Single/Dual/Quad, Micropower,

Ultra-Low-Voltage, Rail-to-Rail I/O Comparators

10

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HYST

REF

GND

R1

R2

MAX965

MAX967

MAX968

MAX969

+1.6V TO +5.5V

Figure 3. Programming the HYST Pin

GND

OUT

R3

R1

R2

R4

MAX966

MAX970

Figure 4. External Hysteresis

Figure 2 illustrates the case in which IN- has a fixed

voltage applied, and IN+ is varied. If the inputs were

reversed, the figure would be the same, except with an

inverted output.

Adding Hysteresis to the

MAX965/MAX967/MAX968/MAX969

To add hysteresis to the MAX965/MAX967/MAX968/

MAX969, connect resistor R1 between REF and HYST,

and connect resistor R2 between HYST and GND

(Figure 3). If additional hysteresis is not required,

connect HYST to REF. When hysteresis is added, the

upper and lower trip points change by the same

amount in opposite directions. The hysteresis band (the

difference between the upper and lower trip points,

and REF. The HYST input voltage range is from REF

down to (REF - 50mV). This yields a hysteresis band

from ±1mV to a maximum of ±50mV. Calculate the val-

ues of R1 and R2 for the desired hysteresis band with

the following formulas:

not exceed the REF source capability (12µA typical),

and is significantly larger than the HYST leakage cur-

are good choices. If 2.4M

mated as:

R1(k

In the MAX967/MAX968/MAX969, the HYST pin pro-

grams the same hysteresis for all comparators in the

package.

Due to the internal structure of the input developed for

ultra-low-voltage operation, the hysteresis band varies

with common-mode voltage. The graph Programmed

Hysteresis vs. Common-Mode Voltage in the Typical

Operating Characteristics shows this variation. Notice

that the hysteresis band increases to almost twice the

calculated value toward the ends of the common-mode

range. This is apparent only when programming addi-

tional hysteresis using the HYST pin. The hysteresis

band is constant when HYST is connected to REF.

Adding Hysteresis to the MAX966/MAX970

The MAX966/MAX970 do not have a HYST pin for pro-

gramming hysteresis. Hysteresis can be generated with

three resistors using positive feedback (Figure 4). This

method generally draws more current than the method

using the HYST pin on the MAX965/MAX967/MAX968/

MAX969. Also, the positive feedback method slows

hysteresis response time. Use the following procedure

to calculate the resistor values:

1) Select R3. The leakage current of IN+ is under 5nA,

so the current through R3 should be at least 500nA

to minimize errors caused by leakage current. The

R3. Taking into consideration the two possible out-

put states and solving for R3 yields two formulas:

and

Use the smaller of the two resulting resistor values.

the two resistor values are 2.4M

Ω and 7.6mΩ. For

R3, choose the 2.2M

Ω standard value.

example, choose 50mV.