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LED Driver IC Current Setting and Switching Frequency

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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?

voltagecurrentdc-dc-converterboostled-driverShareEditFollowFlagedited 7 hours agoasked 7 hours agoNewbie88444 silver badges2424 bronze badges

  • 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. – tlfong01 7 hours ago   
  • 1Sure, I have edited the question. Could you please help me with the additional information which I have provided in the edited question? – Newbie 7 hours ago
  • Perhaps you can also list the newbies tutorials explaining engineering trade offs and cost benefit analysis in LED drivers. I would suggest at least this short intro: components101.com/diodes/1-watt-led – tlfong01 7 hours ago   
  • 1Sure, thank you. Could you help me with my questions? – Newbie 7 hours ago
  • You NiChia LED is for back lit. What sort of back lit is it for? – tlfong01 7 hours ago   
  • Please let me know if there are terms in the Components 101 tutorial you don’t understand, and the reasons for using current drivers and negative feedback etc. Perhaps you don’t need to use the boost LED driver you suggested. You need to give a reason of choosing that particular driver, among the perhaps tens of drivers. – tlfong01 6 hours ago   
  • Me lazy hobbyist have been using simple drivers of package SOT23 5 or 6 pins and I do my own SMD reflow soldering. Do you already have samples of assembled modules? – tlfong01 6 hours ago   
  • 1The 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 – Newbie 6 hours ago
  • Ah yes, so it is for a LCD panel. Do you have any commercial products your are do reverse engineering, and they are using the driver you are copycating? – tlfong01 6 hours ago   
  • Let us continue this discussion in chat. – tlfong01 6 hours ago   
  • 1Can someone help me with this question – Newbie 6 hours ago

Please avoid extended discussions in comments. Would you like to automatically move this discussion to chat?Add a comment

1 Answer

ActiveOldestVotes2

You already have the datasheet. Make use of it.

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

enter image description here
  • Pick your overvoltage level. (VOUT,OVPVOUT,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 VfVf for your LED string.
  • Pick a value for one of ROVPHROVPH or ROVPLROVPL 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:

enter image description here
enter image description here
  • If you dim by PWM or don’t dim at all, then you set VADIM>1.2VVADIM>1.2V and use ILED=300mVRVFBILED=300mVRVFB
  • If you dim by analog input, then use the same equation (ILED=300mVRVFBILED=300mVRVFB) to set the resistor for the current at maximum brightness. The current through the LEDs is then given by ILED=0.3(VADIM−200mVRVFBILED=0.3(VADIM−200mVRVFB. You then vary VADIMVADIM between 300mV and 1.2V to control the brightness (or 0 for off.)

You’ll have to solve ILED=300mVRVFBILED=300mVRVFB 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.ShareEditFollowFlagedited 4 hours agoanswered 5 hours agoJRE45.8k88 gold badges7474 silver badges124124 bronze badges

  • 1Thank 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? – Newbie 4 hours ago
  • 2You only need one value. You use the same equation (ILED=300mVRVFBILED=300mVRVFB) to calculate the maximum current. The dimming just reduces from that maximum. – JRE 4 hours ago
  • 1Thank 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? – Newbie 4 hours ago
  • 228/13.5=2.06. Times 300mA, so input current over 600 mA is about right. Probably over 700 because of losses. – JRE 4 hours ago
  • 1Thank 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? – Newbie 4 hours ago
  • 1And just a clarification. If we set the Rvfb resistor to 1 ohms. And 300mA of current flows through the LED. The Voltage across the Rvfb and the differential input pins of the IC would be 0.3A * 1ohms = 0.3V which is the maximum voltage that the pins can take. Is it OK to not have any margin for this voltage level ? – Newbie 4 hours ago
  • 1It won’t damage the chip. The ranges given are for normal operation. The thing to watch out for is the absolute voltage between the feedback pins and ground. If you exceed that then you will damage the chip. – JRE 3 hours ago

Add a comment


LED Driver IC Current Setting and Switching Frequency

NewbieI 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…voltagecurrentdc-dc-converterboostled-driver

tlfong01

tlfong01Page 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. 

Newbie

NewbieSure, I have edited the question. Could you please help me with the additional information which I have provided in the edited question? 

tlfong01

tlfong01Perhaps you can also list the newbies tutorials explaining engineering trade offs and cost benefit analysis in LED drivers. I would suggest at least this short intro: components101.com/diodes/1-watt-led

Newbie

NewbieSure, thank you. Could you help me with my questions? 

tlfong01

tlfong01You NiChia LED is for back lit. What sort of back lit is it for?Please let me know if there are terms in the Components 101 tutorial you don’t understand, and the reasons for using current drivers and negative feedback etc. Perhaps you don’t need to use the boost LED driver you suggested. You need to give a reason of choosing that particular driver, among the perhaps tens of drivers.Me lazy hobbyist have been using simple drivers of package SOT23 5 or 6 pins and I do my own SMD reflow soldering. Do you already have samples of assembled modules? 

Newbie

Newbie5:36 AMThe 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 

tlfong01

tlfong01Ah yes, so it is for a LCD panel. Do you have any commercial products your are do reverse engineering, and they are using the driver you are copycating?So let us carrying on chatting here. Locking down lunch for me now. See you later. 

Newbie

Newbiehi  1 hour later…

tlfong01

tlfong016:57 AMHi there. 

Newbie

Newbiecould you help me with the answers for my questions 

tlfong01

tlfong017:09 AMI think you need to build up your knowledge and skills first. You might like to tell me if you know the following: (1) Ohm’s Law, (2) How to use Arduino to blink a LED, (3) How to play with LCD1602 or similar with black lit. You might also like to let me know if it is a College level project, and you only have two weeks to complete the project.If you don’t know how to use a multi-meter, then is is mission impossible. You might also like to read the following chat record to see how other newbies are learning the tricks: chat.stackexchange.com/transcript/119380/2021/2/5

.END

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