A small, cheap, CC LED driver module that is worth a look... (5/5)

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LDO6AJSA.pngProduct:  2.58—6V@1.5A max CC LED Driver Module
Price:   ~2.10 USD / ea. qty: 10   including shipping (2022-09-12)
Rating:  5/5             pending revision based on lifespan
Seller:  CZB6721960 Store      on AliExpress

Another cheap and cheerful offering from an anonymous Chinese manufacturer that isn't fit for aerospace but certainly has a spot in your own LED projects. The little board provides a PWM controlled constant current driver using the CN7511 chip. It is just about perfect for getting power to those LEDs (up to a healthy 1.5A@6V) with dimming or simple digital control. Quite a bit of functional and stress testing, and a fairly substantial burn-in of two weeks on the bench at 6V@350mA makes me reasonably confident this will be a reliable module. Definitely recommended with the current information.

Driving LEDS is a constant source of discussion* in the AAC forums. Of course there are many methods producing various quality of results and costing varying amounts. While simple current limiting resistors are always going to be the simplest and cheapest, they are also only a way to make sure the LEDs don't become SEDs (Smoke Emitting Diodes). Very frequently, the problem to be solved also includes turning them on and off using a GPIO pin, or providing a dimming facility. This means more parts and more complexity that mere resistors and that can add up fast.
*no pun intended but they just happen on their own...

Enter the mysteriously named LDO6AJSA LED driver module. This very simple little board will cost you a touch more than two bucks American and provides the switching and dimming in a tiny 10x20mm package with four pads on the edge at 2.54mm centers to accept the usual pin header.

The board is based on the CN7511, a monolithic constant current LED driver rated for 1.5A at 2.8—6V.

LDO6AJSA.png

The board has only two passives on it: a variable resistor connected to pin 1 (ISET) which sets the output current, in series with a 1K2 resistor providing the lower limit of the setting. The four pads, left to right are:

  • VI which is connected to pins 3 and 4 of the chip, both VCC;
  • CE which is connected to pin 8, Chip Enable;
  • G which connects to pin 2, GND; and,
  • LED attached to pins 5 and 6, also called LED on the chip which is IOUT.

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The PCB exactly follows the manufacturer's guidelines concerning the thermal path for the chip, featuring a wide pads for sinking heat transferred from the die through the internal copper lead frame, to the leads. There is also a substantial copper flood on the reverse of the PCB with 21 vias to provide a good thermal path from the obverse side with the leads are soldered. You might note the nearly 100% coverage of the copper on the PCB.

The chip features a thermal regulation system that limits current to the LED if the CN7511's junction temperature reaches ~135℃. Instead of turning off the LED, it dims it to protect the chip without losing the light. Additionally, overcurrent protection prevents downgrading your LEDs to the aforementioned SEDs.

The result is that in extended tests operating at 400mA into a 12Ω 5W resistor the chip was about 8℃ above ambient, warm to touch, while the load resistor was about 80℃ above ambient, an order of magnitude hotter. This was confidence building concerning long term reliability since heat plays such an important role in component lifespan.

Actual use is very simple. Supply at least 2.8V + LED VF + voltage drop to VI on the board, pull CE high (or provide a ≤2kHz PWM signal), and the LED(s) attached between LED and G will light, either as set by the VR attached to ISET or using it as a limit according to the duty cycle of the PWM signal on CE. (NOTE: you can buy the CN7511 and its variants on AliExpress to use on your own PCB, something I expect to do in the future.)

The chip is perfectly happy with parallel LEDs so long as they are within the current limits. The application note advises "in order to make sure the current is evenly distributed among the LEDs, a robust resistor in series with each LED is recommended", the same advice so often given by members here.

Another nice little trick in the datasheet is using a logic level signal to provide fixed dimming. Adding a parallel resistor connected to circuit ground via a small N-Channel MOSFET lets you make specific dimming levels controllable by logic, a GPIO, or even a mechanical switch combined with the preceding or alone (though in that case you could omit the NMOS FET).

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a scheme for step-wise dimming from the datasheet

Puling CE low with turn off the LED output and reduce current consumption to the around 1㎂. This is done using the in-built power MOSFET. Even if dimming is in your plans, the ability to use a logic level GPIO to switch 1.5A of LED load is pretty handy. Of course, ISET lets you choose your current limit so with an on-off scenario this chip is pretty slick as well.

If you would like to use a potentiometer to dim the LEDs rather than PWM—a common request in the forums—you can remove the on in the board and connect to its pads. If this is desired, keep in mind the application note cautions RSET should be located as close to the board as possible, with stray capacitance limited, so some empirical testing might be required for your application.

Given these constraints, it might be a really good application for a digital potentiometer. There are some have persistent memory, and some, like the X9C102 will also use the output of a rotary encoder directly. A pretty neat arrangement for a noisy environment or a longer distance than can be accommodated by direct connection to the chip.

The bottom line for me is a new tool in the box for managing LEDs. I picked them up as a candidate for use in a project developing my own, low cost WIFi connected undershelf lighting for my new studio-lab. These, along with stupid cheap ESP-01 modules, MQTT, and Node-RED will be the first prototype for a version with the ESP8266 module and the CN7511 on my own board.

Considering the cost, features, and tested performance of the module, it is highly recommended if you need to control some LEDs reliably and simply.

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Ya’akov
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