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ICL7665BCPA

P1-P3P4-P6P7-P9P10-P12
Basic Over/Undervoltage
Detection Circuits
Figures 3, 4, and 5 show the three basic voltage detec-
tion circuits.
The simplest circuit, depicted in Figure 3, does not
have any hysteresis. The comparator trip-point formulas
can easily be derived by observing that the comparator
changes state when the V
SET
input is 1.3V. The exter-
nal resistors form a voltage divider that attenuates the
input signal. This ensures that the V
SET
terminal is at
1.3V when the input voltage is at the desired compara-
tor trip point. Since the bias current of the comparator
is only a fraction of a nanoamp, the current in the volt-
age divider can be less than one microamp without los-
ing accuracy due to bias currents. The ICL7665A has a
2% threshold accuracy at +25°C, and a typical temper-
ature coefficient of 100ppm/°C including comparator
offset drift, eliminating the need for external poten-
tiometers in most applications.
Figure 4 adds another resistor to each voltage detector.
This third resistor supplies current from the HYST out-
put whenever the V
SET
input is above the 1.3V thresh-
old. As the formulas show, this hysteresis resistor
affects only the lower trip point. Hysteresis (defined as
the difference between the upper and lower trip points)
keeps noise or small variations in the input signal from
repeatedly switching the output when the input signal
remains near the trip point for a long period of time.
The third basic circuit, Figure 5, is suitable only when the
voltage to be detected is also the power-supply voltage for
the ICL7665. This circuit has the advantage that all of the
current flowing through the input divider resistors flows
through the hysteresis resistor. This allows the use of
higher-value resistors, without hysteresis output leakage
having an appreciable effect on the trip point.
Resistor-Value Calculations
Figure 3
1) Choose a value for R11. This value determines the
amount of current flowing though the input divider,
equal to V
SET
/ R11. R11 can typically be in the
range of 10kΩ to 10MΩ.
2) Calculate R21 based on R11 and the desired trip
point:
V
TRIP
– V
SET
V
TRIP
– 1.3V
R21 = R11
(
———————
)
= R11
(
——————
)
V
SET
1.3V
ICL7665
Microprocessor Voltage Monitor with
Dual Over/Undervoltage Detection
_______________________________________________________________________________________ 7
Figure 3. Simple Threshold Detector Figure 4. Threshold Detector with Hysteresis
ICL7665
OUT1
OUT2
SET2
SET1
R21
R11
R22
R12
V
IN1
V+ V
IN2
HYST1
HYST2
R31
R32
V+
OUT1
V
L1
V
U1
0V
V+
V
U2
OUT2
V
IN1
0V
V
L2
V
IN2
ICL7665
Microprocessor Voltage Monitor with
Dual Over/Undervoltage Detection
8 _______________________________________________________________________________________
Figure 4
1) Choose a resistor value for R11. Typical values are
in the 10kΩ to 10MΩ range.
2) Calculate R21 for the desired upper trip point, V
U,
using the formula:
V
U
- V
SET
V
U
– 1.3V
R21 = R11
(
——————
)
= R11
(
—————
)
V
SET
1.3V
3) Calculate R31 for the desired amount of hysteresis:
(R21) (V+ – V
SET
) (R21) (V+ – 1.3V)
R31 = ————————— = —————————
V
U
– V
L
V
U
– V
L
or, if V+ = V
IN
:
(R21) (V
L
– V
SET
) (R21) (V
L
– 1.3V)
R31 = ————————— = —————————
V
U
– V
L
V
U
– V
L
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 100kΩ 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 10kΩ and
10MΩ.
2) Calculate R21:
V
L
– V
SET
V
L
– 1.3V
R21 = R11
(
——————
)
= R11
(
—————
)
V
SET
1.3
3) Calculate R31:
V
U
– V
L
R31 = R11
(
—————
)
V
SET
4) As in the other circuits, all three resistor values may
be scaled up or down in value without changing V
U
and V
L
. V
U
and V
L
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 V
SET
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 (V
U
= 5.55V, V
L
= 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 V
SET
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
V
SET
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, V
IN
= V+
ICL7665
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 I
OUT
/ C (in this case, 200mV/ms).
Since the 7805 will continue to provide a 5V output at
1A until V
IN
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 2
1
⁄
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 V
SET2
equals the V
SET2
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 V
SET1
is 1.3V when the
NPN transistor’s temperature reaches the low-tempera-
ture limit.
P1-P3P4-P6P7-P9P10-P12

ICL7665BCPA

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IC VOLT MONITOR LP W/DET 8-DIP
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