Microprocessor Voltage Monitors

with Programmable Voltage Detection

4

_______________________________________________________________________________________

V+

HYST

THRESH

MAX8211

MAX8212

OUT

GND

V+

R3

R2

R1

Figure 3. Basic Overvoltage/Undervoltage Circuit

in the networks that set up the trip voltage, since the

comparator input (THRESH pin) is a low-leakage

MOSFET transistor. This further reduces system current

drain. The tolerance of the internal reference has also

been significantly improved, allowing for more precise

voltage detection without the use of potentiometers.

The available current from the HYST output has been

increased from 21µA to 10mA, making the hysteresis

feature easier to use. The disparity between the HYST

output and the voltage required at THRESH to switch

the OUT pin has also been reduced in the MAX8211

from 8mV to 0.1mV to eliminate output “chatter” or

oscillation.

Most voltage detection circuits operate with supplies of

15V or less; in these applications, the MAX8211/

MAX8212 will replace ICL8211/ICL8212s with the per-

formance advantages described above. However, note

that the CMOS parts have an absolute maximum sup-

ply-voltage rating of 18V, and should never be used in

applications where this rating could be exceeded.

Exercise caution when replacing ICL8211/ICL8212s in

closed-loop applications such as programmable

zeners. Although neither the ICL8211/ICL8212 nor the

MAX8211/MAX8212 are internally compensated, the

CMOS parts have higher gain and may not be stable

for the external compensation-capacitor values used in

lower-gain ICL8211/ICL8212 circuits.

__________Applications Information

Basic Voltage Detectors

Figure 3 shows the basic circuit for both undervoltage

detection (MAX8211) and overvoltage detection

(MAX8212). For applications where no hysteresis is

needed, R3 should be omitted. The ratio of R1 to R2 is

1.15V is applied to the THRESH pin. Since the com-

parator inputs are very low-leakage MOSFET transis-

tors, the MAX8211/MAX8212 can use much higher

resistors values in the attenuator network than can the

bipolar ICL8211/ICL8212. See Table 1 for switching

delays.

Table 1. Switching Delays

Voltage Detectors with Hysteresis

To ensure noise-free output switching, hysteresis is

frequently used in voltage detectors. For both the

MAX8211 and MAX8212 the HYST output is on for

threshold voltages greater than 1.15V. R3 (Figure 3)

controls the amount of current (positive feedback) sup-

plied from the HYST output to the mid-point of the resis-

tor divider, and hence the magnitude of the hysteresis,

or dead-band.

Figure 2. MAX8212 Block Diagram

P

N

1.15V

REFERENCE

HYST

OUT

THRESH

V+

TYPICAL DELAYS

MAX8211

MAX8212

40µs

250µs

1.5ms

3ms