LMC6464BIN National Semiconductor, LMC6464BIN Datasheet - Page 10

• Manufacturer Part Number: LMC6464BIN
• Description: Operational Amplifier (Op-Amp) IC
• Manufacturer: National Semiconductor

Specifications of LMC6464BIN

• No. Of Amplifiers: 4
• Bandwidth: 50kHz
• No. Of Pins: 14
• Peak Reflow Compatible (260 C): No
• Single Supply Voltage Min (+v): 3V
• Leaded Process Compatible: No
• Amplifier Type: Operational
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Application Information
2.0 Rail-to-Rail Output
The approximated output resistance of the LMC6462/4 is
180
sourcing and 83
swing can be estimated as a function of load using the calcu-
lated output resistance.
3.0 Capacitive Load Tolerance
The LMC6462/4 can typically drive a 200 pF load with V
5V at unity gain without oscillating. The unity gain follower is
the most sensitive configuration to capacitive load. Direct ca-
pacitive loading reduces the phase margin of op-amps. The
combination of the op-amp’s output impedance and the ca-
pacitive load induces phase lag. This results in either an un-
derdamped 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 cir-
cuit in Figure 4 .
FIGURE 3. Input Current Protection for Voltages
FIGURE 5. Pulse Response of the LMC6462
sourcing, and 130
FIGURE 4. Resistive Isolation of
Exceeding the Supply Voltage
a 300 pF Capacitive Load
Circuit Shown in Figure 4
sinking at V
sinking at V
S
= 5V. The maximum output
S
(Continued)
= 3V, and 110
DS012051-7
DS012051-8
DS012051-9
S
=
10
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 de-
termined by the system designer for the desired pulse re-
sponse. Increased capacitive drive is possible by increasing
the value of the capacitor in the feedback loop.
The pulse response of the circuit shown in Figure 6 is shown
in Figure 7 .
4.0 Compensating for Input Capacitance
It is quite common to use large values of feedback resis-
tance with amplifiers that have ultra-low input current, like
the LMC6462/4. Large feedback resistors can react with
small values of input capacitance due to transducers, photo-
diodes, and circuits board parasitics to reduce phase
margins.
FIGURE 6. LMC6462 Non-Inverting Amplifier,
Compensated to Handle a 300 pF Capacitive
FIGURE 7. Pulse Response of
LMC6462 Circuit in Figure 6
and 100 k
Resistive Load
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DS012051-10