BULK Capacitor Design Problem

[umesh49]

Hi,
I have been desiging a circuit for which customer is providing 5V regulated VCC to operate. As a general practice we put a bulk capacitor at VCC input line. I had been thinking it is mostly matter of experiance and we placed a capacitor around 10uF as consider on previous design.
Now our customer asked to justify this value and I am not sure how to calculate the value of bulk capacitors for such application (If I would use a LDO on board then simply I would refer the data sheet which suggest some values, but fortunatly this is not the case). My application is drawing a DC current from supply.
I would appreciate if any one can help me regarding this.

Thanks
Umesh.

 

[emresel]

Dear colleague;
I think it is a little bit related to your load current. If it is changing time to time, then your supply voltage might has some ripple on it, so you need bulk caps.
In your case (you said) it is DC current, in that case it shouldn't be matter 10uf or 100uf. It would be enough to filter some undesired enviromental noise.
But keep in mind that only a resistive passive load can consume DC current.
Generally all other sinks have some ripple in current.
By the way your power cct is Linear or switching type?

hopefully give any idea...

 

[umesh49]

Actually my load is not DC but I am placing bulk capacitor close in ICs which are drawing switching load, so I actual current drawn from VCC supply line is DC.

 

[chuckey]

" so I actual current drawn from VCC supply line is DC." I think you mean "mainly DC", unless you have an infinite capacitor any change of current through it produces a change of voltage across it. What you are trying to do is to make the change of voltage on your Vcc line small enough to be ignored. For some TTL logic one volt of noise can be ignored, for a microvolt instrumentation amplifier one millivolts may be too much. Also what you are trying to do is to reduce the PSU's impedance due to the leads to your circuit. Any piece of wire will look like an inductance, so relying on the PSU's decoupling capacitors may result in a significant impedance to any AC current component, hence you are trying to contain these within your circuit by your decoupling capacitor.
Frank