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Topic: My Board Design with AL8805 has half the expected drive current (Read 673 times) previous topic - next topic

ron_sutherland

I have been tinkering with a board of my own design, it has AL8805 led drivers like the Sparkfun PicoBuck and FemtoBuck. It seemed like a good choice since I have had good luck with other Diodes Inc parts.

My [RPUlux] version ^1 [layout] has less noise on the 0V plane than the first version. The ADC readings from the MCU look right (I sort of figured those were noise related so I ignored that problem).

[RPUlux]: https://github.com/epccs/RPUlux/
[layout]: https://github.com/epccs/Eagle/tree/master/RPUlux

The problem is I am still getting 160mA with .3 Ohm sense from the LED driver (AL8805). Looking at the Diodes example circuit when the switch is off. I see the current flows through D1 and into C6 mostly (some goes to C3 bypass). The magnetic fields from this current are far from the sense lines.



However when the switch is on the current flow is right under the sense and the magnetic field from that induces a voltage on the sense lines, thus causing the chips sensor input voltage to be different than the sense resistor voltage. It looks to me like the induced voltage works against the voltage on the resistor, so the induced voltage makes the sensed voltage seem less than it is.



This must be how Diodes characterized there part. In order to duplicate there datasheet values guess I would need to run the switched current under the current sense resistor the way they have done. This seems like a questionable way to do the sense for a switch mode power supply.

Wawa

I designed a 16-channel LED driver board, but used the PT4115 (almost the same chip as the AL8805).
The sense resistor is about 8mm away from the (shielded) inductor.
I don't have the problems you mentioned. LED current with a 0.3ohm resistor is ~333mA (I use 0.2ohm for 500mA).
Layout (without groundplane) of two channels attached. The location of the sense resistor is circled.

The board uses a 16-channel PCA9685 PWM chip, and a WiFi ESP-12 is mounted underneath.
No other connections than (24volt) power and LED wiring.
A single WeMos D1 mini master controls several LED slaveboards, with commands broadcasted via UDP.
Leo..


ron_sutherland

Your board could light the total house (or yard), that is very cool. Do you have it for sale?

The PT4115 almost looks pin compatible. They are not on Mouser or Digikey, I am not sure of a reliable place to get them.

Wawa

Not for sale. I just made some boards for myself.
This is a new WiFi version of a previous Mega/ethernet controlled board (in use 24/7).
This WiFi board is in the programming/testing phase.

PT4115 chips are $0.10 on ebay. Never had any problems or failures with those chips.
I use CDRH74R inductors (<$0.20 ebay) for LED currents <=500mA.
Leo..

ron_sutherland

Adding another data point from the PicoBuck.

https://github.com/sparkfun/PicoBuck

Can anyone confirm that they provide 330mA to the LED (I am sure it does... but)?

Picobuck with AL8805 switch On current flow (note on the bottom plane it is not easy to know where the current would flow).



Picobuck with the AL8805 switch open (e.g. current flow through the diode).



This setup does not look like the switching current flows between the sense pin inputs, but it is sure suspicious that the LED current flows between the sense pins. There would be a ramp on that current flow and it will induce a voltage on the current sense input. This has the feel of playing darts with a blindfold, and a trampoline in the background, and a turntable for the launching platform.

Wawa

This setup does not look like the switching current flows between the sense pin inputs
Current flows through sense resistor R1, and the two top pins of the chip are connected to both sides of R1.
R4 can be added across R1 with the solder bridge (if you want 660mA LED current).
The question mark connects to the bottom ground plane.
The whole top copper-colouted plane is V-in (supply).
Leo..

ron_sutherland

Just to clarify, I guess when saying "switching current" I am thinking about the AC currents. The magnetic fields from DC currents don't do very interesting things like inducing voltages. So what I was getting at was that the LED current flowing under the U1 package (U1 is the AL8805) has a ramp (e.g. some AC current) that may induce voltages between the U1 package sense pins.

ron_sutherland

I have added more bypass to the AL8805 VIN pin and seen no change. Changing the LED bypass so one side went to ground rather than the LED had no change. I am at a loss so I removed the tombstone hacks and fly wires (so it is back to how it came out of reflow, except the capacitors are now thermal shocked) and changed the sense resistor to 0.15 Ohm. Now I have 330 mA, guess 0.075 Ohm would give 660mA, and 0.1 Ohm 500mA. The ramps on the LED current look OK, the ADC's on the MCU are giving what seems to be good values, so I guess this is how it will work. For reference, this is how my layout looks with the switching currents shown while AL8805 switch is on.



And, the AL8805 switch open.


Wawa

That's weird. Since 0.3ohm is very low, I put my sense resistor directly on the pins of the chip (no tracks or vias).
I used thickfilm 1206 resistors.
Leo..

ron_sutherland

@Wawa, does your copper pour go under the sense resistor (i.e. between the pads)?


Wawa

@Wawa, does your copper pour go under the sense resistor (i.e. between the pads)?
Yes. And all vias you see, except for the two between '1u' and '0.2' are connected to a big ground plane on the other side of the board.
The via pairs close together connect to a 120mil wide horizontal power trace on the other side.
Leo..

ron_sutherland

OK, I am going to redo. I think it is important to get it to work like the datasheet. I recall reading something about slop compensation many years back and maybe the connection going under the sense helps to do something like that.

Thank you Leo, I very much appreciate your help.

ron_sutherland

OK, this is what I am thinking for the AL8805 LED drivers on the next board iteration. The main thing is that when the switch is conducting the current returns to the bypass between the sense pins. I will go over it a few times before I order the board, there is always a few mistakes to find.


Wawa

Looks good, but why the rounded pads for smd resistors/caps
Leo..

ron_sutherland

The story has faded from my memory but it started with an IPC paper about 20 years ago I think. Nowadays it tells me that I have looked at that pad and fixed it if needed, I have been burned a few times with pads that I did not check. Anyway it solders nice, the solder wets the corners better, that was what the IPC paper said if memory serves (I think it does).

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