Do we have information available for the Switch Current vs. Input Voltage for the LMR62421? The datasheet is still in Preliminary status.
I have a customer who is using the LM2735 for Vin=5V and Vout=22V, but it can only supply 400mA max at these ratings. They would like to have 500mA available.
I am also considering the LM3224 as an alternative. Let me know if you have any other suggestions.
The LMR62421 will have the same current limit as the LM2735. Sorry, no help there.
The LM3224 has a max Vout of 20V, I believe SW abs max is 22V.
The only other product I know of is the LM2586, which is a much lower frequency and larger package 3A boost.
You may have to go with a controller like the LM3481.
Thanks for the quick reply, Allan.
I used Webench to search for solutions for 4.5-5.5Vin, 22Vout @ 550mA, and it suggested the LM2587-ADJ and LM2588-ADJ, but not the LM2586. I just want to confirm that in a boost configuration, the LM2586 will be a viable option. I am not very familiar with Webench yet, and if it tends to give conservative results, etc.
To be honest, I did not run through the calculations. Webench does give conservative values, which should give you more confidence in the results. I would suggest entering a lower current value such as 450mA. Then select the LM2586 and increase the default inductor value to allow for more current. Then you may be able to increase the current to 550mA. If Webench doesn't allow it, then it is potentially a borderline selection and I would recommend using the LM2587 or 88.
To follow up on this, the boost parts are typically rated for switch current. A basic calculation for the switch current, which is approxmately equal to the input current is as follows:Pin = Pout/Efficiencyor Iin = Isw = (Vout * Iout)/(Vin * Efficiency)For this case if we assume 80% efficiency and VinMin which is worst case:Isw = (22 * .55)/(4.5 * .8) = 3.4AAdditionally any inductor ripple current and a safety margin needs to be added in.
For the LM2585 and LM2586 the switch current rating is 3A so they cannot support this design.
WEBENCH Design Center
Thanks for the follow-up, Jeff!
About how much safety margin should typically be added to the switch current?
You should design using the minimum current limit value, 5A for the LM2587. When you calculate switch current, including inductor ripple, a typical safety margin is +10-20%.
Its really up to you and depends on the application. Basically, you don't want the part to hit current limit during normal operation. So if you expect heavy load transients or high temperatures (which decrease the inductance) you may want to have a higher margin.
Thanks for the tips, guys! Much appreciated.
Bryan, Centralized AFA
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