Calculating Power Dissipation - What I need for a power supply

Thread Starter

matthej

Joined Oct 10, 2020
35
Hi,

I have a general question that I have always struggled with. I want to calculate what I need for a power supply to supply enough current to operate, lets say a simple LVC buffer (see attached). From the datasheet (running at 25 degC). Lets also assume it is driving the same buffer:

Vcc = 3.3V, Icc=500uA, Iol=4mA, Ioh=-4mA, Iin=1uA

So the simplest way is to just do Pd=Vcc*Icc = 3.3*500uA= 1.65mW.

But I am guessing that you need to add in the Ioh and the Iin(l) values too? So if it is just 1 buffer, you get an additional:

= Vcc*ioh + Vcc*Iin(l) = 3.3*4mA + 3.3*1uA = 13.2mW and then add to the 1.65mW to get your final answer which = 14.8mW.

The input capacitance of the receiver is 4pF and lets say this toggles at 1Khz.
Can anyone help me with this?
https://docs.rs-online.com/adfa/A700000006908799.pdf
 
Last edited:

MrChips

Joined Oct 2, 2009
23,073
No. You have ignored the most important part, switching frequency.
Power consumption increases with frequency and capacitance on output pins.
 

MrChips

Joined Oct 2, 2009
23,073
You are misinterpreting IOH and IOL. These are the output driving capabilities of the output pin. These will be determined by the load and yes must be taken into account. If the load is another CMOS gate then a typical input load is 1μA max at 0Hz. That is a drop in the bucket compare with 40μA.
 

Thread Starter

matthej

Joined Oct 10, 2020
35
You are misinterpreting IOH and IOL. These are the output driving capabilities of the output pin. These will be determined by the load and yes must be taken into account. If the load is another CMOS gate then a typical input load is 1μA max at 0Hz. That is a drop in the bucket compare with 40μA.
I guess what I am saying is how can Icc=40uA but Ioh can be in the mA range? I am assuming the Icc doesn't account for the Ioh values. So, for making sure my power supply was adequate current wise, I would need to add in Ioh and Iin values as well if they were driving another type of load which would increase the Icc value then.
 
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