Slew rate limiting in amplifiers for cars
Slew-rate control enables on-the-fly gate resistor adjustments during operation, thus simultaneously improving system efficiency and electromagnetic interference EMI. Both series implement slew-rate control functionality. It is a single-channel isolated gate driver family with up to 18 A output current and wide output supply voltage range. The slew rate can be adapted on-the-fly during operation based on gate resistor changes, enabling system output power to be optimized without changing the BOM and without compromising EMI behavior.
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5.4: Slew Rate and Power Bandwidth
As noted in the previous section, general-purpose op amps contain a compensation capacitor that is used to control the open loop frequency response. The signal developed across this capacitor will be amplified in order to create the final output signal. In essence, this capacitor serves as the load for the preceding stage inside of the op amp.
Like all stages, this one has a finite current output capability. Due to this, the compensation capacitor can be charged no faster than a rate determined by the standard capacitor charge equation:.
By definition, this parameter is called slew rate SR. The base unit for slew rate is volts per second, however, given the speed of typical devices, slew rate is normally specified in volts per microsecond. Slew rate is very important in that it helps determine whether or not a circuit can accurately amplify high-frequency or pulse-type waveforms. The resulting slew rate would be:. This means that the output of the op amp can change no faster than 3.
It would take this op amp about 3 microseconds for its output signal to change a total of 10 V. It can go no faster than this. The ideal op amp would have an infinite slew rate.
Although this is a practical impossibility, it is possible to find special high-speed devices that exhibit slew rates in the range of several thousand volts per microsecond.
Slew rate is always output-referred. This way, the circuit gain need not be taken into account. Slew rate is normally the same regardless of whether the signal is positive or negative going. There are a few devices that exhibit an asymmetrical slew rate. One example is the It has a slew rate of 0. In reality, the rising and falling edges are limited by the slew rate. The has a slew rate of 0. The resulting waveform is still recognizable as a pulse, however.
Here, the pulse width is only 3 microseconds, so the doesn't even have enough time to reach the high level. In 3 microseconds, the can only change 1. By the time the gets to 1. This same effect can occur if the amplitude of the pulse is increased. Obviously, then, pulses that are both fast and large require high slew rate devices.
Note that a op amp would produce a nice output in this example. Slew rate limiting produces an obvious effect on pulse signals. Slew rate limiting can also affect sinusoidal signals. All that is required for slewing to take place is that the signal change faster than the device's slew rate.
If the rate of change of the signal is never greater than the slew rate, slewing will never occur. To find out just how fast a given sine wave does change, we need to find the first derivative with respect to time. The maximum rate of change will occur when the sine wave passes through zero i. To find this maximum value, substitute 0 in for t, and solve the equation. From this, it is apparent that high-amplitude, high-frequency signals require high slew rate op amps in order to prevent slewing.
We can rewrite our Equation in a more convenient form:. A further rearranging yields. This frequency is commonly referred to as the power bandwidth. Note that slew rate calculations are not dependent on either the circuit gain or small-signal bandwidth.
This is a very important point! The effects of slewing can be either subtle or dramatic. Small amounts of SID are very difficult to see directly on an oscilloscope and require the use of a distortion analyzer or a spectrum analyzer for verification.
Heavy slewing will turn a sine wave into a triangular wave. A is used as part of a motor control system. If the highest reproducible frequency is 3 kHz and the maximum output level is 12 V peak, does slewing ever occur? For this application, the is twice as fast as it needs to be. If the calculation produced a smaller value, say 2 kHz, then slewing is a possibility for certain signals. An audio pre-amplifier needs to reproduce signals as high as 20 kHz.
The maximum output swing is 10 V peak. What is the minimum acceptable slew rate for the op amp used? For this design, a would not be fast enough. The aforementioned through would certainly be satisfactory, whereas the would probably be overkill. The circuit is configured with a 2 volt input at 20 kHz and a gain of 5. This will yield the worst case output of 10 volts at 20 kHz.
For the first Transient Analysis, a is used. Note how the output waveform is essentially triangular. It is also below the expected peak output level. Clearly, this waveform is severely slewed, and results in undesired distortion and a reduction in audio quality. The second simulation is performed using the faster LF In this case, the simulation shows a full 10 volt peak output with no discernable distortion. The LF would certainly meet the circuit requirements. There is a convenient way of graphically determining whether the output of an op amp will be distorted.
