Application Information

The LMV7235/LMV7239 are single supply comparators with

45ns of propagation delay and only 65µA of supply current.

The LMV7235/LMV7239 are rail-to-rail input and output. The

typical input common mode voltage range of −0.2V below

the ground to 0.2V above the supply. The LMV7235/

LMV7239 use a complimentary PNP and NPN input stage in

and the NPN stage senses common mode voltage near V

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 input 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 damage to the input stage.

The LMV7239 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 65µA at 5V, thus

conserving power consumption.

The LMV7235 has an open drain that requires a pull-up

resistor to a positive supply voltage for the output to switch

properly. When the internal output transistor is off, the output

voltage will be pulled up to the external positive voltage.

COMPARATOR WITH HYSTERESIS

The basic comparator configuration may oscillate or produce

a noisy output if the applied differential input voltage is near

the comparator’s offset voltage. This usually happens when

the input signal is moving very slowly across the compara-

tor’s switching threshold. This problem can be prevented by

the addition of hysteresis or positive feedback.

INVERTING COMPARATOR WITH HYSTERESIS

The inverting comparator with hysteresis requires a three

resistor network that is referenced to the supply voltage V

CC

of the comparator, as shown in Figure 1. When V

inverting input is less than V

inverting node of the comparator (V

voltage is high (for simplicity assume V

V

R1||R3 in series with R2. The lower input trip voltage V

defined as

V

When V

low, very close to ground. In this case the three network

resistors can be presented as R2||R3 in series with R1. The

upper trip voltage V

V

The total hysteresis provided by the network is defined as

Delta V

To assure that the comparator will always switch fully to V

CC

and not be pulled down by the load the resistors, values

should be chosen as follow:

R

www.national.com

9