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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+