Question asked by phoenix710 on Jul 31, 2015
Latest reply on Aug 4, 2015 by phoenix710

All:

I have successfully prototyped a new product based on the ADP2303 (and based very closely on the recommended 4 layer layout). It's output is set at 5.0V, and uses a 4.7 uH inductor (as spec'd by the Table 11 in the datasheet). Thus far, while I've tested with varying output currents, I was most concerned with the maximum current of 3.0 A and didn't really do much testing at really light loads.

My questions concern the proper use/interpretation of the formula for inductance (pg 18 of the datasheet):

Since my goal is to have a limited number each of fixed-output supplies (5.0V, 3.3V, and 2.5V, for  example), I started running thru some Excel spreadsheets based on the above formula to get a "feel" for what inductor values/ranges would be needed for each board. I found, somewhat to my surprise, that the inductor size increases dramatically as the output current drops. As an example, for Vin = 20.0, Vout = 5.0, the inductor values I calculated are 6.3 uH @ 3 A out, 12.6 uH @ 1.5 A, 37.8 uH @ 0.5A, and 189 uH 2 0.1 A.

I've rechecked the math and my spreadsheet and am convinced that this is exactly what the formula predicts. (The "left half" of the formula dominates the increase, due to Iload being in the denominator.) But does this mean that if I choose an inductor size, such as the 4.7 uH inductor for Vout = 5.0 and Vin approx 12 V, that if my product is run at a 200 mA load that the inductor is going to actually be much too small? (The formula indicates that the inductor should be 72 uH for these specs.) Will this cause premature failure of my product (probably either the converter or the inductor)? Conversely, is there any issue (other than the obvious parts cost "waste") if I choose a large inductor "to be safe"?

If this scenario (increasing inductance for decreasing load) is correct, how is it possible to design and implement a fixed output buck converter based on the ADP2303? Have I missed something really basic? What are the best practices for the design of a power supply that would be stable and reliable over a wide current range (from very small load to full load)?

Thanks very much for whatever assistance is offered.

Dave