It involves graphing output levels versus frequency. The two major distortion causes are clipping and slewing. We start with a grid measuring frequency on the horizontal axis, and output voltage on the vertical axis. The first step involves plotting the output level limit imposed by clipping. Clipping is dependent on the circuit's power supply and is independent of frequency. The output level cannot swing above this line because clipping will be the result.
Everything below this line represents unclipped signals. The second step is to plot the slewing line. Plot this point on the graph.
This new point lets you graphically determine the slope of the slew limiting line. Everything above this line represents slewed signals, and everything below this line represents non-slewed signals.
As long as the desired output signal falls within the lower intersection area of the two lines, the signal will not experience either slewing or clipping. A quick glance at the graph allows you to tell what forms of distortion may affect a given signal.
What is the effective slew rate of the system? You might think that it is set by the slowest device at 0. The fact is that the system slew rate could be set by any of the devices, and it depends on the gains of the stages. The one thing that you can say immediately is that the system slew rate will never be faster than the final device. It may be less than this, however. The trick to finding the effective system slew rate is to start at the output of the first stage, and then determine the maximum rate of change for the following stages in sequence.
Looking at stage 1, its maximum output rate is 0. This is the maximum rate of change going into stage 2. Therefore the is the limiting factor at this point. This signal is then applied to stage 3, which has a gain of 3. This is the value used to calculate the system power bandwidth, if needed. The first op amp to slew in this circuit is the , even though it is about 30 times faster than the used in stage 1. The reason for this is that it must handle signals 32 times as large. Note that if the final stage had a larger gain, say 5, the would become the limiting factor.
The important thing to remember is that the front end stages of a system don't need to be as fast as the final stages, as they handle smaller signals. As discussed in Chapter Three, all op amps need some form of frequency compensation in order to ensure that their closed loop response is stable.
In this way, no matter what gain you choose, the circuit will be stable. Although this is very convenient, it is not the most efficient form of compensation for every circuit.
High-gain circuits need less compensation capacitance than a low-gain circuit does. The advantage of using a smaller compensation capacitor is that slew rate is increased. Also, available loop gain at higher frequencies is increased. Therefore, if you are designing a high gain circuit, you are not producing the maximum slew rate and small-signal bandwidth that you might.
Operational Amplifier ICs
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60v Op Amp
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Slew Rate for OP-AMP, Square and Sinusoidal Wave – How to Calculate It
Slew Rate of an Op-Amp is a critical factor in determining the overall performance of the electronic circuit. The maximum possible rate of change of output voltage of Op-Amp with respect to time is referred to as Slew Rate. In other words, it can be defined as the parameter that describes the rate of variation of output voltage per specified unit time, when the inputs change rapidly. It is directly proportional to temperature i. Amplifiers with higher SR have higher current consumption.
NCS20034: Quad Op-Amp, 7 MHz, High Slew Rate, Rail-to-Rail Output
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Module 6.4
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What Is Amplifier Slew Rate & Does It Affect Performance?
Ever wonder what slew rate was all about? No, it has nothing to do with how a drunk person slurs their speech. This article gets to the basics of Amplifier Slew rate and what it means. When reviewing the spec sheet of an amplifier, one potentially unfamiliar term you may run into is slew rate. There are a lot of gobbledygook explanations floating around the web which seem to misunderstand the basic premise, so we at Audioholics are here to clear the air. In short, slew rate has little to do with how an amplifier produces dynamics so much as its ability to effectively maintain output into higher frequencies.
The 1. The low input offset voltage 4 mV max allows the op amp to be used for current shunt monitoring. Additional features include no output phase reversal with overdriven inputs and ultra low input bias current of 1 pA. The NCS family is the ideal solution for a wide range of applications and products. Market Leadtime weeks : Contact Factory. Rail to Rail.
An ideal op-amp is a voltage amplifier with infinite input impedance, zero output impedance, and infinite bandwidth. In the real world, however, these parameters are limited by small imperfections in semiconductor manufacturing, effects of temperature, and so on. There is one parameter that is much overlooked but stands out in the above regard, and that is the slew rate.
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