Datasheet

LMV7219
www.ti.com SNOS458F –APRIL 2000–REVISED MARCH 2013
APPLICATION SECTION
LMV7219 is a single supply comparator with internal hysteresis, 7ns of propagation delay and only 1.1mA of
supply current.
The LMV7219 has a typical input common mode voltage range of −0.2V below the ground to 1V below V
cc
. The
differential 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 inputs 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
CC
and ground,
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 has 7mV of internal hysteresis to counter
parasitic effects and noise. The hysteresis does not change significantly with the supply voltages and the
common mode input 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 internal 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 resistors. The fixed internal hysteresis eliminates these resistors.
Figure 17. Input and Output Waveforms, Non-Inverting Input Varied
Additional Hysteresis
If additional hysteresis is desired, this can be done with the addition of three resistors using positive feedback, as
shown in Figure 18. The positive feedback method slows the comparator response time. Calculate the resistor
values as follows:
1) Select R3. The current through R3 should be greater than the input bias current to minimize errors. The
current through R3 (I
F
) at the trip point is (V
REF
- V
OUT
) /R3. Consider the two possible output states when solving
for R3, and use the smaller of the two resulting resistor values. The two formulas are:
R3 = V
REF
/I
F
(1)
when V
OUT
= 0
R3 = V
CC
- V
REF
/I
F
(2)
when V
OUT
= V
CC
2) Choose a hysteresis band required (V
HB
).
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