Go Down

Topic: AMC7140 constant current linear regulator (Read 123 times) previous topic - next topic


Nov 11, 2020, 01:19 pm Last Edit: Nov 11, 2020, 01:44 pm by gene-pavlovsky
Hello guys,

I've bought a 10-pack of AMC7140 LED drivers to make some high power LED lamps. This regulator was recommended here on the forum a few times, e.g. by DrAzzy and Wawa. So I hope people who have used it already can answer a few questions about it. As usual, I talk too much, so this post is long. I've marked my questions in bold so that other parts may be skipped :)

This device comes from a Chinese company and unfortunately the datasheet is no on the same level as most other datasheets I've seen. It doesn't even have a functional block diagram of the device, but I assume it's a linear regulator as there is no external inductor which a buck converter would have.

The first page of the datasheet contains a circuit diagram with an output capacitor, however there is no mention of this capacitor (type and value) anywhere else in the document. Other circuit diagrams don't have it either. I was wondering how necessary it is, and what function does it have. A picture of the circuit is attached.

In the datasheet for AMC7135, they do describe the output capacitor, which can be omitted in some cases (depending on lengths of PCB traces/wiring). "Typically, capacitance of 0.1uF ~ 1uF is recommended and 1uF is needed when L2 is much longer than 3cm."
Can I apply the same reasoning to AMC7140? Would I use the same capacitance value recommendations, or would AMC7140 need a larger capacitor?

The device can be dimmed in two ways, either by adjusting the current setting resistor, or by PWM control of the Enable pin.

In the first case, a potentiometer Rset1 can be wired in series with a fixed resistor Rset2 (880 Ohm in a diagram in the datasheet). If a 10K pot is used, the minimum current will be Iout=500*Iset=500*1.2V/Rset=500*1.2/(880+10000)~=55mA. Max current (@Rset=880) is 682mA.

The datasheet doesn't provide details regarding PWM control of the Enable input. To avoid LED flickering, which can be a problem around machine tools (a "strobe light" effect is highly undesirable) or for photo/video shooting, so it's probably best to use a relatively high PWM frequency. What is the highest (recommended) PWM frequency that AMC7140 can handle?

I will next describe my particular current project in a separate message, if that's interesting to anyone.



The project I'm planning is a dimmable ring-shaped LED light for a lathe. It will be mounted on a flexible arm or a gooseneck tube, so that it can be slipped over a drill chuck or a boring tool to get a good view when drilling or boring.

I've ordered CREE XT-E Q5 4000K 90CRI High Efficacy LEDs for this project (from Mouser, non stocked, ~12 weeks factory lead time...). The high efficacy version has Vf<=2.85 (@350 mA). Their max rated current is 1500 mA, although Vf~=3.15 at this current.

I have 24V switching power supplies available in 1.75A and 1A. According to datasheet, AMC7140 drops out 0.5V at 700 mA, meaning that total Vf for the LED string should be <23.5V.

I could use two AMC7140 in parallel, to drive a string of 7 LEDs in series at <=1400 mA. At this current each LED will give around 330 Lm max, for a total of 2310. Probably overkill for a close-up task light, although it will be dimmable after all.

Or I could use a single AMC7140 to drive a string of 8 LEDs in series. I can then afford max Vf per LED of ~2.93. According to a Vf/If curve from CREE (attached), I might even be able to drive them at 700 mA. The curve represents the typical Vf at Tj=85C, though. Will have to test the real max current. My guess is 500-600 mA is more realistic, this will produce around 160 Lm per LED, for a total of 1280. Probably more than bright enough for this light.

A third option would be two AMC7140, each driving a separate string of 8 LEDs at ~500 mA.

For dimming I will probably use a pot, in which case I could just use the Rset method of dimming. But I haven't ruled out an Arduino Nano or Pro Mini and using a touch control and PWM for dimming.

The LEDs will be mounted on 8mm aluminum-core star boards, which will be glued with thermal adhesive to a circular recess turned in the face of an aluminum ring. A milky-white acrylic diffuser ring will cover the LEDs to prevent glare and protect them from the environment. I will try to turn it as a close fit, but it would be nice to come up with a better sealing arrangement. I wonder if I can find a large enough o-ring. Or maybe use some (non-corrosive) RTV silicone sealant.


having a linear driver I would avoid PWM, I wouldn't want my lathe going stroboscopic and freezing movement... or perhaps you would to see a small mark on the object.  enable would work fine for touch sensing.  Dimming is best done with the resistor.  An added feature of PWM might be interesting.   

Bad Boys Race Our Young Girls But Violet Generally Wins - Get Some Now :) - ELI ICE man

Go Up