One of the challenges when designing an op-amp circuit is verifying that the op-amp is stable when driving capacitive loads. The OPAx994 has a unique architecture that features Unlimited Capacitive Load Drive (UCLD), which is used to prevent sustained oscillations on the amplifier's output signals when driving large capacitive loads. This is achieved by maintaining an acceptable phase margin as the size of the capacitive load increases.

An op-amp circuit that is unstable will have an unpredictable or unexpected output with poor transient performance. This typically results in large overshoots and ringing when changes occur on the input or load, but may also result in sustained oscillations. One common cause of instability in op-amps can occur when connecting a load capacitor, CL, to the output of the amplifier. This instability is a result of the op-amp's internal output resistance, Zo, that creates a secondary pole with CL.

The OPAx994 family's UCLD has a proprietary output compensation structure that is able to sense the capacitance on the output and adjust internal pole and zero structures to achieve acceptable phase margins. This behavior is unique to UCLD devices and allows the op-amp to remain stable under larger capacitive loads compared to traditional amplifiers.

To keep an acceptable phase margin, UCLD devices lower the gain bandwidth product under larger capacitive loads. The OPAx994 is specified to have a gain bandwidth product of 24MHz without significant capacitive load, but this value will begin to decrease at the point where a traditional amplifier would begin to become unstable. This tradeoff is what extends the output drive capability. OPAx994 is designed with a wide gain bandwidth product to make sure there is headroom for many general-purpose applications with higher capacitive loads.

Figure 6-1 Extending Capacitive Load Drive With the OPA994

Figure 6-2 Phase Margin vs Capacitive Load