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Design >> Mixed-Signal Design >> Allowable voltage ripple in MPU supply voltage
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Message started by jbrydon on Aug 9^th, 2011, 4:23pm

Title: Allowable voltage ripple in MPU supply voltage
Post by jbrydon on Aug 9^th, 2011, 4:23pm

In an MPU with a supply voltage range of 3.3 to 5 volts I want to use the output of a switch mode chip that has a ripple of about a volt within that range to provide the supply voltage. Datasheets are silent on the allowable ripple, both magnitude & frequency. Texas Instruments says that's not a parameter they quote. Any ideas on deciding the allowable parameters (other than experiment) ?

Title: Re: Allowable voltage ripple in MPU supply voltage
Post by raja.cedt on Aug 10^th, 2011, 12:34am

O/P voltage ripple is the one of the spec for any regulator, because it desides decap size. This come from load specifications.
Thanks.

Title: Re: Allowable voltage ripple in MPU supply voltage
Post by jbrydon on Aug 11^th, 2011, 1:20am

I'm sorry, you have misunderstood my question. I wish to determine what voltage ripple my MPU will tolerate on its supply.

Title: Re: Allowable voltage ripple in MPU supply voltage
Post by raja.cedt on Aug 11^th, 2011, 4:15am

hello it depends on the spec yaar. Please check the load data sheets yaar.

Title: Re: Allowable voltage ripple in MPU supply voltage
Post by jbrydon on Aug 12^th, 2011, 6:33pm

If you read my original post you will see that the whole point of this is that MPU manufacturers are silent (as far as I can determine) as to the tolerable ripple allowed on the power pins of their MPUs. Please don't reply again if you still don't understand the problem.

Title: Re: Allowable voltage ripple in MPU supply voltage
Post by ywguo on Aug 13^th, 2011, 6:12pm

Hi Jbrydon,

Does the ripple of switch regulator exceed the spec of MPU? I think MPU must have minimum and maximum voltage specs.

Yawei

Title: Re: Allowable voltage ripple in MPU supply voltage
Post by jbrydon on Aug 13^th, 2011, 10:34pm

The ripple is within the supply voltage range of the MPU. My worry is that a rapidly varying supply, although within the DC limits will still cause problems. Imagine, for example, if you had a legal range of 3.3 to 5v and the supply was 4v with a fast ±0.5v square or sawtooth wave superimposed.

Title: Re: Allowable voltage ripple in MPU supply voltage
Post by harpoon on Aug 16^th, 2011, 1:48am

It is not easy to spec the ripple and what effect it has on the MPU.

It depends on what the MPU is doing internally and how the MPU uses those supplies.

Here is a stab at answering the question :-
a) If the MPU is in charge of some really tough PHY (e.g. USB2, PCIe, etc ...), then it is crucial to provide as clean a supply as possible, else eye diagram degradation creeps in ... (though this may not apply to the 3.3V pins)

b) To get an idea of what is tolerable, look at the power management circuit reference of the MPU and check what the ripple spec of the regulator powering the MPU is. Generally 0.5%-1% maybe ?

c) If the MPU has internal regulators (e.g. takes the 3.3V-5V input and drops it down to 1.2V internally), then there will be some spec/info regarding what the internal regulator can tolerate.

d) In your example of 3.3-5V input, wouldn't there be a decoupling cap on that pin ? This decoupling cap will smooth out your sawtooth and provide a semi-stable supply to the MPU.

hope this helps ...

Title: Re: Allowable voltage ripple in MPU supply voltage
Post by jbrydon on Aug 16^th, 2011, 10:22pm

Yup - all good ideas. I'm also thinking of speaking to ARM direct to see whether they issue guidance notes for integrators as to what inputs internal voltage regulators can tolerate.