LMX331 datasheet(8/12 Pages) MAXIM

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General-Purpose, Low-Voltage,

Single/Dual/Quad, Tiny-Pack Comparators

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1) Find output voltage when output is high:

VOUT(HIGH) = VDD - ILOAD ✕ RL

2) Find the trip points of the comparator using these

formulas:

VTH = VREF + ((VOUT(HIGH) - VREF)R2) / (R1 + R2)

VTL = VREF(1 - (R2 / (R1 + R2)))

where VTH is the threshold voltage at which the com-

parator switches its output from high to low as VIN rises

above the trip point, and VTL is the threshold voltage at

which the comparator switches its output from low to

high as VIN drops below the trip point.

3) The hysteresis band will be:

VHYST = VTH - VTL = VDD(R2 / (R1 + R2))

In this example, let VDD = 5V, VREF = 2.5V, ILOAD =

50nA, RL = 5.1k

Ω:

VOUT(HIGH) = 5.0V - (50 ✕ 10-9 ✕ 5.1 ✕ 103

Ω) ≈ 5.0V

VTH = 2.5V + 2.5V(R2 / (R1 + R2))

VTL = 2.5V(1 - (R2 / (R1 + R2)))

Select R2. In this example, we will choose 1k

Ω.

Select VHYST. In this example, we will choose 50mV.

Solve for R1:

VHYST = VOUT(HIGH)(R2 / (R1 + R2)) V

0.050V = 5(1000 / (R1 + 1000)) V

where R1

≈ 100kΩ, VTH = 2.525V, and VTL = 2.475V.

Choose R1 and R2 to be large enough as not to exceed

the amount of current the reference can supply.

The source current required is VREF / (R1 + R2).

The sink current is (VOUT(HIGH) - VREF) ✕ (R1 + R2).

Choose RL to be large enough to avoid drawing excess

current, yet small enough to supply the necessary cur-

rent to drive the load. RL should be between 1k

Ω and

10k

Ω.

Board Layout and Bypassing

Use 0.1µF bypass capacitors from VDD to VSS. To max-

imize performance, minimize stray inductance by

putting this capacitor close to the VDD pin and reduc-

ing trace lengths. For slow-moving input signals (rise

time > 1ms), use a 1nF capacitor between IN+ and IN-

to reduce high-frequency noise.

Chip Information

LMX331/LMX331H TRANSISTOR COUNT: 112

LMX393/LMX393H TRANSISTOR COUNT: 211

LMX339/LMX339H TRANSISTOR COUNT: 411

LMX331

IN+

IN-

VDD

VOUT

VSS

VREF

VIN

VDD

OUT

Figure 2. Adding Hysteresis with External Resistors

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