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1720

Dual/Quad, 4.5ns, Single Supply 3V/5V Comparators with Rail-to-Rail Outputs

Linear Technology 

LT1720/LT1721

APPLICATIONS INFORMATION

The exact amount of hysteresis will vary from part to part

as indicated in the specifications table. The hysteresis level

will also vary slightly with changes in supply voltage and

common mode voltage. A key advantage of the LT1720/

LT1721 is the significant reduction in these effects, which

is important whenever an LT1720/LT1721 is used to de-

tect a threshold crossing in one direction only. In such a

case, the relevant trip point will be all that matters, and a

stable offset voltage with an unpredictable level of hyster-

esis, as seen in competing comparators, is of little value.

The LT1720/LT1721 are many times better than prior com-

parators in these regards. In fact, the CMRR and PSRR

tests are performed by checking for changes in either trip

point to the limits indicated in the specifications table.

Because the offset voltage is the average of the trip points,

the CMRR and PSRR of the offset voltage is therefore

guaranteed to be at least as good as those limits. This

more stringent test also puts a limit on the common mode

and power supply dependence of the hysteresis voltage.

Additional hysteresis may be added externally. The rail-to-

rail outputs of the LT1720/LT1721 make this more pre-

dictable than with TTL output comparators due to the

LT1720/LT1721’s small variability of VOH (output high

voltage).

To add additional hysteresis, set up positive feedback by

adding additional external resistor R3 as shown in Figure

3. Resistor R3 adds a portion of the output to the threshold

set by the resistor string. The LT1720/LT1721 pulls the

outputs to the supply rail and ground to within 200mV of

the rails with light loads, and to within 400mV with heavy

loads. For the load of most circuits, a good model for the

voltage on the right side of R3 is 300mV or VCC – 300mV,

for a total voltage swing of (VCC – 300mV) – 300mV =

VCC – 600mV.

With this in mind, calculation of the resistor values needed

is a two-step process. First, calculate the value of R3 based

on the additional hysteresis desired, the output voltage

swing, and the impedance of the primary bias string:

R3 = (R1R2)(VCC – 0.6V)/(additional hysteresis)

Additional hysteresis is the desired overall hysteresis less

the internal 3.5mV hysteresis.

The second step is to recalculate R2 to set the same

average threshold as before. The average threshold before

was set at VTH = (VREF)(R1)/(R1 + R2). The new R2 is

calculated based on the average output voltage (VCC/2)

and the simplified circuit model in Figure 4. To assure that

the comparator’s noninverting input is, on average, the

same VTH as before:

R2′ = (VREF – VTH)/(VTH/R1 + (VTH – VCC/2)/R3)

For additional hysteresis of 10mV or less, it is not uncom-

mon for R2′ to be the same as R2 within 1% resistor

tolerances.

This method will work for additional hysteresis of up to a

few hundred millivolts. Beyond that, the impedance of R3

is low enough to effect the bias string, and adjustment of

R1 may also be required. Note that the currents through

the R1/R2 bias string should be many times the input

currents of the LT1720/LT1721. For 5% accuracy, the

current must be at least 120µA(6µA IB ÷ 0.05); more for

higher accuracy.

VREF

R2

R1

R3

+

1/2 LT1720

–

INPUT

1720/21 F03

VREF

R2′

VTH

R3

R1

+

1/2 LT1720

–

VAVERAGE

=

VCC

2

1720/21 F04

Figure 3. Additional External Hysteresis

Figure 4. Model for Additional Hysteresis Calculations

9

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