±5V, ±12V (±15V) Dedicated

Microprocessor Voltage Monitors

_______________________________________________________________________________________

7

Adding hysteresis to a comparator creates two trip

points–one for the input voltage rising and one for the

input voltage falling.

When the voltage at the

MAX8215/MAX8216 auxiliary comparator’s (noninvert-

ing) input falls, the threshold at which the comparator

switches equals the reference voltage connected to the

comparator’s inverting input. However, when the volt-

age at the noninverting input rises, the threshold

equals the reference voltage plus the amount of hys-

teresis voltage built into the part. The trip point is

somewhat more accurate when the hysteresis voltage

is not part of the threshold voltage (i.e., when the input

voltage is falling) because the tolerance of the hystere-

sis specification adds to the tolerance of the trip point.

Overvoltage and Undervoltage

Detection Circuits

Figure 2 shows connection of the auxiliary comparator

as either an undervoltage or overvoltage comparator.

Hysteresis makes this circuit more accurate when the

input voltage is dropping as opposed to rising. Figure

3 illustrates the comparator’s operation. The input volt-

age’s direction determines at which of two trip points

the comparator switches. Thus, the diagram includes

arrows that indicate whether the input voltage is rising

or falling. The formulas are provided for determining

trip-point voltages for specified resistors and for ease

in calculating appropriate resistor ratios for particular

trip points.

The MAX8215/MAX8216 comparator outputs correctly

display a low level down to 0.8V supply voltage. This is

useful in undervoltage applications where the monitored

pin. See the section

Monitoring the Supply Voltage.

9

RB

1.24V REFERENCE

DOUT

MAX8215

MAX8216

VDD

DIN

+VS

RA

7

180k

Figure 2. Undervoltage/Overvoltage Comparator Using the

Auxiliary Comparator

GND

INPUT VOLTAGE

OUTPUT

VOLTAGE

GND

RA

RB

(

)

TO DETERMINE THE TRIP VOLTAGES

FROM PARTICULAR RESISTOR

VALUES:

TO CALCULATE THE REQUIRED

RESISTOR RATIOS FOR PARTICULAR

TRIP VOLTAGES:

RA

RB

(

)

RA

RB

(

)

RA

=

- 1

RB

RA

=

- 1

RB

Figure 3. Undervoltage/Overvoltage Detector Waveforms and

Formulas

MAX8215

MAX8216

14

8

PGND

0.1

µF

OUT1

UNDERVOLTAGE

DOUT

7

13

9

OVERVOLTAGE

3

2

R1

GND

+5V

DIN

R2

COMPARATOR'S SPECIFIED TRIP LEVEL. THE AUXILIARY

R2

Figure 4. Monitoring Supply Powering the MAX8215/MAX8216

with Undervoltage and Overvoltage Comparators