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1719

4.5ns Single/Dual Supply 3V/5V Comparator with Rail-to-Rail Output

Linear Technology 

LT1719

APPLICATIONS INFORMATION

Hysteresis

The LT1719 includes internal hysteresis, which makes it

easier to use than many other similar speed comparators.

The input-output transfer characteristic is illustrated in

Figure 3 showing the definitions of VOS and VHYST based

upon the two measurable trip points. The hysteresis band

makes the LT1719 well behaved, even with slowly moving

inputs.

VOH

VHYST

(= VTRIP+ – VTRIP–)

VHYST/2

VOL

0

VTRIP–

VOS

=

VTRIP+

+

2

VTRIP–

∆VIN = VIN+ – VIN–

VTRIP+

1719 F03

Figure 3. Hysteresis I/O Characteristics

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

LT1719 is the significant reduction in these effects, which

is important whenever an LT1719 is used to detect 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 hysteresis, as

seen in competing comparators, is useless. The LT1719 is

many times better than prior comparators in these re-

gards. 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 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.

VREF

R3

R2

+

R1

LT1719

–

INPUT

1719 F04

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.

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:

R2′ = (VREF – VTH)/(VTH/R1 + (VTH – VS/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.

9

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