So I bought some AMC7135 from FastTech for a project. The datasheet wasn’t too clear, but after some research and poking I believed I had one working as follows:
But then I soldered up a stack of 3 and the current dropped to only 0.135 amp. It made no sense, so I decided I damaged them by overheating them with the solder iron. So I ordered my OSH Park board and reflowed them on. It turns out the same problem exists in the PCB version. So I must have the pinout wrong. Can someone explain what the heck I am doing wrong?
Pin on the left you labeled as OUT is not OUT, is IN and should be connected to LED-
It appears to be connected as you suggest. I added a link to the datasheet in the OP.
If what is next to the “350ma” label is the led ( I had mistaken it with a switch, sorry ) pinout seems correct to me.
It’s a couple test points that I drew.
You seem to agree that calling that pin OUT instead of IN or REG is confusing 
There is no IN ?
Only VDD , GND and OUT. VDD is supply voltage, i.e. positive from power source. Same positive is the positive to the led.
OUT is connected to the LED -, i.e. the amc7135 is controlling the current on the minus side, it ‘passes trough’ the chip.
GND is just GND or minus.
I might have figured it out. I think the 7135 fails to do it’s job if the load is too low resistance. Like an amp meter.
Just to be sure :
When you say you stack them you mean they are wired in parallel, with all out pins connected together to the led, all gnd pins pins connected together to the (-) and all command pins connected together to the (+) ?
Wirering the 7135 in serial might result in low current like you describe because then the dropout voltage of all the 7135 are added
That’s wrong.
The leds will get his negative from the chip, it needs the get the positive from the source.
It’s possible that it will only work with a led (or any load higher than an ammeter or multimeter)
Think of it like a mosfet. If voltage + is supplied on the VDD pin then current flows threw GND and OUT but is limited to 350ma per chip.
VDD would go to battery + and led +, they are shared. GND would go to battery - , OUT goes to led - .
The VDD pin can be PWM’ed for lower led output but the current is the same just a percentage of off time in a second. You probably already Know that.
Yea, the datasheet suggests it needs at least a 0.2v difference to work with 
The datahseet does indeed label the “IN” pin as “OUT”. Regardless of labeling your schematic is correct.
Are you sure you have enough voltage? 7135s have a minimum dropout voltage. The voltage “4.0” in your case has to cover the voltage over the LED at plus the minimum dropout voltage over the 7135, which is around 0.12V according to the sheet. I’ve never checked myself. If it doesn’t have the 0.12V dropout voltage it can’t do anything.
The 0.2V from the datasheet are between the Out and gnd pins, say that's what the 7135 needs for himself to be able to regulate the current
Example :
if the supply is 3.4V and the led eats 3.0V there is 0.4V left for the 7135 and it's ok but with the same led and a 3.1V supply then there is only 0.1V left for the 7135 and the current will decrease.
If you short the led it should still work (ie limit the current to 350mA) because then the 7135 will have the full supply voltage wich is higher than 0.2V (except that the 7135 will dissipate all the power and will heat fast)
If you share the Oshpark files maybe one of us can help you find the problem.
Or PM me and I’ll see if I can help.
All right, I’m back.
So the project is just to make a 1 amp dis-charger for 18650 cells. I thought (3) 7135 would be a simple solution.
I have learned by starting this thread that yes, my schematic was correct.
But here’s the issue:
Scenario #1
- Power up the (3) 7135 (with a 0.6 ohm load connected).
- Frustration. It is now drawing 135ma and will stay that way.
Scenario #2
- Power up the (3) 7135 with the load disconnected.
- Connect the 0.6 ohm load. This creates a 0.68v drop.
- Satisfaction. It is now drawing 350ma per 7135.
- Things get a little hot.
SO yay for the learning, crap for the project.
Try a bypass cap to VDD. Sometimes the control circuit needs to settle first before hitting it with full load. Same like trying to start a car’s engine if it’s still in-gear.
The state of VDD on boot does not change the results, so idk if that would work or not. But I’ve abandon the 7135 because of the intense heat they create. I expected the heat to be acceptable, it was not.
It’s a linear regulator. All that power has to go somewhere.
Well I figured if they survived inside a hot stick of aluminum, they would be even better on a bare board. But they even seemed to thermal-shutdown when run for a few minutes.