SOT23, 1.8V, Nanopower, Beyond-the-Rails

Comparators With/Without Reference

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11

THRESHOLDS

OUT

IN+

IN-

HYSTERESIS

BAND

Figure 2. Threshold Hysteresis Band

MAX917

MAX919

OUT

R3

R1

R2

Figure 3. MAX917/MAX919 Additional Hysteresis

Additional Hysteresis (MAX917/MAX919)

The MAX917/MAX919 have a 4mV internal hysteresis

with three resistors using positive feedback (Figure 3).

Unfortunately, this method also slows hysteresis re-

sponse time. Use the following procedure to calculate

resistor values.

1) Select R3. Leakage current at IN is under 2nA, so

the current through R3 should be at least 0.2µA to

minimize errors caused by leakage current. The cur-

Considering the two possible output states in solving

resistor values. For example, when using the

1.2MΩ and 3.8MΩ. Choose a 1.2MΩ standard value

for R3.

example, choose 50mV.

3) Calculate R1 according to the following equation:

For this example, insert the values

R1 = 1.2MΩ (50mV/5V) = 12kΩ

comparator switches its output from low to high as

choose 3V.

5) Calculate R2 as follows:

R2 = 1/[3.0V/(1.2V · 12kΩ) - (1 / 12kΩ) -

(1/1.2MΩ)] = 8.05kΩ

For this example, choose an 8.2kΩ standard value.

6) Verify the trip voltages and hysteresis as follows:

+ (1 / R3)]

Additional Hysteresis (MAX918/MAX920)

The MAX918/MAX920 have a 4mV internal hysteresis

band. They have open-drain outputs and require an

external pullup resistor (Figure 4). Additional hysteresis

can be generated using positive feedback, but the for-

mulas differ slightly from those of the MAX917/

MAX919. Use the following procedure to calculate

resistor values.

the two resulting resistor values.

3) Calculate R1 according to the following equation:

voltage at which the comparator switches its output

5) Calculate R2 as follows:

R2

1/ V

/ V

R1

1

R1

1

R3

THR

REF

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