LMC6035 Datasheet, PDF (12/29 Pages) National Semiconductor (TI) – Low Power 2.7V Single Supply CMOS Operational Amplifiers

LMC6035, LMC6035-Q1, LMC6036
SNOS875G – JANUARY 2000 – REVISED APRIL 2013
APPLICATION NOTES
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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 TI's CMOS amplifiers. The circuit of
Figure 43 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 600Ω 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.
Figure 43. Differential Driver
The circuit in Figure 43 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” arrangement 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 44 and Figure 45 represent measurements taken directly at the output (relative to
GND) of U1A, in Figure 43. Figure 44 illustrates the output swing capability of the LMC6035, while Figure 45
provides a benchmark comparison. (The benchmark op amp is another low voltage (3V) op amp manufactured
by one of our reputable competitors.)
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