Hello everyone,
We’ve developed a buck-boost LED driver based on the MP24833A chip, inspired by the EV24833-A-N-00A design. We purchased the reference design and modified it to limit the current to 650mA. The driver accommodates an input voltage range from 8V to 35V, while our LED strip operates at approximately 21V when at 650mA.
The primary challenge we’ve encountered is the size of the power inductor. Currently, we’re using the same Wurth 7447709680 inductor as the evaluation board, which has a maximum current rating of 3.2A and a total height of 10mm. Given our space constraints, we need an inductor with a lower DCR but reduced height.
Could anyone suggest an appropriate inductor with a reduced height that meets our current requirements? Your insights would be greatly appreciated.
What is your Vin? I find Coilcraft has a fantastic tool for picking inductors. They have a buck-boost calculator. Suggests for instance XAL1080-333 8mm tall 33uh and 35mohm of resistance
Hello @andrea.falcucci,
I understand your concerns regarding space constraints and requirement of lower DCR inductor. A lower DCR inductor helps in enhancing the efficiency by reducing the power losses. However, this comes at the cost of size of the inductor in terms of core and wire size(AWG). Is efficiency your concern too apart from space constraint?
Also, It is not recommended to deviate from the suggested EVB design which might affect the part’s performance (mostly current ripple).
Best,
Saquib
Hi @jshannon . The voltage range is 8-35V.
33uH seems to be so small to keep current ripple low.
Hi @Saquib.FAE and thank you for the prompt answer.
Since the huge voltage range we have to cover (8V-35V) and since the maximum current requested (650mA) I’m trying to evaluate the correct inductor to keep ripple within the range suggested by your datasheet (30% - 60%).
I’m landing to these results for two possible current consumption based on the specs of LEDs.
8V @ 530mA → 100uH with I avg = 1.77A and Ripple of 17%
35V @ 530mA → 100uH with I avg = 0.86A and Ripple of 72%
8V @ 650mA → 100uH with I avg = 2.17A and Ripple of 14%
35V @ 650mA → 100uH with I avg = 1.05A and Ripple of 59%
I found a possible inductor (B82477R4104M100 from TDK) with a current rating of 2.35A and a saturation of 2.9A that seems to be quite good. The DCR is 120mOhm.
Sounds good or I’m completely wrong in my mental path?
Thanks
Well I just plunked your numbers into their calculator. I would point out the high current case is when Vin is low. As soon as Vin rises abs value of ripple goes down as does the peak current
I guess the 33uH value is giving you about 59% ripple at 8V
There are a lot of ripple-o-phobes in the world. I don’t see anything that forbids 100% ripple in this IC. The only limit would be the switch current limit.
If you want a smaller physical inductor, and you have some room with the switch current limit, then this is the game you probably have to play.
Maybe I’m one of the ripple-o-phobes 
33uH and 59% ripple is fine but how it will perform the same inductor, since it will buck/boost, at 35V?
If I’m not completely wrong, the ripple will be higher than 100%.
Aside that, I’m not finding the MP24833 in the calculator… need to dig better inside.
Hello @andrea.falcucci,
The calculations look fine but B82477R4104M100 has higher DCR which will lead to more losses eventually affecting your efficiency.
Best,
Saquib
@jshannon thanks!
I’ll take a look even if I’m not able to match my calculation from the MPS datasheed to the one done by Coilcraft website… I’ll study both better
Hi @Saquib.FAE
Little confused because, as reported by @jshannon , the Coilcraft website suggest a 33uH as a good coil for buck/boost with our requirements with a deltaIL of 35%
At the same time, using 33uH on the formulas on MP24833A datasheet, carry ripple of about 200% (!!!) in case of VIN 35V, Vout 21V @ 530mA.
So little confused. Which is your suggestion for the coil to reach our goal?
About inductor power lossed, I’m roughly calculating them with Iavg^2 * DCR. Is it correct?
Thanks
I got no idea what the real peak current will be, I do trust the CC calculator. The peak is reported as 3A bigger value inductors will of course lower the peak at the expense of bigger physical size. Resistive loss is best done with Irms^2*DCR. The minimum peak current for the part is 3A so you might want to run the calculator constraining the ripple to 30% which would probably give you 47uH inductors. CC is really good with samples
Hi @andrea.falcucci,
I would not recommend choosing an inductor with ripple current beyond the mentioned limitations(60%) and this might affect your operating conditions as well. You can opt for a higher inductance value to stay within the current ripple limits.
Power Losses of an inductor include core loss and copper loss. The copper loss consists of both DC and AC resistive losses. The Power loss from DCR is Irms^2*DCR where Irms is the rms value of the peak-peak ripple current applied to the inductor.