6. Verify the threshold voltages with these formulas:

Board Layout and Bypassing

Power-supply bypass capacitors are not needed if the

supply impedance is low, but 100nF bypass capacitors

should be used when the supply impedance is high or

when the supply leads are long. Minimize signal lead

lengths to reduce stray capacitance between the input

and output that might cause instability. Do not bypass

the reference output.

_______________Typical Applications

Auto-Off Power Source

Figure 5 shows the schematic for a 40mA power supply

that has a timed auto power-off function.

The

comparator output is the switched power-supply

output. With a 10mA load, it typically provides a

quiescent current. This circuit takes advantage of the

four key features of the MAX921: 2.5µA supply current,

an internal reference, hysteresis, and high current

output. Using the component values shown, the three-

resistor voltage divider programs the maximum ±50mV

of hysteresis and sets the IN- voltage at 100mV. This

gives an IN+ trip threshold of approximately 50mV for

IN+ falling.

The RC time constant determines the maximum power-

on time of the OUT pin before power-down occurs.

This period can be approximated by:

R x C x 4.6sec

For example: 2M

Ω x 10µF x 4.6 = 92sec. The actual

time will vary with both the leakage current of the

capacitor and the voltage applied to the circuit.

Window Detector

The MAX923 is ideal for making window detectors

(undervoltage/overvoltage detectors). The schematic

is shown in Figure 6, with component values selected

for an 4.5V undervoltage threshold, and a 5.5V

overvoltage threshold. Choose different thresholds by

changing the values of R1, R2, and R3. To prevent

chatter at the output when the supply voltage is close

to a threshold, hysteresis has been added using R4

and R5. OUTA provides an active-low undervoltage

indication, and OUTB gives an active-low overvoltage

indication. ANDing the two outputs provides an active-

high, power-good signal.

The design procedure is as follows:

1. Choose the required hysteresis level and calculate

values for R4 and R5 according to the formulas in

the

Hysteresis (MAX921/MAX923) section. In this

example, ±5mV of hysteresis has been added

at the

the input resistor divider.

V

rising :

V

V

R1

1

R1

1

R2

1

R3

V

falling :

V

V

R1

V

R3

IN

THR

REF

IN

THF

THR

=×

×

+

+









=−

×+

()

Ultra Low-Power,

Single/Dual-Supply Comparators

______________________________________________________________________________________

11

MAX921

OUT

IN-

HYST

REF

V-

GND

IN+

V+

VBATT -0.15V

10mA

21

4

5

6

8

3

100k

1.1M

47k

4.5V TO 6.0V

MOMENTARY

SWITCH

7

R

C

Figure 5. Auto-off power switch operates on 2.5µA quiescent

current.