Microprocessor Voltage Monitor with

Dual Over/Undervoltage Detection

8

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Figure 4

1) Choose a resistor value for R11. Typical values are

in the 10k

Ω to 10MΩ range.

using the formula:

R21 = R11

1.3V

3) Calculate R31 for the desired amount of hysteresis:

(R21) (V+ – 1.3V)

R31 = ————————— = —————————

R31 = ————————— = —————————

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 100k

Ω 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 10k

Ω and

10M

Ω.

2) Calculate R21:

R21 = R11

1.3

3) Calculate R31:

R31 = R11

4) As in the other circuits, all three resistor values may

three resistors, if the absolute values are such that

the hysteresis output resistance and the leakage

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-

(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 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

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.

ICL7665

OUT1

OUT2

SET2

SET1

R21

R11

HYST1

HYST2

OUT1

OUT2

V+

GND

OVERVOLTAGE

UNDERVOLTAGE

R31

R32

R22

R12