LP8863-Q1: Calculation for Feedback Resistor in SEPIC Design

Hi All,

is there any update for my question?

Thank you

Hi, Ully

Could you pls share your result here? You can change R1 to match R2.

Hi Zhiqian,

The result for R2 is 52kOhm,

i also use the R1 Recommendation from datasheet, 560KOhm, and VOUT 14V

I also curious why the recommendation for R1 is 560kOhm, where did you get this value? is there any formula?

Hi, Ully

The formula is the same as the the Boost mode.

LP8863 is different from the normal design of switching device in boost output.

Pls use below formula to calculate min and max voltage

Hi Zhiqian,

I have calculated the MIN and Max BOOST using that formula and the Feedback resistor recommendation, 
the one that i would like to know is how to calculate the Feedback resistor, because the datasheet only provide the recommendation value, So where this value come from ( RFB1 560Kohm RFB2 170kOhm) is there any calculation to get the value of RFB1/RFB2? 
What if we do not follow the recommendation value, what will affect the LED Driver, that's why i want to know how to get that value

Thank you

Certainly, here are the points turned into individual questions:

1. How can I calculate the feedback resistor for my LED driver circuit if the datasheet only provides recommended values (e.g., RFB1 560 kOhm, RFB2 170 kOhm)? What methodology or formula should I use to determine the values of RFB1 and RFB2 for my LED driver circuit if they are not provided in the datasheet?

2. Why are specific values recommended for RFB1 and RFB2 (e.g., RFB1 560 kOhm, RFB2 170 kOhm) in the LED driver datasheet? What factors influence these recommendations? so there must be an equation for that right?

3. What are the potential consequences or effects on the LED driver's performance if I choose to deviate from the recommended values of RFB1 and RFB2?Are there any considerations I should keep in mind when selecting alternative values for RFB1 and RFB2 to ensure optimal performance of the LED driver circuit?

Hi, Ully

You can find the answer in this section of the datasheet, and datasheet recommend to use 170k for RFB2.

It is the same formula as the boost topology.

AN-1484 Designing A SEPIC Converter can help you design the inductor, cap and other component, for FB you need to use the formula in the datasheet.

Hi Zhiqian, 

I have tried to calculate using that formula, 

I m using VMaxBOOST = 24V( Our VBOOSTMAX) , 26.89V (VBOOSTMAX Result If I used the RFB1/RFB2 Recommendation),

using a known VBOOST value, I used the formula to get RFB2, but the result is still not 170kOhm.


I Know that datasheet recommendation is 170kOhm and 560Ohm, but i still don't got it, where is this value come from? 

Hi, Ully

Where do you find the recommendation value of resistor?

This two value is related with the boost output requirement.

Hi Zhiqian

I got the recommendation value from LP8863 Datasheet

I know that the feedback resistor is related with boost output, that's why i want to know where this recommendation value come from

Hi, Ully

The 170k is recommended, for 560k, it is related with the boost output. 560k is used for the specific value of output requirement.

Hi Zhiqian, 

Okay got it, I tried to calculate the RFB1 using the known value from VBOOST 24V and RFB2 = 170kOhm. And got the result for RFB1 = 499kOhm (Same as our resistor setting)

From this, I have a question, from datasheet, the SEPIC output voltage is 6V -24V right? So what about the 560kohm recommendation? meanwhile if I am using 24V (maximum value) then using the formula given by you (datasheet)  i got the resistor value 499kohm for RFB2.

And also i am still curious, where the 170kOhm come from, why TI give the recommendation value 170kohm for SEPIC Design?

Hi, Ully

170kOhm is from our internal control logic, our device doesn't have an external COMP pin.

For the resistor design, Pls calculate the maximum and minimum voltage depending on the formula.

560kOhm can also cover the 6-24V.

Hi Zhiqian,

sorry i still have a question, you said that the RFB1 is related to the specific value of output requirement.

and you give me this formula right:

If i used RFB1 560KOhm, and RFB2 170kOhm, the result for VBOOST is 26V.

But the output specification for SEPIC from the datasheet is 6V-24V right?

So I am really confused.

please help me to understand it.

Thank you

Hi, Ully

lets explain it during the afternoon meeting.

Hi Zhiqian,

Alright, thank you

Hi Ully

pls let me know if you still have any questions.