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Ultra-Low-Power, Open-Drain,

Single/Dual-Supply Comparators

MAX971鈥揗AX974/MAX981鈥揗AX984

__________Applications Information

Hysteresis

Hysteresis increases the comparators鈥?noise margin by

increasing the upper threshold and decreasing the

lower threshold (Figure 2).

R

H

V+

Hysteresis (MAX9_1/MAX982/MAX9_3)

To add hysteresis to the MAX9_1, MAX982, or MAX9_3,

connect resistor R1 between REF and HYST, and

connect resistor R2 between HYST and V- (Figure 3). If

no hysteresis is required, connect HYST to REF. When

hysteresis is added, the upper threshold increases by

the same amount that the lower threshold decreases.

The hysteresis band (the difference between the upper

and lower thresholds, V

HB

) is approximately equal to

twice the voltage between REF and HYST. The HYST

input can be adjusted to a maximum voltage of REF

and to a minimum voltage of (REF - 50mV). The

maximum difference between REF and HYST (50mV)

will therefore produce a 100mV max hysteresis band.

Use the following equations to determine R1 and R2:

V

HB

R1 =

2

脳

I

REF

V

IN

R

PULL-UP

V+

OUT

MAX9_4

V-

GND

V

REF

Figure 4. External Hysteresis

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.

(

)

铮?/div>

V

HB

铮?/div>

1.182 鈥?/div>

铮?/div>

2

铮?/div>

R2 =

I

REF

where I

REF

(the current sourced by the reference)

should not exceed the REF source capability, and

should be significantly larger than the HYST input

current. I

REF

values between 0.1碌A and 4碌A are

usually appropriate. If 2.4M鈩?is chosen for R2 (I

REF

=

0.5碌A), the equation for R1 and V

HB

can be

approximated as:

R1 (k

鈩?/div>

) = V

HB

(mV)

When hysteresis is obtained in this manner for the

MAX982/MAX9_3, the same hysteresis applies to both

comparators.

Window Detector

The MAX9_3 is ideal for making window detectors

(undervoltage/overvoltage detectors). The schematic is

shown in Figure 5, with component values selected for a

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. Taken

alone, OUTA would provide an active-low undervoltage

indication, and OUTB would give an active-low

overvoltage indication. Wired-ORing 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 (MAX9_1/MAX982/MAX9_3)

section.

In this example, 卤5mV of hysteresis has been added

at the comparator input

(V

H

= V

HB

/ 2). This means

that the hysteresis apparent at V

IN

will be larger

because of the input resistor divider.

Hysteresis (MAX972/MAX9_4)

Hysteresis can be implemented with any comparator

using positive feedback, as shown in Figure 4. This

approach generally draws more current than circuits

using the HYST pin on the MAX9_1/MAX982/MAX9_3,

and the high feedback impedance slows hysteresis. In

addition, because the output does not source current,

any increase in the upper threshold is dependent on

the load or pull-up resistor on the output.

10

______________________________________________________________________________________

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