LMC6035IBPX datasheet(12/20 Pages) NSC

12 / 20 page

1.0 Application Notes

1.1 Background

The LMC6035/6 is exceptionally well suited for low voltage

applications. A desirable feature that the LMC6035/6 brings

to low voltage applications is its output drive capability — a

hallmark for National’s CMOS amplifiers. The circuit of Fig-

ure 1 illustrates the drive capability of the LMC6035/6 at 3V

of supply. It is a differential output driver for a one-to-one

audio transformer, like those used for isolating ground from

the telephone lines. The transformer (T1) loads the op amps

with about 600

Ω of AC load, at 1 kHz. Capacitor C1 functions

to block DC from the low winding resistance of T1. Although

the value of C1 is relatively high, its load reactance (Xc) is

negligible compared to inductive reactance (X

I) of T1.

The circuit in Figure 1 consists of one input signal and two

output signals. U1A amplifies the input with an inverting gain

of −2, while the U1B amplifies the input with a non-inverting

gain of +2. Since the two outputs are 180˚ out of phase with

each other, the gain across the differential output is 4. As the

differential output swings between the supply rails, one of

the op amps sources the current to the load, while the other

op amp sinks the current.

How good a CMOS op amp can sink or source a current is

an important factor in determining its output swing capability.

The output stage of the LMC6035/6 — like many op

amps — sources and sinks output current through two

complementary transistors in series. This “totem pole” ar-

rangement translates to a channel resistance (R

dson) at each

supply rail which acts to limit the output swing. Most CMOS

op amps are able to swing the outputs very close to the

rails — except, however, under the difficult conditions of low

supply voltage and heavy load. The LMC6035/6 exhibits

exceptional output swing capability under these conditions.

The scope photos of Figure 2 and Figure 3 represent mea-

surements taken directly at the output (relative to GND) of

U1A, in Figure 1. Figure 2 illustrates the output swing capa-

bility of the LMC6035, while Figure 3 provides a benchmark

comparison. (The benchmark op amp is another low voltage

(3V) op amp manufactured by one of our reputable competi-

tors.)

Notice the superior drive capability of LMC6035 when com-

pared with the benchmark measurement — even though the

benchmark op amp uses twice the supply current.

Not only does the LMC6035/6 provide excellent output swing

capability at low supply voltages, it also maintains high open

loop gain (A

VOL) with heavy loads. To illustrate this, the

LMC6035 and the benchmark op amp were compared for

their distortion performance in the circuit of Figure 1. The

graph of Figure 4 shows this comparison. The y-axis repre-

sents percent Total Harmonic Distortion (THD plus noise)

across the loaded secondary of T1. The x-axis represents

the input amplitude of a 1 kHz sine wave. (Note that T1 loses

about 20% of the voltage to the voltage divider of R

L (600

Ω)

and T1’s winding resistances — a performance deficiency of

the transformer.)

01283044

FIGURE 1. Differential Driver

01283045

FIGURE 2. Output Swing Performance of

the LMC6035 per the Circuit of Figure 1

01283046

FIGURE 3. Output Swing Performance of Benchmark

Op Amp per the Circuit of Figure 1

LMC6035IBPX Datasheet(PDF) 12 Page - National Semiconductor (TI)

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