MP2459GJ-P Inductor Sizing

DC-DC Power Converters
#1

Hi, I am looking to use MP2459GJ-P buck regulator to create a +24V output from +48V input, and also a +5V output from a +24V input. The output current requirement is 0.2A.

Using the inductor equation provided in the formula, I get values around 420 uH and 140 uH respectively for the 2 circuits. These are very large inductors.
image

When using the online tool, here https://www.monolithicpower.com/en/design-tools/design-tools/dc-dc-designer-online.html?PN=MP2459/, it gives me a value of 47 uH for both circuits. However, it seems possibly that the inductor value is maxing out at 47 uH. When I adjust the ripple current %, the inductor value remains at 47 uH. I would expect the inductor value to decrease with a higher ripple current, and vice versa.

Could you please let me know what is the correct inductor value to use, and whether I can create the supplies I need with this part. It seems to me that the Vin to Vout difference is too great for the amount of current we need, though everything is within the recommended operating specs per the data sheet. Supply Voltage VIN … 4.5V to 55V
Output Voltage VOUT …0.81V to 0.95×VIN

Thank you! Please let me know what other information I can provide.

Jeff

#2

Rules of thumb sometimes lead you astray. This is a peak current mode controller and the rule of thumb is inductor current ripple of 20-40%. If you have too little ripple current they the inductor is large and the PWM tends to be jittery. If you have too much ripple current you may not be able to make your required output power. In your case you can tolerate much more ripple than normal Your desired Iout is 0.2A and the parts internal current limit is 1A 100% ripple should work just fine. The best thing to do is go get a demo board and try different inductors. Coilcraft has an excellent tool for finding inductors on their website. They are also good with samples.

#3

Hi Jeffrey,

We would recommend using the 47uH and then tweaking the inductance value as needed during testing. But please note that the inductor current ripple might be higher since the inductance is lower.

Here is the typical application circuit that we have in our evaluation board, you can refer to this schematic for the sizing of components.