# Can one CC boost LED driver be used for 2 different loads separately?

I have a constant current boost LED controller (TPS92691) which I am designing to output 4A switching between two LED strings. This is with a 2S lipo battery. One string has a stack forward voltage of 13.2V at 4A and the other 11.2V at 4A. Initially coming into this, I thought that the boost controller would output 4A and deal with the two different stack voltages. But when I went into the data sheet and started calculating parameters, you come across calculating the Duty cycles of the controller for D , Dmax and Dmin.

As for Dmax and Dmin that's easy as I can take the Vo(max) as 13.2 and Vo(min) as 11.2. But for D the question came up of whether what I picked for D mattered, and it did! In calculations for compensator parameters to estimate small signal model parameters such as dc gain, zero frequency...D is required.

Which further got my doubting if I can even use one constant current output boost converter for 2 different loads - the boost output is switched between the two loads (at distinct intervals of time)...least to say I'm rather confused.

Also, commercial CC drivers usually mention as "x-yV output" which means that the output voltage has a compliance range where CC operation will be maintained in constant current mode right? But then how does one use the calculations in the datasheet to design around this compliance range?

• Why does this question need more clarity? It's a pretty clear cut question trying to clarify why Duty cycle calculation is voltage specific when most LED drivers marketed have a output voltage compliance range for which constant current is applicable. Commented Apr 23 at 23:17
• The first equation has a smudge. Correct it. Commented Apr 24 at 0:36
• @MOSFET That's not a smudge, I took off the values in the application example to only have the formulas... Commented Apr 24 at 0:37
• Like all compensation schemes, you're going to be trading off transient response for stability. It's just a matter of how sluggish a response can you tolerate. The datasheet recommends 60 deg phase margin with 10dB of gain margin. Design for worst case because that is where you will have the least stability - If you hit that operating point, all other operation points will be stable. Without even doing the math, putting a large enough origin pole capacitor will make the loop unconditionally stable. The transient response will be poor. But stable. You can do better with a pole-zero added. Commented Apr 24 at 1:36
• @MOSFET - Hi, Thanks for trying to help. However that comment appears to be an answer to the original question, which is not an allowed use of comments (please see here for why answering in a comment can be harmful & here for more details of the allowed uses of comments). Therefore please consider posting an actual answer and deleting that comment. TY Commented Apr 27 at 12:42

It may help you to look up a little more of the operating principles of the type of converter you're building, but essentially duty cycle sets the output voltage. For example, if you have a 24v source and a 12v LED string, a buck converter will operate at a 50% duty cycle. If you change it out for a 6v string the duty cycle will be 25% in order to obtain 25% of the voltage. Alternatively for a boost converter, you can use the equation you posted in your question to get the duty cycle, or to calculate what the output voltage would be for a given duty cycle.