LMV7271MFX datasheet(10/16 Pages) NSC

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Application Notes

BASIC COMPARATOR

A comparator is often used to convert an analog signal to a

digital signal. As shown in Figure 2, the comparator com-

pares an input voltage (V

IN) to a reference voltage (VREF). If

IN is less than VREF, the output (VO) is low. However, if VIN

is greater than V

REF, the output voltage (VO) is high.

RAIL-TO-RAIL INPUT STAGE

The LMV727X has an input common mode voltage range

CM) of −0.1V below the V

− to 0.1V above V+. This is

achieved by using paralleled PNP and NPN differential input

pairs. When the V

CM is near V

+, the NPN pair is on and the

PNP pair is off. When the V

CM is near V

−, the NPN pair is off

and the PNP pair is on. The crossover point between the

NPN and PNP input stages is around 950mV from V

+. Since

each input stage has its own offset voltage (V

OS), the VOS of

the comparator becomes a function of the V

CM. See curves

for V

OS vs. VCM in Typical Performance Characteristics sec-

tion. In application design, it is recommended to keep the

CM away from the crossover point to avoid problems. The

wide input voltage range makes LMV727X ideal in power

supply monitoring circuits, where the comparators are used

to sense signals close to ground and power supplies.

OUTPUT STAGE

The LMV7271 and LMV7272 have a push-pull output stage.

This output stage keeps the total system power consumption

to the absolute minimum. The only current consumed is the

low supply current and the current going directly into the

load. When the output switches, both PMOS and NMOS at

the output stage are on at the same time for a very short

time. This allows current to flow directly between V

+ and V−

through output transistors. The result is a short spike of

current (shoot-through current) drawn from the supply and

glitches in the supply voltages. The glitches can spread to

other parts of the board as noise. To prevent the glitches in

supply lines, power supply bypass capacitors must be in-

stalled. See section for supply bypassing in the Application

Notes for details.

HYSTERESIS

It is a standard procedure to use hysteresis (positive feed-

back) around a comparator, to prevent oscillation, and to

avoid excessive noise on the output because the comparator

is a good amplifier of its own noise.

Inverting Comparator with Hysteresis

The inverting comparator with hysteresis requires a three

resistor network that is referenced to the supply voltage V

of the comparator (Figure 3). When V

IN at the inverting input

is less than V

A, the voltage at the non-inverting node of the

comparator (V

IN < VA), the output voltage is high (for sim-

plicity assume V

O switches as high as VCC). The three

network resistors can be represented as R

1||R3 in series with

2. The lower input trip voltage VA1 is defined as

When V

IN is greater than VA (VIN > VA), the output voltage is

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

resistors can be presented as R

2//R3 in series with R1. The

upper trip voltage V

A2 is defined as

The total hysteresis provided by the network is defined as

∆V

A =VA1 -VA2

A good typical value of

∆V

A would be in the range of 5 to

50mV. This is easily obtained by choosing R

3 as 1000 to 100

times (R

1||R2) for 5V operation, or as 300 to 30 times

1||R2) for 1.8V operation.

LMV7271

20064025

20064017

FIGURE 2. LMV7271 Basic Comparator

LMV7271MFX Datasheet(PDF) 10 Page - National Semiconductor (TI)

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