LMC6035IMMSLASHNOPB datasheet(18/44 Pages) TI

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7.2 Typical Applications

7.2.1 Differential Driver

The LMC603x are an excellent choice for low-voltage applications. A desirable feature that the LMC603x bring to

low-voltage applications is the output drive capability—a hallmark for TI's CMOS amplifiers. The circuit of Figure

7-3 illustrates the drive capability of the LMC603x at 3V of supply. These devices are used as 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 600Ω of ac load, at 1kHz. Capacitor C1 functions to block dc

from the low winding resistance of T1. Although the value of C1 is relatively high, the capacitive load reactance

(XC) is negligible compared to inductive reactance (XI) of T1.

–

10μF

100k

200k

100k

VIN

1.5V

600

VOUT

U1A

U1B

1:1

(600

to 600 )

Figure 7-3. Differential Driver

The circuit in Figure 7-3 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. The two outputs are 180°

out of phase with each other; therefore, 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 output swing

capability. The output stage of the LMC603x—like many op amps—sources and sinks output current through two

complementary transistors in series. This totem pole arrangement translates to a channel resistance (Rdson) at

each supply rail that 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 LMC603x

exhibit exceptional output swing capability under these conditions.

The scope photos of Figure 7-4 and Figure 7-5 represent measurements taken directly at the output (relative to

GND) of U1A, in Figure 7-3. Figure 7-4 illustrates the output swing capability of the LMC6035, while Figure 7-5

provides a benchmark comparison. (The benchmark op amp is another low-voltage (3V) op amp manufactured

by one of our reputable competitors.)

LMC6035, LMC6036

SNOS875H – JANUARY 2000 – REVISED DECEMBER 2024

www.ti.com

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