Slew rate of the OPAMP for DC application

[ananthesh bhat]

Hello,

I'm designing an OPAMP for the voltage reference. Can anybody explain me what I should consider the value for slew rate of the OPAMP?
I'm keeping this slew rate value as the reference value to decide upon the current requirements of my OPAMP.

 

[D.A.(Tony)Stewart]

Bipolar output amplifiers come in a wide range of products from 0.5mA to 5A and wide range of slew rates.

So your assumption depends on application.

 

[erikl]

I'm designing an OPAMP for the voltage reference. Can anybody explain me what I should consider the value for slew rate of the OPAMP?

I think this - and the closed loop bandwidth of the OPAMP - depends on the necessary speed of control of your voltage reference.

 

[dick_freebird]

Slew rate should exceed the characteristics of incoming
supply ripple by a comfortable margin if you want the
reference output to not follow it. You might relieve this
requirement by pre-regulating the local supply to the
reference, scraping off the HF ripple and letting the op
amp deal with only the DC variability of the crude pre-reg.

 

[ananthesh bhat]

I think this - and the closed loop bandwidth of the OPAMP - depends on the necessary speed of control of your voltage reference.

Hello,
Can you please explain the speed of control of the voltage reference?

 

[erikl]

Can you please explain the speed of control of the voltage reference?

See dick_freebird's answer above:

Slew rate should exceed the characteristics of incoming supply ripple by a comfortable margin

Means the same: you want to compensate the incoming supply ripple, which means your control speed (slew rate, bandwidth) must be higher than that of the incoming voltage changes which you want to compensate.

 

[ananthesh bhat]

See dick_freebird's answer above:


Means the same: you want to compensate the incoming supply ripple, which means your control speed (slew rate, bandwidth) must be higher than that of the incoming voltage changes which you want to compensate.

Thanks for the clarification.
Yes. I agree with that. Can I assume a slew rate to be of value 1V/us and 30 Mhz as the unity gain bandwidth for my OPAMP? I will be using it for the bang gap circuit.

 

[erikl]

Can I assume a slew rate to be of value 1V/us and 30 Mhz as the unity gain bandwidth for my OPAMP?

As mentioned several times: you have to know the frequency or slew rate of your input ripple (input voltage changes). The slew rate resp. bandwidth of your regulating OPAMP should be greater.

 

[ananthesh bhat]

As mentioned several times: you have to know the frequency or slew rate of your input ripple (input voltage changes). The slew rate resp. bandwidth of your regulating OPAMP should be greater.

Hey,

I agree that you are saying I should keep input as the reference. My question is if I'm taking input to the opamp as DC, then what can you say about slew rate requirement? I'm deciding the value of the tail current of opamp based on this slew rate. So this is my starting point in the design.

You mentioned that I should look for the ripple in the power supply. But the point is, I'm not connecting the power supply to my opamp input terminals. So what I should consider as slew rate in this scenario.

I went through quite a lot of the papers regarding the same topic. Finally I'm concluding that my slew rate requirement should be near to 1V/us and there will be no constraints on the bandwidth.

Correct me if this conclusion is wrong!!

 

[leo_o2]

Hi ananthesh,

Slew rate of opamp might influence the start-up time of the voltage reference. When it is power off, it takes some time to start up the target value. If more current is burned, your reference voltage will start up faster.

 

[ananthesh bhat]

Hi ananthesh,

Slew rate of opamp might influence the start-up time of the voltage reference. When it is power off, it takes some time to start up the target value. If more current is burned, your reference voltage will start up faster.

Hello Leo,

I got to implement a start up circuit since my architecture is a self biased one. So start up time will be handled by this start up circuit anyway.

 

[D.A.(Tony)Stewart]

Slew Rate and DC application seems like an Oxymoron .

1st you decide what load and slew rate you need on power up ready then you will know the current requirements or visa versa.