# Some help interpreting formula variables in Texas Instruments TPS565201 Buck regulator datasheet

I am working on a custom PCB for a robotics project, and so far it is going well. I already learned how to work with 3.3v/5v LDO voltage regulators, but now I need a high current 6V DC solution for the motors. I found the Texas Instruments TPS565201 as a solution, which would work great with 3S/4S LiPo batteries, and would give me the 6V/5A that I would need for my project.

I read the datasheet and used the typical application circuit to implement the IC on my PCB. The only step left is to choose the right coil. I used the formulas in the datasheet to calculate the values required for my project. Since I don't have much experience doing this, I did this in Excel to do the calculations more easily. I ran into some variables in the formula that I could not find a definition/explaination of (only an example value, but I want to understand what I am doing instead of just using that), could you guys maybe help me understand what these variables are?

The formula variables I need to calculate to proceed:

• Lout
• Cout
• LO

These can be found on pages 13 and 14 of the TI datasheet

Unfortunately I could not find anything in the datasheet that explains what the variables should be/mean, which makes me assume that these variables should be common knowledge maybe? Could you guys guide me in the right direction?

Thanks in advance!

## 2 Answers

For switching regulator designs, especially as a less experienced designer, I would recommend sticking closely to the datasheet, reference designs, and, in the case of TI, you can use their WebBench tool. Note that $$\L_\mathrm{O} = L_\mathrm{out}\$$. The values are defined on the first page of the datasheet:

In your case, $$\V_\mathrm{out}\$$ is $$\6\ \mathrm{V}\$$, so according to Table 2, you should choose $$\L_\mathrm{out} = 3.3\ \mu \mathrm{H}\$$ and $$\C_\mathrm{out} = 44\ \mu \mathrm{F}\$$. That capacitor value is two $$\22\ \mu \mathrm{F}\$$ in parallel, mentioned just above section 8.2.2.3. Given those values, you can find the peak current in the inductor to choose the right one.

• Amazing! I was not aware of the WebBench tool, this will help a lot. I will stick to the specified partlist for now. Thanks for the quick and clear reply! Commented Aug 28, 2020 at 16:17
• @Stathis91 No worries, happy to help :) Welcome to SE too - feel free to upvote this answer if it was helpful. You can also mark it as an answer, but it's often good to wait a day or so to see if anyone else has a better answer. Commented Aug 28, 2020 at 16:19

Motor current on start can be 10x full load and 100x no- load full RPM, give or take 20%.

I think you would be better off surge current wise , controlling acceleration to prevent over-current or simply use a full bridge rated for 10x current and limit the over-voltage by the same amount in PWM max.

SMPS drivers make poor motor drivers unless it can drive stall current or acceleration is limited to rated current.

• The 2 motors (6V) of my project have 1.5A stall current each, and a no-load current of 0.10 A. I will also attach two small 12g servo's that do not use much current when stalled. 5A should be enough headroom to drive these all together right? The motor driver in my project also has a current sensor for each motor, this will allow me to disable the motors (or reduce the PWM signal to less power) via code when stalled. Commented Aug 29, 2020 at 8:47
• Will you have full control of braking by hi or low side FETs with PWM to reduce the BEMF energy generated driving the regulator with inertial energy stored? Commented Aug 29, 2020 at 12:23
• I am not entirely sure if I understand that fully, but I am using a motor driver board that adds an additional set of complexity between the motor and the 6V regulator (pololu.com/product/1213, based on 2x NXP MC33926), it seems to add reverse voltage protection (motors only, not for logic), over-current protection and over-temperature protection. Would this be enough to make sure that the 6V regulator is safe? Commented Sep 1, 2020 at 10:18
• Safe perhaps depending on how aggressive your g control is Commented Sep 2, 2020 at 0:27