Thread Starter

Dan Aronin

Joined Dec 14, 2018
37
Hi all, a driver circuit needed for lighting a 30W LED* (one color) -

I had a look at the XL6005 datasheet and wanted to make sure I understand it properly based on the following diagram**:
Untitled.png

1. To drive the LED with 300mA I should make RS = 0.73 ohm (ILed=0.22/0.73=0.3A)
2. To limit the max voltage of the output I can put a Zener diode (optional)
3. dimming the voltage output can be achieved by a PWM signal through the EN pin.

Please correct me if I'm wrong!

Reference:
Led - https://www.ebay.com/itm/4w-10w-20w...hash=item3d412b38c8:m:maXqAHUMQRoDbZROCkkkJMQ

XL6005 - http://www.xlsemi.com/datasheet/XL6005 datasheet.pdf

*Ebay choices - Wattage:30W Qty:1
**Figure 11 taken from datasheet page 9.
 

bertus

Joined Apr 5, 2008
21,314
Hello,

The dimming is done in fig 10 of the datasheet.
The zener shoud be 1.3 times the Vf of the led, as given in the text above fig 11.

Bertus
 

ronsimpson

Joined Oct 7, 2019
1,179
The zener shoud be 1.3 times the Vf of the led, as given in the text above fig 11.
Is it optional or mandatory?
If the LEDs are open. (not installed) The power supply is a boost and it will boost the voltage up until something brakes. In the unlikely event the LEDs are not right the Zener limits the output voltage to save the IC and D1.
Optional.
I normally pick a Zener to save the IC not 1.3X the LED voltage. The IC = 60V max. The diode = 60V. The Feed back voltage is 0.2V. I would pick a Zener some where larger than 1.3X and smaller than (60-0.2V). I don't know what your boost voltage is. I have used a 47V Zener before. (Cout & C2 can also die from too much voltage) Honestly, I look in inventory and see what Zener we use often in other products. If we have too many 51V Zener diodes and no 47V diodes then …. ….

Choose the Diode (or not) because you know why the diode and what it does.
 

Thread Starter

Dan Aronin

Joined Dec 14, 2018
37
Optional.
If the LEDs are open. (not installed) The power supply is a boost and it will boost the voltage up until something brakes. In the unlikely event the LEDs are not right the Zener limits the output voltage to save the IC and D1.
Optional.
I normally pick a Zener to save the IC not 1.3X the LED voltage. The IC = 60V max. The diode = 60V. The Feed back voltage is 0.2V. I would pick a Zener some where larger than 1.3X and smaller than (60-0.2V). I don't know what your boost voltage is. I have used a 47V Zener before. (Cout & C2 can also die from too much voltage) Honestly, I look in inventory and see what Zener we use often in other products. If we have too many 51V Zener diodes and no 47V diodes then …. ….

Choose the Diode (or not) because you know why the diode and what it does.
Thanks!
Maybe you can explain how to set the boosted voltage output? What sets it how to calculate the value?
 

ronsimpson

Joined Oct 7, 2019
1,179
Maybe you can explain how to set the boosted voltage output?
The LED sets the output voltage.
A LED is much like a Zener diode. At low voltage there is no current. At some point the diode pulls current.
The PWM IC does not directly set the voltage. The IC monitors the current. (LEDs are a current device not a voltage device)
The IC pushes up the voltage until there is LED current. (current is measured by 0.22 volts across RS) If the LED is not here the voltage will go up too high and brake something at 60 volts.

A white LED will have about 3V per LED. Some LEDs have more than LED inside. So maybe 6V or 9V or 12V. This voltage is not accurate. It changes with temperature. It is not the same with every LED. Some might be 3.3V and some 2.5V. That is why we use current not voltage on a LED.
 

Thread Starter

Dan Aronin

Joined Dec 14, 2018
37
The LED sets the output voltage.
A LED is much like a Zener diode. At low voltage there is no current. At some point the diode pulls current.
The PWM IC does not directly set the voltage. The IC monitors the current. (LEDs are a current device not a voltage device)
The IC pushes up the voltage until there is LED current. (current is measured by 0.22 volts across RS) If the LED is not here the voltage will go up too high and brake something at 60 volts.

A white LED will have about 3V per LED. Some LEDs have more than LED inside. So maybe 6V or 9V or 12V. This voltage is not accurate. It changes with temperature. It is not the same with every LED. Some might be 3.3V and some 2.5V. That is why we use current not voltage on a LED.
Thanks! That helps a lot :)
So you mean the circuit acts like a voltage and current limiter - as current flows the LED draws more and more voltage (that goes up) according to its resistance (that changes with heat).
If I put a Zener which limits the voltage less than the LED max voltage, it will not go fully bright.
Please correct me if I'm wrong.

Maybe you know similar ics with more detailed documentation (maybe similar or DIP package)
 

Thread Starter

Dan Aronin

Joined Dec 14, 2018
37
Thanks! That helps a lot :)
So you mean the circuit acts like a voltage and current limiter - as current flows the LED draws more and more voltage (that goes up) according to its resistance (that changes with heat).
If I put a Zener which limits the voltage less than the LED max voltage, it will not go fully bright.
Please correct me if I'm wrong.

