LMC66 datasheet(13/24 Pages) NSC

13 / 24 page

Application Information

1.0 Input Common-Mode Voltage

Range

The LMC6681/2/4 has a rail-to-rail input common-mode volt-

age range.

Figure 1 shows an input voltage exceeding both

supplies with no resulting phase inversion on the output.

The absolute maximum input voltage at V

+ = 3V is 300 mV

beyond either supply rail at room temperature. Voltages

greatly exceeding this absolute maximum rating, as in

Figure

2, can cause excessive current to flow in or out of the input

pins, possibly affecting reliability. The input current can be

externally limited to ±5 mA, with an input resistor, as shown

Figure 3.

2.0 Rail-to-Rail Output

The approximated output resistance of the LMC6681/2/4 is

Ω sourcing, and 50Ω sinking at V

S = 3V. The maximum

output swing can be estimated as a function of load using the

calculated output resistance.

3.0 Low Voltage Operation

The LMC6682 operates at supply voltages of 2.2V and 1.8V.

These voltages represent the End of Discharge voltages of

several popular batteries. The amplifier can operate from 1

Lead-Acid or Lithium Ion battery, or 2NiMH, NiCd, or

Carbon-Zinc batteries. Nominal and End of Discharge of

Voltage of several batteries are listed below.

Battery Type

Nominal Voltage

End of Discharge

Voltage

NiMH

1.2V

NiCd

1.2V

Lead-Acid

1.8V

Silver Oxide

1.6V

1.3V

Carbon-Zinc

1.5V

1.1V

Lithium

2.6V–3.6V

1.7V–2.4V

At V

= 2.2V, the LMC6681/2/4 has a rail-to-rail input

common-mode voltage range.

Figure 4 shows an input volt-

age extending to both supplies and the resulting output.

The amplifier is operational at V

S = 1.8V, with guaranteed in-

put common-mode voltage range, output swing, and CMRR

specs.

Figure 5 shows the response of the LMC6681/2/4 at

S = 1.8V.

DS012042-5

FIGURE 1. An Input Signal Exceeds the LMC6681/2/4

Power Supply Voltages with No Output Phase

Inversion

DS012042-6

FIGURE 2. A ±7.5V Input Signal Greatly

Exceeds the 3V Supply in

Figure 3,

Causing No Phase Inversion Due to R

DS012042-7

FIGURE 3. Input Current Protection for

Voltages Exceeding the Supply Voltage

DS012042-8

FIGURE 4. The Input Common-Mode Voltage

Range Extends to Both Supplies at V

S = 2.2V

DS012042-9

FIGURE 5. Response of the LMC6681/2/4

at V

S = 1.8V

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