±5V, ±12V (±15V) Dedicated

Microprocessor Voltage Monitors

8

_______________________________________________________________________________________

Monitoring the Supply Voltage

The supply voltage to these devices can also be moni-

tored by the 5V dedicated comparator and the auxiliary

comparator. Figure 4 shows a circuit that monitors the

undervoltage conditions. The +5V comparator checks

for undervoltage conditions while the auxiliary compara-

tor monitors overvoltage conditions. In general, no extra

supply bypassing circuitry (other than the normally rec-

ommended 0.1µF capacitor) is required when perform-

ing this function. However, using resistor values higher

than 100k

Ω at the auxiliary comparator’s input requires

attention to eliminate potential oscillations. Also, partic-

ularly low pull-up resistor values on DOUT contribute to

the likelihood of the auxiliary comparator’s oscillation.

See the section

Eliminating Output Oscillation.

Microprocessor Reset

____________Circuit with Time Delay

It is often necessary to reset a microprocessor (µP) when

its supply voltage drops below a certain level. Figure 5’s

circuit generates a low output when the monitored volt-

age drops below the 5V monitor’s threshold. Additionally,

this output remains low for 200ms after the supply voltage

goes above the threshold. µP reset circuits typically

include this feature because it gives the µP time to be

fully reset after power has been restored, and allows any

capacitors in associated circuitry time to charge. Figure

6 shows this circuit’s waveforms and formulas.

Figure 7 shows Figure 5’s µP reset circuit, but with the

monitored supply also powering the MAX8215. Figure

6’s waveforms and equations also apply to this circuit.

The MAX8215/MAX8216 comparator outputs correctly

display a low level down to a 0.8V typical supply voltage.

Unused Inputs

When the uncommitted comparator within the

MAX8215/MAX8216 is not used, tie the unused input to

either the positive supply or ground. This prevents noise

generation due to the comparator output switching from

one logic state to another (due to noise at the input).

Output Pull-Up Resistors

Pull-up resistors are required at the outputs of each

comparator. Resistor values should not be less than

2.7k

save power by using higher values, e.g.,

≥100kΩ. Use

of higher-value resistors also minimizes the possibility

of oscillations due to a spurious feedback (see the sec-

tion

Eliminating Output Oscillation).

Input Voltage Limitation

If the voltages at the various inputs are kept within the

absolute maximum ratings, the device is not damaged.

However, high input voltages within this range can

cause the reference voltage to move. To prevent the

reference voltage from changing, limit the +5V input to

+17V; the -5V and -15V inputs to +1V; and the +15V

input to +60V. Negative input voltages within the

MAX8215

MAX8216

14

8

PGND

0.1

µF

OUT1

DOUT

7

13

9

RESET

3

2

GND

+5V

DIN

+5V

680k

680k

1

µF

Figure 5. Microprocessor Reset Circuit with 200ms Time Delay

GND

INPUT VOLTAGE

OUTPUT

VOLTAGE

(OUT1)

GND

(

)

GND

OUTPUT

VOLTAGE

(DOUT)

IN THIS CASE, IT IS EQUAL TO THE INTERNAL REFERENCE VOLTAGE.

Figure 6. Microprocessor Reset with Time Delay Waveforms