MAX9647_12 Datasheet, PDF(9/11 Page) Maxim Integrated Products – General-Purpose, Low-Voltage, Tiny Pack Comparators

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MAX9647/MAX9648

General-Purpose, Low-Voltage,

Tiny Pack Comparators

Additional hysteresis can be generated with two resis-

tors using positive feedback (Figure 2). Use the following

procedure to calculate resistor values:

1) Find output voltage when output is high:

VOUT(HIGH) = VDD - ILOAD x 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 compara-

tor 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 is:

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

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

50nA, and RL = 5.1kI.

VOUT(HIGH) = 5.0V - (50 x 10-9 x 5.1 x 103I) â 5.0V

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

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

Select R2. In this example, choose 1kI.

Select VHYST. In this example, choose 50mV.

Solve for R1.

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

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

where R1 â 100kI, 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) x (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 1kI and

10kI. Choose R1 to be much larger than RL to avoid

lowering VOUT(HIGH) ir raising VOUT(LOW).

Board Layout and Bypassing

Use 0.1FF bypass capacitors from VDD to VSS. To maxi-

mize performance, minimize stray inductance by putting

this capacitor close to the VDD pin and reducing trace

lengths. For slow moving input signals (rise time > 1ms),

use a 1nF capacitor between IN+ and IN- to reduce high

frequency noise.

IN+

THRESHOLDS

HYSTERESIS

VTH

IN-

VHYST

BAND

VTL

OUT

Figure 1. Threshold Hysteresis Band (Not to Scale)

VREF

VIN

IN+

IN-

VDD

OUT VOUT

MAX9647

VSS

Figure 2. Adding Hysteresis with External Resistors

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