LMV321-N, LMV321-N-Q1, LMV358-N, LMV358-N-Q1
LMV324-N, LMV324-N-Q1
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SNOS012I –AUGUST 2000–REVISED FEBRUARY 2013
Figure 50. Output Swing of LM324
CAPACITIVE LOAD TOLERANCE
The LMV321-N/LMV358-N/LMV324-N can directly drive 200 pF in unity-gain without oscillation. The unity-gain
follower is the most sensitive configuration to capacitive loading. Direct capacitive loading reduces the phase
margin of amplifiers. The combination of the amplifier's output impedance and the capacitive load induces phase
lag. This results in either an underdamped pulse response or oscillation. To drive a heavier capacitive load, the
circuit in Figure 51 can be used.
Figure 51. Indirectly Driving a Capacitive Load Using Resistive Isolation
In Figure 51 , the isolation resistor RISO and the load capacitor CL form a pole to increase stability by adding more
phase margin to the overall system. The desired performance depends on the value of RISO. The bigger the RISO
resistor value, the more stable VOUT will be. Figure 52 is an output waveform of Figure 51 using 620Ω for RISO
and 510 pF for CL..
Figure 52. Pulse Response of the LMV324 Circuit in Figure 51
The circuit in Figure 53 is an improvement to the one in Figure 51 because it provides DC accuracy as well as
AC stability. If there were a load resistor in Figure 51, the output would be voltage divided by RISO and the load
resistor. Instead, in Figure 53, RF provides the DC accuracy by using feed-forward techniques to connect VIN to
RL. Caution is needed in choosing the value of RF due to the input bias current of theLMV321-N/LMV358-
N/LMV324-N. CF and RISO serve to counteract the loss of phase margin by feeding the high frequency
component of the output signal back to the amplifier's inverting input, thereby preserving phase margin in the
overall feedback loop. Increased capacitive drive is possible by increasing the value of CF . This in turn will slow
down the pulse response.
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