Maybe you know similar ics with more detailed documentation (maybe similar or DIP package)
One more aspect I thought of - if I apply constant current (300mA) to a high power LED (let's say which is rated 20v@300mA) and a voltage of 50V (limitation) will it drop only the necessary 20v (like current draw in voltage source)?
 

ronsimpson

Joined Oct 7, 2019
1,179
Applying 300mA to your 20V LED; The voltage across the LED will be about 20V. The data sheet will say some thing like V=20 typical, 25 max, ? min. If you have a 50V max set by a Zener it will have no effect when thing are working right.

Here is a picture of a tractor working hard. Gas is going to the engine. If the hitch broke and there is no load for the tractor to pull, there would be too much gas. The engine would go too fast. There is a governor that limits the engine RPMs. It will reduce the gas to a very small amount.
1585746909901.png
In a car you want to go the speed limit +5 more. lol With a tractor you want to go the right speed to get the work done. Slow if the ground is hard and faster if the ground is soft. This changes as you drive. My tractor, I can pull 60 horse power and the speed is what it is I do not care. (bad example)
 

Thread Starter

Dan Aronin

Joined Dec 14, 2018
37
Applying 300mA to your 20V LED; The voltage across the LED will be about 20V. The data sheet will say some thing like V=20 typical, 25 max, ? min. If you have a 50V max set by a Zener it will have no effect when thing are working right.

Here is a picture of a tractor working hard. Gas is going to the engine. If the hitch broke and there is no load for the tractor to pull, there would be too much gas. The engine would go too fast. There is a governor that limits the engine RPMs. It will reduce the gas to a very small amount.
View attachment 203126
In a car you want to go the speed limit +5 more. lol With a tractor you want to go the right speed to get the work done. Slow if the ground is hard and faster if the ground is soft. This changes as you drive. My tractor, I can pull 60 horse power and the speed is what it is I do not care. (bad example)
Ummmm, sure :)
I reckon then 50V@300mA won't burn the LED because it will draw only 20V@300mA, will it?
(Our imaginary LED - no datasheet available)
 

Audioguru again

Joined Oct 21, 2019
2,976
So you mean the circuit acts like a voltage and current limiter - as current flows the LED draws more and more voltage (that goes up) according to its resistance (that changes with heat).
If I put a Zener which limits the voltage less than the LED max voltage, it will not go fully bright.
Please correct me if I'm wrong.
No and no.
An LED sets its own voltage that barely increases as the current is increased. It is a diode, it is not a resistor. If you limit the voltage then the current might be zero then it does not produce any light.

LEDs cannot be made identical. An LED rated at 3V will have some at 2.5V, others at 3.5V and some at voltages in between.
Here is a graph showing a typical white LED that is 3V when it draws a current of 75mA. Its voltage is just a little higher at 3.25V when its is drawing a current that is 5 times (!) higher:
 

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Thread Starter

Dan Aronin

Joined Dec 14, 2018
37
OK, to be more precise I meant that the voltage output of the XL6005 doesn't have to be controlled (set to some values) because the LED will draw how much current it needs or unless you want to dim it. (Let's say if we apply 50V on 20V max LED, it will consume only 20V and not the whole 50V and burn out), although current has to be limited.
 

ronsimpson

Joined Oct 7, 2019
1,179
I think you know but you may have said it backwards.

You can not put 50V on a 20V LED. It will take 1000s of amps to get there.
If you put 300mA into the LED the voltage will be about 3V. Because the LED voltage is not accurate we measure current.

Resistor "RS" is used to measure LED current. Just like there is a low value resistor in your amp-meter.
The IC looks at the voltage across RS and adjusts so there is 0.22 volts across RS. The IC does not know what the voltage is. It only knows what the current is.
 

Thread Starter

Dan Aronin

Joined Dec 14, 2018
37
I think you know but you may have said it backwards.

You can not put 50V on a 20V LED. It will take 1000s of amps to get there.
If you put 300mA into the LED the voltage will be about 3V. Because the LED voltage is not accurate we measure current.

Resistor "RS" is used to measure LED current. Just like there is a low value resistor in your amp-meter.
The IC looks at the voltage יום RS and adjusts so there is 0.22 volts across RS. The IC does not know what the voltage is. It only knows what the current is.
Dear @ronsimpson according to your answer, I must set manually the output voltage of the XL6005 to match the LED voltage requirements.
But, some people here say that no such thing is needed because the LED will draw only the voltage it needs. (With constant current)
Btw, regular diodes have voltage drop of let's say 0.7V even when 5V is applied to them, will the LED behave the same (with different voltage)?
 

Audioguru again

Joined Oct 21, 2019
2,976
Dear @ronsimpson according to your answer, I must set manually the output voltage of the XL6005 to match the LED voltage requirements.
No, the XL6005 has a current output. The LED sets its own voltage.

But, some people here say that no such thing is needed because the LED will draw only the voltage it needs. (With constant current)
Yes.

Btw, regular diodes have voltage drop of let's say 0.7V even when 5V is applied to them, will the LED behave the same (with different voltage)?
No, you NEVER apply a voltage to a diode or LED, instead you apply a current then the diode or LED sets its own voltage.
Of course the voltage feeding the current-limiting device must be high enough to have some voltage across the current-limiting device (so that Ohm's Law sets the current) plus the voltage set by the diode or LED.
 
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