# Calculating Power Dissipation - What I need for a power supply

#### 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?

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.

#### crutschow

Joined Mar 14, 2008
26,739
Also what is the output load (besides the capacitance)?

#### matthej

Joined Oct 10, 2020
35
Updated my question above.

#### MrChips

Joined Oct 2, 2009
23,073
Updated my question above.
If the load is a similar driver SN74LVC244A and running at 1kHz I am going to say Icc max is 40μA.

#### matthej

Joined Oct 10, 2020
35
If the load is a similar driver SN74LVC244A and running at 1kHz I am going to say Icc max is 40μA.
So you just took the Icc of the device. What about Ioh for example? Wouldn't that be factored in too? Also if Icc is 40uA, how are you able to source mA of current from Ioh?

#### 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.