25˚C (Continued)

Application Section

LMV7219 is a single supply comparator with internal hyster-

esis, 7ns of propagation delay and only 1.1mA of supply cur-

rent.

The LMV7219 has a typical input common mode voltage

range of −0.2V below the ground to 1V below V

ential input stage is a pair of PNP transistors, therefore, the

input bias current flows out of the device. If either of the input

signals falls below the negative common mode limit, the

parasitic PN junction formed by the substrate and the base

of the PNP will turn on, resulting in an increase of input bias

current.

If one of the inputs goes above the positive common mode

limit, the output will still maintain the correct logic level as

long as the other input stays within the common mode range.

However, the propagation delay will increase. When both in-

puts are outside the common mode voltage range, current

saturation occurs in the input stage, and the output becomes

unpredictable.

The propagation delay does not increase significantly with

large differential input voltages. However, large differential

voltages greater than the supply voltage should be avoided

to prevent damages to the input stage.

The LMV7219 has a push pull output. When the output

switches, there is a direct path between V

causing high output sinking or sourcing current during the

transition. After the transition, the output current decreases

and the supply current settles back to about 1.1mA at 5V,

thus conserving power consumption.

Most high-speed comparators oscillate when the voltage of

one of the inputs is close to or equal to the voltage on the

other input due to noise or undesirable feedback. The

LMV7219 have 7mV of internal hysteresis to counter para-

sitic effects and noise. The hysteresis does not change sig-

nificantly with the supply voltages and the common mode in-

put voltages as reflected in the specification table.

The internal hysteresis creates two trip points, one for the

rising input voltage and one for the falling input voltage. The

difference between the trip points is the hysteresis. With in-

ternal hysteresis, when the comparator’s input voltages are

equal, the hysteresis effectively causes one comparator-

input voltage to move quickly past the other, thus taking the

input out of the region where oscillation occurs. Standard

comparators require hysteresis to be added with external re-

sistors. The fixed internal hysteresis eliminates these

resistors.

Additional Hysteresis

If additional hysteresis is desired, this can be done with the

addition of three resistors using positive feedback, as shown

in

Figure 2. The positive feedback method slows the com-

parator response time. Calculate the resistor values as fol-

lows:

1) Select R3. The current through R3 should be greater than

the input bias current to minimize errors. The current through

R3 (I

possible output states when solving for R3, and use the

smaller of the two resulting resistor values. The two formulas

are:

R3=V

(when V

R3=V

(V

2) Choose a hysteresis band required (V

3) Calculate R1, where R1 = R3 X(V

4) Choose the trip point for V

voltage (V

high as V

5) Calculate R2 as follows:

6) Verify the trip voltage and hysteresis as follows:

Propagation Delay (t

DS101054-17

DS101054-18

FIGURE 1. Input and Output Waveforms, Non-Inverting

Input Varied

www.national.com

8