driving LEDs with PAM2861 pulse width modulated chip.

How many LEDs are in the circuit , how are they wired and what is the supply voltage? Is the Vset pin floating?

 

How many LEDs are in the circuit , how are they wired and what is the supply voltage?

3 LEDs in series, 12 volt

 

A quick sanity check is to take your power supply and connect it to a resistor (or several resistors in parallel to spread the heat) such that it will draw about 1 A and see what the supply's meter shows.

If it shows about 600 mA, then you know the problem. If it shows 1 A, then it might still be a problem with the supply and/or meter.

One thing that might be causing a problem would be if the supply can't deliver the peak current that the circuit needs. The LED driver works by pumping up the voltage using an inductive kick from the inductor and the result is that you get pulses of current that are well above the 1 A target and that are spaced out so that they average out to 1 A. If the supply can't provide those peak currents, then the driver can't meet the target and it's just doing the best it can.

What voltage is your supply set for and how much current is it supposed to be able to deliver?

Do you have a suitable capacitor across the supply terminals at the driver chip? What size and kind? It needs to be a low-ESR capacitor.

What value inductor are you using?

supply 12 volt, 3 amp power supply
10uF, 50v, x7r and a .1uF, 50v, x7r
47uH inductor

 

3 LEDs in series, 12 volt

22 posts and still no schematic. A well-drawn schematic will give us a wealth of information and reduce the need to guess or play 20 questions. Give us a break.

 

22 posts and still no schematic. A well-drawn schematic will give us a wealth of information and reduce the need to guess or play 20 questions. Give us a break.

the schematic is in the second response.

 

the schematic is in the second response.

So your configuration is exactly what the data sheet shows for a typical application?
What about the diode?

 

So your configuration is exactly what the data sheet shows for a typical application?
What about the diode?

yes.
ss24

 

Tried another resistor?

 

supply 12 volt, 3 amp power supply
10uF, 50v, x7r and a .1uF, 50v, x7r
47uH inductor

Try cutting the current significantly to reduce the load on the supply. Say make it a 1 Ω resistor. See if the meter on the supply shows closer to 100 mA or is it still showing significantly less.

Also, do that sanity check on the supply that I suggested.

Another sanity check would be to rig up a 555 circuit with a transistor switch to a load and set it to produce a 50% duty cycle squarewave at about the same frequency as the LED driver. Then see if the meter's reading is consistent with the expected current. If not, lower the frequency to a few hundred Hz and see if the meter now shows a more reasonable reading. If that's the case, then the problem is likely the frequency response of the supply's meter.

 

yes.
ss24

Exactly - except for D1. Anything else that's different? What is the value of L1? Rs? A single LED? I recall you mentioning 3.

 

3 LEDs in series, 12 volt

supply 12 volt, 3 amp power supply

More information!
It looks like you have 12Volts of LEDs and a 12V supply. Not going to work.
If that is really true you are not regulating and the LED current is low.

PWM in general; Vin=24V, Vout =12V. The current at 24V will be about 1/2 that at the 12V point. If this is true, you will measure 0.6A at 24V to get 1A at 12V. The duty cycle will be at about 50%. The current resistor reads 0.1V.

If you really are running 12V of LEDs from a 12V supply then, Vin=12V at 0.6A, Vout=11V at 0.6A and the PWM=100% duty cycle. The voltage across 0.1 ohm resistor is 0.06V.

 

Exactly - except for D1. Anything else that's different? What is the value of L1? Rs? A single LED? I recall you mentioning 3.

are you saying i shouldn't use ss24 at D1 ?
L1 = 47uH
Rs = 0.1^
2 parallel strings of 2 LEDs

 

1. if i run the PAM2861 chip at 1 amp output, will each string get 0.5 amp?
2. is it a good idea to connect the strings after the first LED as shown? Someone said it helped balance the current in both strings?
thanks

 

More information!
It looks like you have 12Volts of LEDs and a 12V supply. Not going to work.
If that is really true you are not regulating and the LED current is low.

PWM in general; Vin=24V, Vout =12V. The current at 24V will be about 1/2 that at the 12V point. If this is true, you will measure 0.6A at 24V to get 1A at 12V. The duty cycle will be at about 50%. The current resistor reads 0.1V.

If you really are running 12V of LEDs from a 12V supply then, Vin=12V at 0.6A, Vout=11V at 0.6A and the PWM=100% duty cycle. The voltage across 0.1 ohm resistor is 0.06V.

i'm sorry, i have 2 LEDs per string. had my mind on another circuit.

 

is it a good idea to connect the strings after the first LED as shown?

No.

Someone said it helped balance the current in both strings?

It's bad enough that you have parallel strings of LEDs with no ballast resistor for each string. The extra connection exacerbates the problem of unmatched forward voltages.

How bad this is depends on LED specifications (continuous current, peak current, forward voltage spread) and how you're driving them.

Parallel LEDs in inexpensive flashlights is a common practice, but that doesn't mean it's good. I have several where most of the LEDs no longer light.

 

22 posts and still no schematic. A well-drawn schematic will give us a wealth of information and reduce the need to guess or play 20 questions. Give us a break.

please excuse my ignorance. i figured out how to post my schematic.
now where do i measure current going through each string?
thanks

 

are you saying i shouldn't use ss24 at D1 ?

No. I'm saying that your statement that your circuit is exactly like the referenced schematic is incorrect. There are so many exceptions that you should post a schematic of your actual configuration.

EDIT: Moderation merged two threads. The schematic hadn't been posted before my post...

 

i figured out how to post my schematic.

Schematics are more useful if you provide component values. With an inductor, I'd expect a Schottky diode somewhere.

Cropped unnecessary information for you (we don't need the grid either):


The symbol for U1 isn't very useful. Do you have an option to turn off those unnecessary terminal numbers on the two terminal devices?

 

No, and no.

If you want to control the current in two parallel strings, you need two controllers.

That said, you can ameliorate the problem by putting a series resistor in each string.

 

now where do i measure current going through each string?

With the way you have then connected, you can't.