Driver LED LNG420EG

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Hello!I plan on using the solution LNK420EG for LED driver.Are 36 LEDs in series, I will need an output voltage of 125V and currenthave to vary according to the resistance of an LDR (2K ~ 27K). The variation ofCurrent will be according to the resistance of LDR 180 ~ 450mA.This is the best option? And putting the LDR will work well for FB?Let's eat at the beginning about 300 pcs / month and would like to know a representateHere in Brazil, already use some items for switching power supply, but the HITECH OF BRAZIL, it sells the items.
Thank you!


For an output of 125V, the best suitable topoly would be buck-boost specially it it is  non-isolated.  you can use the reference.  Then add your LDR in sink some current on the FB pin.

You can use the atttached approach to vary the output current.

I hope this helps.. 


Thank you! A great solution, I liked!But, I need to isolate the primary from the secondary! Attached is the solution I built using the PI EXPERT 9.Do you think the solution attached will not work?Associating with LDR R14 I can vary the output current?Initially I thought to use a flyback power, and to control the LEDs a micro-controller + BUCK, but I thought the safest solution POWER (LinkSwitch).Thank you!


It will work, just add the circuit to share some current to the FB pin using the LDR. 

By the way the .uds file looks like corrupted, I am having difficulty to open it.  Good thing you attached a pdf schematic.

Please make sure to have some heatsink. 

Nice project, you can easily power-up this design.  Use the PIXL if you need some tuning in case the output current is a little bit off.  The calculation is based from some assumptions.  Actual verification is required.


Hi!Rode a prototype for testing using Design Kit RDK-290.Without triggering circuit LDR alone and changing the value of R14 to decrease and increase the output current work, but the output voltage falls when the carrente tail off. (Iout = 360mA Vout = 117V) (Iout = 220mA Vout = 109.5 V) What can I do to keep the output voltage and only change the output current?Triggering the LDR circuit voltage and output current was very unstable, between 85 ~ 265VAC. It's good to put a resistor between BP and FB?Gotta keep the LEDs flashing on a cycle on / off CONTINUOUS. Added a key to bringing down the V pin to ground, as shown in the Annex. Got a problem with respect to lifetime of LNK?Follow the photo mounted with plate Kit
Thank you!


It is a normal characteristic for the LED voltage to drop once the drive current is reduced.  Please refer to this sample specification of an LED (page 7).  The driver has no control in the LED voltage, it is a physical characteristic of an LED. 

Can you tell me what is the objective/spec for  LDR?  If the objective is to adjust the output current depending on the light then it should work.  Make sure that the LDR circuit threshold is property calculated.

On off operation should not be a problem.  The device is rugged enough for this kind of application.  The heatsink is used to cool-down the device becuase the power  loss is high at 70W application.  You can reduce the size of the heatsink depending on your system level of application.



Good morning!That's right, the LDR has the function of miminuir output current according to light.I'll do some more tweaking, but I'm happy with the results.Thank you very much for your help!

Hello!I'm trying to reduce the current in nighttime use, the ideal would be 150mA. I made adjustments to reduce the current, but was unstable depending on the input voltage. Input - 85Vac / output - 106mA Input - 110vac / output - 135mA Input - 220vac / output - 190mA Input - 265VAC / output - 202mA What can I do to keep current more stable? Thank you!

hi Jhonny,


Jono is currently out on a business trip and I will be the one to assist you on your question, atleast until he comes back this week.


Regarding your question about line regulation, can you replace the LDR with a fix resistor value based on what the nominal output current you desire at the nominal input voltage at either low line or high line and check the line regulation?


I just want to check if the LDR is causing such a deviation in line regulation or is it the really the circuit's characteristics even without the LDR.



Hello Crumb,  Thank you for helping me! The test was done without the LDR only decrease the value of the Zener. With 250mA does not oscillate much, but I need to have a better precision 120mA. Thank you!

hi Jhonny,


Please calculate the operating IFB at 120mA. If IFB falls below 85uA, LNK-PH will go into duty-cycle reduction mode which could affect line/load regulation. 

What is your maximum output power? You can try to use smaller device and increase the operating range of IFB in case your current IFB is very low.


Hello Crumb,
Maximum power is 125v @ 350mA.
Attached is the draft schematic to better assess.
Thank you!

At 43W, I think it's better to use LNK-419 so that the operating IFB at maximum power is higher and hope that at the lower power, the IFB does not go below 85uA. 

Assuming a bias voltage of 20V, when the transistor is fully off, then the IFB is around 120uA. If it's fully ON, then the IFB with 12V zener is 70 uA which is already below 85uA. That's why I suggest you use a more appropriate LNK-PH device and check if you see an improvement. Try not to operate below 85uA IFB.


Hello all right?  Had not seen your reply, sorry! Ok, I'll try with LNK-419. Thank you!

Hello, worked very well with LNK-419. Thank you for your help!

Hello, could improve the stabilization of the current, made ​​some changes in the transformer and it was good!now implemented the circuit ON / OFF, but the source goes into protection mode, then added a 100nF capacitor to GND pin V improved the power on and off commands as the CI555. But interfere with the current and voltage output decreasing somewhat.I am using an optical coupler in place of the transistor.What can I do to make the opto does not influence?
Thank you!