AMC7140 constant current linear regulator

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 :slight_smile:

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=500Iset=5001.2V/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.

Regards
Gene

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2020-11-11 12_49_47-AMC7140_D.pdf - [Microsoft Word - DD032-AMC7140_D.doc] - SumatraPDF.png

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.