LNK304GN

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RUa
LNK304GN led driver
Hello!
Could you help me wiht my led design, please? My circuit diagram is on the picture.
My parameters are:
LinkSwitch-TN: LNK304GN
Rfb = 300
Rbias = 2k
Rsense = 22
Csense = 10u
Co = 10u
L = 2.8 mH
My load is one led chain (12 leds, 2.86V for led). Need current 100 mА. So, output voltage should be Vo = 34,3 V.
My problem is that if I put Rsense = 22 Ohm, I got current Io = 90 mА and the circuit works fine; But when I put Rsense = 20 Ohm for current 100mA, my circuit has failure and led chain starts blinking with near 800 ms period (it seems LNK304GN restarting). What is wrong with my circuit?
with respect,
Oleg

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Hi Oleg,

The blinking LED may be caused by auto-restarting of the device LNK304GN. It enters auto-restart if it sees an output overload, output short, or open loop. If the FB pin is not pulled HIGH for 50ms, the device disables the switching.

Please see the following possible solutions:
1. Remove 1 LED to your led string, it might be seeing an output overload.
2. Monitor FB voltage reference to Source pin. FB voltage should be 1.65V.

Questions:
1. Are you using a zener diode at VRFB? If yes, what is the zener voltage?
2. What is your input voltage?

Thanks. Please let me know if this helps.

Other references.
https://www.power.com/sites/default/files/product-docs/lnk302_304-306.pdf

PI-Zek

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Hi, PI-Zek,

Thanks for your reply!

your solutions:
1)remooving 1 LED helps (I got Io=100mA, Vo = 32.2V), but input input voltage range is near ~180...220 VAC(my input circuit is in the attachment). when VAC is below 180V (256Vin after bridge), the circuit becomes unstable(blinking).
2)when circuit is stable, I have 1.613V FB voltage.

your questions:
1. I'm not using an optional circuit with zener diode.
______________________
I think I have to divide my 1 led chain to 2 led chains in parallel to reduce output voltage Vo, but it is very inconvenient...

or may be I can try to enlarge Rfb resistor...

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Hi Oleg,

1. What is your target output voltage? If you divide the LEDs into two identical string they will have the same voltage each string. The output current will be divided, in your case, 50mA per string (if identical).

2. Is your input voltage at universal, 85-265VAC? Does this mean the unit is not working at low-line only? (below 180VAC).

Try the ff suggestions:
1. FB voltage threshold is at 1.54V. You can adjust FB resistors if it reaches the threshold at low-line.
2. Monitor the BP voltage. BP voltage should be at 5.8V. IF BP voltage is below 5.6V. Increase BP cap or add a pull-up resistor to supply current to BP. Rpull-up is tied to Vo to BP). (500k-1M)

Thanks.

Hi, PI-Zek,

1) I wish my target output voltage to be near Vo = 34-35V, but, unfortunately,
coldn't achieve stable circuit work if Vo > 25V for universal voltage range 85...265 VAC.

2) Yes, my input voltage at universal, 85-265VAC and unit is not working at low-line (ment below 180VAC, if Rsense = 20 Ohm, wish Io = 100 mА, Vo = 34,4V ).

Thank you for your help, PI-Zek, but I have no time to later research. I hope playing with FB & BP resistors could help (may be later I will try), but now dividing the LEDs into two identical strings works fine (unfortunately, currents in the strings are a little differ, even they are identical).

Hi,

This part:
but now dividing the LEDs into two identical strings works fine. --> Does this mean the unit works fine at all input line (85-265VAC)? That is great news!

Yes you can also use parallel LEDs. They may not have similar current due to difference in their forward voltage but that is ok. You can check if the LED specification can handle 100mA and if LEDs are all working fine. If the LED cannot handle the current the voltage drop will differ. This is a possible reason why the circuit works fine using parallel LEDs vs single string.

Thanks. Good luck!