LMC7111 datasheet(12/18 Pages) NSC

12 / 18 page

Application Information (Continued)

Low Supply Current. The typical 25 µA supply current of

the LMC7111 extends battery life in portable applications,

and may allow the reduction of the size of batteries in some

applications.

Wide Voltage Range. The LMC7111 is characterized at

2.7V, 3V, 3.3V, 5V and 10V. Performance data is provided at

these popular voltages. This wide voltage range makes the

LMC7111 a good choice for devices where the voltage may

vary over the life of the batteries.

2.0 Input Common Mode

Voltage Range

The LMC7111 does not exhibit phase inversion when an in-

put voltage exceeds the negative supply voltage.

The absolute maximum input voltage is 300 mV beyond ei-

ther rail at room temperature. Voltages greatly exceeding

this maximum rating can cause excessive current to flow in

or out of the input pins, adversely affecting reliability.

Applications that exceed this rating must externally limit the

maximum input current to ±5 mA with an input resistor as

shown in

Figure 1.

3.0 Capacitive Load Tolerance

The LMC7111 can typically directly drive a 300 pF load with

S = 10V at unity gain without oscillating. The unity gain fol-

lower is the most sensitive configuration. Direct capacitive

loading reduces the phase margin of op-amps. The combi-

nation of the op-amp’s output impedance and the capacitive

load induces phase lag. This results in either an under-

damped pulse response or oscillation.

Capacitive load compensation can be accomplished using

resistive isolation as shown in

Figure 2. This simple tech-

nique is useful for isolating the capacitive input of multiplex-

ers and A/D converters.

4.0 Compensating for Input

Capacitance when Using Large

Value Feedback Resistors

When using very large value feedback resistors, (usually

> 500 k

Ω) the large feed back resistance can react with the

input capacitance due to transducers, photodiodes, and cir-

cuit board parasitics to reduce phase margins.

The effect of input capacitance can be compensated for by

adding a feedback capacitor. The feedback capacitor (as in

Figure 3), C

f is first estimated by:

1 CIN ≤ R2 Cf

which typically provides significant overcompensation.

Printed circuit board stray capacitance may be larger or

smaller than that of a breadboard, so the actual optimum

value for C

F may be different. The values of CF should be

checked on the actual circuit. (Refer to the LMC660 quad

CMOS amplifier data sheet for a more detailed discussion.)

5.0 Output Swing

The output of the LMC7111 will go to within 100 mV of either

power supply rail for a 10 k

Ω load and to 20 mV of the rail for

a 100 k

Ω load. This makes the LMC7111 useful for driving

transistors which are connected to the same power supply.

By going very close to the supply, the LMC7111 can turn the

transistors all the way on or all the way off.

6.0 Biasing GaAs RF Amplifiers

The capacitive load capability, low current draw, and small

size of the SOT23-5 LMC7111 make it a good choice for pro-

viding a stable negative bias to other integrated circuits.

The very small size of the LMC7111 and the LM4040 refer-

ence take up very little board space.

DS012352-14

FIGURE 1. R

I Input Current Protection for

Voltages Exceeding the Supply Voltage

DS012352-12

FIGURE 2. Resistive Isolation

of a 330 pF Capacitive Load

DS012352-13

FIGURE 3. Cancelling the Effect of Input Capacitance

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