# LED Driver IC Current Setting and Switching Frequency

I have this Boost LED Driver IC and this IC is new to me. So, I require some help on how to set the current for my LED String of 300mA. 9LEDs with each forward voltage of 3V and forward current of 300mA.

Can someone help me on how the value of RVFB is calculated for a LED Forward current of 300mA? What equation should I use?

And what should I do with the ADIM pin and OVFB pin in this case?

For switching frequency, suppose I have a 51kohm resistor between FSW and ground, my switching frequency is 980kHz? What factors should I consider to select a correct switching frequency?

EDIT :

1. For the switching frequency calculation, I used the equation 4 on page 16.

2. Power LED Datasheet . My requirement is to drive 300mA through an LED String of 9 LEDs.

3. Can someone help me to calculate on what the input current for the LED Driver IC would be, the switching frequency, what to do with the ADIM & OVFB pin and RVFB value calculation?

• Page 5 of datasheet explains the pin functions. Perhaps you can show us your requirements, (1) 1W LED x 9 = 9W, (2) 300 mA, 3V, (3) Switching frequency 980 Hz, ... You might like to give us the following: (4) Link to you power LED, (5) You idea of dimming and over voltage requirement/spec and which page on datasheet you are following, which tutorial you are following for the 980kHz , forward current setting etc. Perhaps you can split your big question into a couple of smaller questions, and start your suggestion, say, first on (a) Current setting, (b) Frequency selection, etc. Feb 17, 2021 at 5:10
• Sure, I have edited the question. Could you please help me with the additional information which I have provided in the edited question? Feb 17, 2021 at 5:16
• Sure, thank you. Could you help me with my questions? Feb 17, 2021 at 5:24
• The LED is for a 3.1inch TFT. I was given this Driver IC to design a boost. But I just need some sort of guidance to design this Feb 17, 2021 at 5:32
• Can someone help me with this question Feb 17, 2021 at 6:12

You already have the datasheet. Make use of it.

Here's the relevant section on the overvoltage protection feedback (OVFB) pin:

• Pick your overvoltage level. ($$\V_{OUT,OVP}\$$) This is a design choice made based on how much voltage would be too much. Set it a bit higher than the worst case $$\V_f\$$ for your LED string.
• Pick a value for one of $$\R_{OVPH}\$$ or $$\R_{OVPL}\$$ and solve the equation to get the other resistor value. Keep in mind the 1 microampere pull up current. You probably want to stay under 100k for each resistor.

For the current, you need to decide how (or if) you will do dimming.

The relevant sections:

• If you dim by PWM or don't dim at all, then you set $$\V_{ADIM}>1.2V\$$ and use $$\I_{LED}= \frac{300mV}{R_{VFB}}\$$
• If you dim by analog input, then use the same equation ($$\I_{LED}= \frac{300mV}{R_{VFB}}\$$) to set the resistor for the current at maximum brightness. The current through the LEDs is then given by $$\I_{LED}= \frac{0.3(V_{ADIM}-200mV}{R_{VFB}}\$$. You then vary $$\V_{ADIM}\$$ between 300mV and 1.2V to control the brightness (or 0 for off.)

You'll have to solve $$\I_{LED}= \frac{300mV}{R_{VFB}}\$$ to get the resistor value when given the current.

The switching frequency is your choice - the chip doesn't care. It will influence your choice of the inductor and the FET, though.

• Thank you. I am varying the ADIM using PWM Input between 0.3V and 1.2V. If I plug the value of ADIM in the second equation, I get Rvfb to be between 0.1ohms and 1ohms. So, what value of Rvfb should I pick? Feb 17, 2021 at 7:33
• You only need one value. You use the same equation ($I_{LED}= \frac{300mV}{R_{VFB}}$) to calculate the maximum current. The dimming just reduces from that maximum.
– JRE
Feb 17, 2021 at 7:44
• Thank you. So, I will choose 1ohm resistor. To help me with my question of input current, I have 9 LEDs. Each having a maximum forward voltage of 3.1V. So, My maximum boost voltage would be 27.9V. Assuming I pick an overvoltage limit of 30V and my input voltage is 13.5V, my worst case maximum input current would be 0.66A ? Is my calculation of input current, correct? Feb 17, 2021 at 7:48
• 28/13.5=2.06. Times 300mA, so input current over 600 mA is about right. Probably over 700 because of losses.
– JRE
Feb 17, 2021 at 7:54
• Thank you for your answer. Just a last question. What should I set the switching frequency to because there are no recommendations from the datasheet. Just a equation is given? Feb 17, 2021 at 7:58