Application Information
1.0 INPUT COMMON-MODE VOLTAGE RANGE
The LMC6462/4 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.
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 LMC6462/4 is
180Ωsourcing, and 130Ωsinking at V
S
= 3V, and 110Ω
sourcing and 83Ωsinking at V
S
= 5V. The maximum output
swing can be estimated as a function of load using the
calculated output resistance.
3.0 CAPACITIVE LOAD TOLERANCE
The LMC6462/4 can typically drive a 200 pF load with V
S
=
5V at unity gain without oscillating. The unity gain follower is
the most sensitive configuration to capacitive load. Direct
capacitive loading reduces the phase margin of op-amps.
The combination of the op-amp’s output impedance and the
capacitive load induces phase lag. This results in either an
underdamped pulse response or oscillation.
Capacitive load compensation can be accomplished using
resistive isolation as shown in Figure 4. If there is a resistive
component of the load in parallel to the capacitive compo-
nent, the isolation resistor and the resistive load create a
voltage divider at the output. This introduces a DC error at
the output.
Figure 5 displays the pulse response of the LMC6462/4
circuit in Figure 4.
Another circuit, shown in Figure 6, is also used to indirectly
drive capacitive loads. This circuit is an improvement to the
circuit shown in Figure 4 because it provides DC accuracy as
well as AC stability. R1 and C1 serve to counteract the loss
of phase margin by feeding the high frequency component of
the output signal back to the amplifiers inverting input,
thereby preserving phase margin in the overall feedback
loop. The values of R1 and C1 should be experimentally
determined by the system designer for the desired pulse
response. Increased capacitive drive is possible by increas-
ing the value of the capacitor in the feedback loop.
01205105
FIGURE 1. An Input Voltage Signal Exceeds
the LMC6462/4 Power Supply Voltage
with No Output Phase Inversion
01205106
FIGURE 2. A ±7.5V Input Signal Greatly Exceeds
the 3V Supply in Figure 3 Causing
No Phase Inversion Due to R
I
01205107
FIGURE 3. Input Current Protection for Voltages
Exceeding the Supply Voltage
01205108
FIGURE 4. Resistive Isolation of
a 300 pF Capacitive Load
01205109
FIGURE 5. Pulse Response of the LMC6462
Circuit Shown in Figure 4
LMC6462 Dual/LMC6464 Quad
www.national.com11