LED Dimming: A High-Power 40V Buck LED Driver with Two Dimming Methods from Diodes Incorporated

Diodes Incorporated has released a high-power 40 V LED controller that's capable of driving up to 3 amps of current.

Diodes Inc. recently announced the AL8843, which is a high-power 40 V hysteretic mode DC/DC step-down converter designed for driving series-connected LEDs. It has an internal 40 V MOSFET, more specifically an NDMOS, which eliminates the need for an external power switch. By the way, if you're not familiar with an NDMOS, it's an N-channel enhancement mode DMOS device, where DMOS stands for double-diffused MOS technology, according to this JEDEC standard. The ZVN4206GV, from Diodes Inc., is an example of such a device.
The figure below, from the AL8843's datasheet, shows a typical application circuit; note that there's no external MOSFET and, furthermore, that few additional components are required.

This implementation looks pleasantly straightforward. Diagram taken from the datasheet (PDF).

High Efficiency Levels...They Aren't Kidding

As stated in the datasheet, the efficiency of this LED driver can go as high as 97%. However, according to the efficiency plots (see the figure below), efficiency values close to 99% are possible, and that is some seriously high efficiency. If these efficiency levels are indeed possible, I'm curious as to why the first page of the datasheet says “up to 97%” instead of “up to 99%.”
By the way, if anyone tells you—as one engineer once told me—that a regulator, or any device for that matter, has an efficiency greater than 100%, you could either educate them (i.e., kindly let them know that efficiency levels greater than 100% are impossible), or simply ignore them.

 

It appears that under certain conditions extremely high efficiency (~99%) can be obtained. Plots taken from the datasheet (PDF).

Two Dimming Methods

The AL8843 provides two dimming methods, both of which use the CTRL (control) pin. The first method, referred to as analog dimming, allows the user to vary the LED intensity from 10% to 100% by applying a DC voltage, between 0.4 V and 2.5 V, to the CTRL pin.

 

Control voltage vs. LED current, for analog dimming. Plot taken from the datasheet (PDF).

Now, if your design requires a dimming range from 0% to 100%, then you'll have to employ the PWM dimming method. And if you're looking for high-accuracy dimming capabilities, then consider using a PWM frequency of 500 Hz or less, as suggested in the section entitled PWM Dimming on page 11 of the datasheet. The figure below shows a linear relationship between the output current and the PWM duty cycle, from 0% to 100%.

 

The PWM dimming method allows for an LED dimming range from 0% to 100%. Plot taken from the datasheet (PDF).

Guidance for Choosing External Components

The datasheet has sections on capacitor selection, diode selection, and inductor selection. Personally, I'm always appreciative when manufacturers provide such information!
For instance, we are told that the value of the input capacitor should be determined by the IC’s input voltage, the peak current, and the LED's cable length. We're also told that 4.7 µF to 10 µF are commonly used values. With regard to the selection of the freewheeling diode, it is suggested that a fast low-capacitance Schottky diode with low reverse leakage current be employed.
I'm sure Diodes Inc. would be happy to provide additional information on any of these component selections if you were to kindly reach out to them.

Evaluation Kit

If you'd like to experiment with the AL8843 without taking the time to design a printed circuit board yourself, then you're in luck, because an eval board is available.
Full disclosure: I wasn't actually able to find the eval board for purchase, but I was able to find its user guide. So, if you're interested in obtaining an eval board, you'll probably have to contact Diodes Inc. directly. The figure below shows the eval board's schematic and PCB.

 

The AL8843EV1 evaluation board and schematic. Images taken from the user guide.

Have you had an opportunity to use this new dimming LED driver in any of your designs? If so, leave a comment and tell us what you think.