
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• Package / Case: SOIC-8
• Minimum Operating Temperature: - 40 C
• Lead Free Status / Rohs Status: No

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.
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 I
Since the 7805 will continue to provide a 5V output at
1A until V
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.
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
Combination Low-Battery Warning and
AC Power-Fail and Brownout Detector
IN
is less than 7.3V, this circuit will give at
Power-Up/Power-Down Reset
_______________________________________________________________________________________
OUT
Low-Battery Disconnect
Power-Fail Warning and
/ C (in this case, 200mV/ms).
Microprocessor Voltage Monitor with
Dual Over/Undervoltage Detection
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 2
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.
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.
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 V
temperature of the NPN transistor reaches the level
selected for the high-temperature alarm. R2 can be
adjusted so that the voltage at V
NPN transistor’s temperature reaches the low-tempera-
ture limit.
Simple High/Low Temperature Alarm
SET2
equals the V
Battery Switchover Circuit
SET2
SET1
trip voltage when the
is 1.3V when the
1
⁄
2
line cycles.
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