How to simulate output noise of LDO - suggestions

output noise of ldo

Hi all,

Can anyone show me how to simulate the output noise of LDO?
Currently, I am using Hspice, and .noise command, but I not sure about the result of waveform ,


Rg,

Added after 2 minutes:

Is there any difference with PSRR and output noise in a LDO?

Thanks,

tdf.

Output noise of LDO

I can answer the PSRR Vs Output Noise part...I am not familiar with noise in HSPICE.

output noise occurs because of the noise present in the active components. E.g. the current and voltage noise (1/f, thermal noise, etc). The PSRR is the rejection of high frequency ripple/noise occuring in the supply.

SO, by definition they are two different quantities of measure

-TB

Re: Output noise of LDO

Hi TB,

Thanks for your reply.
With output noise and PSRR, I knew where they come from.

As my understanding, to see the PSRR, you must put an AC source in the voltage supply and then view the result. Is that right?

The output noise must be viewed in frequency domain, so they also need an AC source in the input, in the LDO case, that is voltage supply too. This test is similar PSRR, do I right? Is that AC source effecting the output noise?

Thanks,

tdf.

Re: Output noise of LDO

Simulate what type of noise, its internal noise, or how much external noise is supressed?

To simulate internal noise, you need to know the noise spectral density of the band gap voltage reference in the chip. A few chips have access to that pin, and you can clean up its noise with a .1 uF cap.

To simulate external noise mitigation, you write the control loop equations for closed loop gain due to an input error. The control loop gain, bandwidth, phase margin, etc, determine how many dB of noise rejection, if there are any noise peaks, etc.

Output noise of LDO

tdf,

Yes. you are right about putting the AC source in supply and viewing the output to study PSRR.

For the noise, you need an an AC input source. You can use the noise analysis function in the analog artist window if you are using cadence. Define input and output terminals. Monitor the output or input referred noise, which is the noise spectral density.

Re: Output noise of LDO

I am not familiar with hspice, but it sounds weird to me that you need to put an ac source for noise analysis. Of course that noise analysis is a small signal analysis but normally you don't need to insert an ac source.

Now, in an LDO, like in any other electronic block, you have diferent sources of noise. You must add the power of them all and chek that the total is below your application limit.

In an LDO, you have internally generated electrical noise. That's the one you estimate with the noise analysis. On top of that, the noise that comes from the reference source (at the plus input of your LDO) will also be transferred to the output with the (large signal gain) of the LDO. On top of that, the noise that is at the supplies (see at what freq it is) will be transferred attenuated by the psr(f).

You must consider them all.

Hi all,
anyboby help me for setup of LDO in ac analysis.

Hi vicky21b,

You need a inductor (1H) in the feedback loop and a large capacitance (1F) on the AC input. And then, do the simulation.

There is some thread that mention this issue, let search it first.

tdf,