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LT1719

APPLICATIONS INFORMATION

–

LT1719S8

1719 F05

′

VREF

VTH

+VS

VAVERAGE =

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 (+VS/2)

and the simplified circuit model in Figure 5. To assure that

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

same VTH as before:

′ = (VREF – VTH)/(VTH/R1 + [VTH – (+VS)/2]/R3)

Figure 5. Model for Additional Hysteresis Calculations

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

rail outputs of the LT1719 make this more predictable than

with TTL output comparators due to the LT1719’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 4. Resistor R3 adds a portion of the output to the

threshold set by the resistor string. The LT1719 pulls the

outputs to + VS 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 +VS – 300mV, for a total

voltage swing of (+VS – 300mV) – (300mV) = +VS

– 600mV.

–

LT1719S8

INPUT

1719 F04

VREF

Figure 4. Additional External Hysteresis

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)(+VS – 0.6V)/(additional hysteresis)

Additional hysteresis is the desired overall hysteresis less

the internal 4mV hysteresis.

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 LT1719. For 5% accuracy, the current must be at least

20 times the input current, more for higher accuracy. This

illustration used an LT1719S8; with an LT1719S6 the same

procedure is used with V+ substituted for +VS.

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