ALED6001 analog Dimming instable at discontinuity points

[Knochi]

Hi,

I did a PCB based on the ALED6001 and using the eDesign Suite.

I want to use analog dimming only and wired it up similar than in the STEVAL-ILL072V1.

Best case i wanted to achieve a 0-100% dimming but it was clear that, due to that curve provided in the datasheet that would not the case. That wouldn't be a big problem if I won't face the big instabilities at the two discontinuity points at 260mV and 1.2V. 

At the lower end the LED flickers and the coil I am using does a humming sound. At the upper end the brightness increases suddenly to full brightness.

Does someone have an idea how this could be solved in a MCU-less design?

I tried both topologies (boost and buck-boost) by modifying one board. Seems the eDesign solution (buck-boost?) works a bit better. Maybe because of the proper calculated compensation circuit?

I attached the schematic. The red lines on the second sheet are my modification from the one topology to the other.

Thanks in advance

David

[Peter BENSCH · Accepted solution]

Welcome @Knochi, to the community!

The limited adjustability of this type of dimming is well known. If the current goes to zero, the load current of the boost controller also goes to zero. This leads to control problems because a boost controller always requires a certain load current. You can hear this effect as a humming sound, as the charged boost coil is not or hardly discharged and tries to get rid of its charge as a high voltage spike when it is switched off.

However, it should also be noted that LED exhibit a colour shift at a significantly lower current than the rated current, which is particularly noticeable with white LEDs. For this reason, PWM dimming is preferred for white LEDs because it avoids such colour shifts. Unfortunately, there are countries where PWM dimming has been patented, although this has been state of the art for decades. However, it may then be possible to switch to parallel dimming, in which a constant current source feeds the LED and a switching transistor in parallel with the LED switches it on and off with the PWM. Parallel dimming has the disadvantage of not saving any current when the light is dimmed because the constant current flows continuously, but the constant current regulator can be highly optimised and the brightness can be continuously adjusted from exactly 0% to 100% with the colour location generally unchanged.

Hope that helps?

Regards
/Peter

[Knochi]

Hi Peter,

so did a redesign with PWM dimming. The PWM generation needs a bit improvement (only allows for 0..50%). I changed C15 to have 100Hz PWM output and not running into the shutdown. But even when i bypass the PWMI and just apply 3.3V the boost is not working. VOut is always equal to Vin. Gate at Q1 is constant high.

When i apply a 50% PWM the input voltage gets shorted and Q1 gets quite hot.

Can you pls point me to some parameters to check?

I have an oscilloscope including current probe. 

Attached schematics.

[Peter BENSCH]

Your description doesn't sound logical: if the gate of Q1 is constantly high, Q1 should be permanently switched on. Then, firstly, the output voltage is short-circuited and, secondly, a large current flows through L1 and Q1, which should cause both to become quite hot.

What saturation current (not the rated current!) does L1 have?

[Knochi]

Found the issue.. looks like I accidentally disconnected the ground from the ALED6001 after ERC and before releasing the design files. 

So ALED6001 was not connected to ground, resulting in some strange behavior including pulling the gate continuously to high.

Fixed that now and tuning the PWM generator circuit.

Thanks anyway 

 

 

[Peter BENSCH]

Great that you found the error and that it now works! Also, thank you very much for the much appreciated feedback!

[Knochi]

So unfortunately I still have problems.

I fixed the ground connections with enamel wire and the circuit works, but only when VIN is below 12V.

When VIN is about 11V I get the expected Power consumption of around 14W

When i lower VIN power consumptions drops as well. e.g. 12W at 9V, 10W at 7V.

I can clear the fault by PWM Low.

Even with 10V VIN the circuit isn't running stable. 

When I operate at 100% PWM, 10V the circuit goes into fault state after one or two minutes.

The Coil and QSW get quite hot but touchable.

ISAT of the coil is 3.5A

The LED is a COB LED with 28V and 0.45A rated current.

[Peter BENSCH]

A few thoughts on this:

• Do you have experience with boost regulators?
• Do you realise that the coil current increases with decreasing input voltage?
• Do you also realise that not only the saturation current plays a role, but also the copper resistance of the coil?
• Have you looked at voltages (L1.VIN, Q1.D, Q1.G, Q1.S) and currents (R3, R6) on an oscilloscope to get to the bottom of your problem?
• So far we have only been able to see a section of your layout around U1, the important current path via L1, Q1, R3, D2, R6, Q2 incl. GND is not visible.