ZHCSIQ4B September 2018 – December 2018 OPA828
In many applications, tight limits on opamp power consumption exist and it is therefore highly desirable that the amplifier’s power consumption remains constant even during fault conditions, such as a large voltage across the inputs or the output hitting the rail. In particular, high slew-rate amplifiers, such as the OPA828 temporarily increase the supply current when the amplifier is slewing. In slew-boosted amplifiers, the presence of a large input signal can present a specific problem, since it applies a large voltage across the amplifier’s inputs. This will activate the slew-boost and can lead to a significant increase in current consumption. In addition, at high supply voltages the large current consumption can lead to significant amplifier self-heating.
OPA828 offers a high slew rate of 150 V/us in combination with a comparably low supply current of 5.5 mA. Like many other amplifiers, this is achieved by a so called slew-boosting method, which temporarily increases the amplifier’s current consumption when the amplifier is slewing. Such a slewing condition is detected by measuring the voltage across the input pins. In quiescent condition, this voltage is very small (equal to the amplifier’s offset). If, on the other hand, an input voltage is changed rapidly, a large voltage will be applied across the inputs and the amplifier output needs to slew. On OPA828, the supply current increase is gradual and proportional to the applied input voltage, ensuring a well-behaved large step response and excellent THD. Because the high slew rate ensures the output re-settles in less than about 300ns, the increased power consumption is absorbed by the decoupling capacitors, and therefore does not additionally load the power supplies.
In OPA828, such an increase in current consumption is avoided by an additional protection circuit, which continuously monitors both the amplifier’s inputs and output. If a large input voltage is detected, the protection circuit checks for the presence of a rapid change in voltage at the output. If the output voltage is not changing, for instance because the output is at a supply rails, the protection circuit will disable the slew-boost circuit after a delay of about 300ns. After the overload condition is removed, the amplifier rapidly recovers to a normal operating condition. This is indicated in Figure 47, where the amplifiers supply current is measured with its decoupling capacitors removed. It can be observed that after 300 ns, the power consumption of the amplifier goes back to quiescent levels. At the same time, the amplifier still has an excellent overload recovery time of less than 55 ns.