TPS40305: Output accuracy for constant current application

Hi

    Definitely a controller for a 2A application would be an overkill. The current regulation accuracy would not be a function of the switching frequency but rather on the overall accuracy of the controller. What is the Vin of the application and the number of series/parallel LEDs with their Vf?

Regards,

Gerold

The Vin is 12V, one diode. Typical Vforward is 3.6V

Hi Timothy,

Is accuracy the most important consideration in your design? As Gerold mentioned, a controller seems like overkill for a 2A, single 3.6V diode. If solution size is important, you could probably use a TPS62130 or TPS82130 (integrated module) for this application as described in this application note.

Regards,
Kris

I have to squeeze the performance out of the TPS40305, 04 or 03. I was asking if the 03 was a better choice based on equations 4 and 5 of the Texas Instruments application note by John Tucker "Understanding output voltage limitations of DC/DC buck converters" slyt293.pdf

www.ti.com/.../slyt293.pdf

I looked at equation 4 and thought that since the minimum output voltage is dependent on frequency then the TPS40303 is a better choice than the TPS40305. Ton min is also the same for all three chips. And the upper limit on voltage is not dependent on frequency. Dmax is 5% higher on the 03 vs 05 but I'm more worried about the low end.  I haven't figured out the ramifications with the 03 vs 05 since I am doing constant current vs constant voltage but maybe somebody on the forum could help out with that question. 

Hi Timothy,

I may not understand the basis of your concerns. With 12V input, and output > ~3.6V across the diode, you will be pretty far from the 70ns minimum controllable on time even with the 1.2MHz switching frequency of the TPS40305. Put another way, assuming +20% tolerance on the 12V supply or a max of 14.4V, you would still be able to have a minimum output voltage of 1.39V (=70ns * 12V * 1.2 * 1.38MHz, where 1.38MHz is the max tolerance on the TPS40305 1.2MHz frequency). This is well below the expected typical 3.6V forward voltage of your LED.

Please let me know if I have misunderstood your concern.

Regards,
Kris

Hi Timothy

   Can you please let us know if you still have further questions on this topic?

Regards,

Gerold