LMC6681 Datasheet, PDF (13/24 Pages) National Semiconductor (TI) – Low Voltage, Rail-To-Rail Input and Output CMOS

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.
DS012042-5
FIGURE 1. An Input Signal Exceeds the LMC6681/2/4
Power Supply Voltages with No Output Phase
Inversion
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
in Figure 3.
2.0 Rail-to-Rail Output
The approximated output resistance of the LMC6681/2/4 is
50Ω 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
1V
NiCd
1.2V
1V
Lead-Acid
2V
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_S = 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.
DS012042-6
FIGURE 2. A ±7.5V Input Signal Greatly
Exceeds the 3V Supply in Figure 3,
Causing No Phase Inversion Due to R_I
DS012042-8
FIGURE 4. The Input Common-Mode Voltage
Range Extends to Both Supplies at V_S = 2.2V
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
V_S = 1.8V.
DS012042-7
FIGURE 3. Input Current Protection for
Voltages Exceeding the Supply Voltage
DS012042-9
FIGURE 5. Response of the LMC6681/2/4
at V_S = 1.8V
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