# High Wattage white led with LM2576 & variable intensity problem

#### Vindhyachal Takniki

Joined Nov 3, 2014
589
High Wattage white led with LM2576 & variable intensity problem

1. I have to drive a white led. Input voltage is 6V-8.4V. I need to vary the intensity using a potentiometer.

2. Using the ckt as in attached image. Not using optional output ripple filter. Ckt shows use of HV version, but I am using normal version of LM2576

3. Directly connected the outout to white led. Max output voltage set to 3.7V. Instead of using 1N5822, I have used SS34 diode.
R1 = 5K fixed
R2 = 10k(pot)
Calculated using formula:
Vout = Vref(1 + R2/R1)
Vref = 1.23V, R1 between 1K & 5K
By fixing r1=5K, R2 value comes out to be 10K

Problem:
1. When I rotate the pot from one end to another, led brightness smoothly decreased.
2. But when it is at max end, i.e output voltage at 3.7V, & slighlty rotate the pot in another direction, suddenly the output voltage drops to 3.05V & there is noise starts from regulator also. Which causes sudden dim in led
And if further move down the pot, the voltage now rises to 3.5V & after that voltage decreases smoothly.

This sudden drop/noise problem only occur when I slightly drop the value from 3.7Vout.

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#### Dodgydave

Joined Jun 22, 2012
10,366
Sounds like too much current being drawn and its starting to self oscillate, try using a bulb or a 3ohm resistor as the load to test, and see if it cures it.

#### Vindhyachal Takniki

Joined Nov 3, 2014
589
At 3.7V, where led works ok, is around 300mA as checked by digital multimeter.
I will place a 1ohm series resistor & check if it resolves the problem.

Otherwise I will use PWM ckt.

#### Sensacell

Joined Jun 19, 2012
3,011
Basic problem:

Your circuit regulates VOLTAGE, what the LED needs is a regulated CURRENT.

The current flow in your circuit will increase dramatically as the voltage increases into the "knee" of the LED's curve, at this point it will cause your regulator to go into overload shutdown mode. It's bad practice to drive LEDs from a voltage source.

Use a chip designed to drive LEDs in constant current mode.

#### Alec_t

Joined Sep 17, 2013
12,906
What you could use is a circuit based on something like this. Use a current sense resistor sized to suit your desired LED current range, with a pot connected to the base of a transistor having its emitter connected to the resistor top and its collector connected to the bottom of the load.

Last edited:

#### Vindhyachal Takniki

Joined Nov 3, 2014
589
THis is one circuit, I found in old local market which works fine. Which IC it could be:

1. Pin1: Input
2. Pin4: Ground
3. Pin7: led output conected

Edit: Its not LM2593, since LM's 7 pin is N_SS

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#### Dodgydave

Joined Jun 22, 2012
10,366
Its neither, search Google for the datasheets the ground pin is different..