I'm looking at using this switching regulator on my board:
My output voltage will be 5V, maximum output current 3A, input voltage 5-16V. The regulator will run at 2.5mhz.
On page 19 of the datasheet it lists an equation for calculating maximum inductor current.
I am considering using this inductor because it fits the reccomended 2.2uH inductance and has a maximum and saturation current of 3A.
The problem is, when I do the calculations on page 19, I arrive at a number that seems wrong.
I tried what I think is a worst case calculation:
IL = Vout * (1-Vout/Vin) / (Lmin * Fsw)
IL = 5* (1-5/16) / (0.0000022 * 2500000)
IL = 0.625
This seems way off. I mean, I know that if I'm drawing 3A from the regulator 3A must be passing through the inductor. So why is the value I get from this calculation so low? Am I off by an order of magnitude here?
Google says a microhenrie is 10^-6 henries, which is 0.000001. Multiply that by 2.2, I get 0.0000022. So the math seems sound. What am I doing wrong?
It says on the first page that the ripple current is load independent. Perhaps this means I should add 0.625 to my 3A load, and either decide that 2.3A is sufficient for my needs, or select a larger inductor capable of handing more current?
Oops, I just realized there was another equation on the previous page in the datasheet, and I'd failed to notice that one of the IL's has a little triangle in front of it and that specifies that it represents the ripple current.
Now off to the Eevblog to find out what ripple current represents. I'm sure I watched Dave give a tutorial on that recently.
Oh, and that other equation is IL = Iout + (/\IL / 2), so if /\IL is 0.625 then at 3A output IL = 3.3A, or I could have a 2.6A output with an inductor that saturates at 3A.
Hard to decide if I should go for the smaller 3A inductor:
Or this larger inductor rated for 5A:
I'm thinking I should just go for the smaller one. Costs half as much, and it actually states that it works at up to 5mhz which the Bourns does not. I assume it does, but it doesn't say, so...
I never understand why DC-DC converter datasheets try to explain
their design equations in poor english with much confusion (random
units is a good one, values in uF or kHz rather than F or Hz), when they
could simply include a 20-line C program that calculates all the
component values completely unambiguously.