OPA828 datasheet(20/47 Pages) TI

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7.3.4 Overload Power Limiter

Many applications have tight limits on op-amp power consumption; therefore, amplifier power consumption must

remain 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 OPAx828 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 because the large input signal applies a large voltage across the amplifier inputs. This large voltage

across the inputs activates the slew-boost and can lead to a significant increase in current consumption. At high

supply voltages, the large current consumption can lead to significant amplifier self-heating.

The OPAx828 offer a high slew rate of 150 V/us in combination with a low supply current of 5.5 mA. Like many

other amplifiers, these characteristics are achieved by a slew-boosting method, which temporarily increases the

amplifier current consumption when the amplifier is slewing. Such a slewing condition is detected by measuring

the voltage across the input pins. In a quiescent condition, this voltage is very small (equal to the amplifier

offset). Alternatively, if an input voltage is changed rapidly, a large voltage is applied across the inputs and

the amplifier output must slew. On the OPAx828, the supply current increase is gradual and proportional to

the applied input voltage, providing a well-behaved large step response and excellent THD. The high slew

rate makes sure the output resettles in less than about 300 ns; therefore, the increased power consumption is

absorbed by the decoupling capacitors, and does not additionally load the power supplies.

In the OPAx828, such an increase in current consumption is avoided by an additional protection circuit that

continuously monitors both the amplifier 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 example, because the output is at supply rails), the protection circuit disables the slew-boost circuit after a

delay of approximately 300 ns. After the overload condition is removed, the amplifier rapidly recovers to a normal

operating condition. This operation is shown in Figure 7-6, where the amplifiers supply current is measured with

decoupling capacitors removed. After 300 ns, the power consumption of the amplifier returns to quiescent levels.

At the same time, the amplifier still has an excellent overload recovery time of < 55 ns.

Time

-12

-18

-10

-12

-8

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-2

Competition

OPA828

Input

Output Railed

Figure 7-6. Supply Current Change With Overloaded Output

OPA828, OPA2828

SBOS671D – SEPTEMBER 2018 – REVISED DECEMBER 2022

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