Author Topic: Adjustable Step-Down DC/DC Constant-Current Control: An LED Controller from Diod  (Read 204 times)


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Adjustable Step-Down DC/DC Constant-Current Control: An LED Controller from Diodes Incorporated

Diodes Incorporated announced a new adjustable constant-current LED step-down (buck) DC/DC converter that accepts an input voltage ranging from 5V to 60V.
Diodes Incorporated recently announced their AL8862, which is a new LED-dimmable buck DC/DC converter that offers a wide input voltage range (from 5V to 60V), provides high efficiencies (up to 97%), and offers two methods of dimming capabilities (analog or PWM).

The figure below shows us that while this IC includes the NDMOS transistor power switch, the designer is responsible for adding the external freewheeling diode (D1), the inductor (L1), the LED current-setting resistor (RSET), and the decoupling capacitor (C1). Diodes Incorporated has provided—starting on page 10 of the datasheet (PDF)—guidelines, requirements, and equations for assisting the designer during the selection process of these required external components.

Although the power switch is integrated into the IC, choosing some external components is still required. This typical applications circuit is from the datasheet (PDF).

Similar to the AL5815/16 Linear Constant-Current Controller
Now, you may be thinking that this constant-current LED controller, or more specifically this particular article, looks awfully familiar. If that's the case, then you may be thinking of this article which discusses Diodes' linear LED constant-current controller—the AL5815/16. While the linear constant-current LED controller (AL5815/16) and the buck constant-current LED controller (AL8862) both have their pros and cons, there's a proper place for each of them.

Which device you choose for your design depends on your design requirements. The AL8862 is limited to 1A maximum current, while the AL5815/6 ICs are limited by whichever external switching transistors you choose.


High Efficiency...up to 97%
As shown in the figures below, achieving high efficiency depends on the IC's input voltage, the IC's output voltage, the value of RSET ,and the inductor L. As an example, look at the first graph below and pick the black [3.3V] and red [9.9V] curves at V=30V. The potential difference between input and output of the black curve is 26.7V at 62% efficiency. The potential difference between input and output of the red curve is 21.1V at 84% efficiency.

For a given output voltage, efficiency decreases as the potential difference between input and output pins increases. But overall, the greatest switching efficiencies occur when there is a small potential difference between input and outputs at high overall voltages.

The IC's efficiency is dependent upon VIN, VOUT, RSET, and L values. Plots taken from the datasheet (PDF).

Two LED-Dimming Methods
Regarding the dimming of LEDs, this IC allows for two techniques, both of which use the CTRL pin.

Technique 1: Analog Dimming
The analog dimming method occurs when a DC voltage is applied to the CTRL input pin. As can be seen in the following figure, the analog dimming method provides a LED-dimming range from 10% to 100%, which corresponds to 0.4V and 2.5V, respectively. For more information on this approach, check out the section entitled Analog Dimming on page 9 of the datasheet.

Using the analog dimming method, a range of 10% to 100% of full brightness can be achieved. Table courtesy of the datasheet (PDF).
Technique 2: PWM Dimming
If your design requirements mandate that LED dimming must include the complete range from 0% to 100%, then consider using the pulse-width modulation (PWM) method. And, as it’s explained in the section entitled PWM Dimming (on page 9), achieving a high LED-dimming resolution can be accomplished with a PWM frequency of less than 500 Hz. However, while higher PWM dimming frequencies are acceptable, the LED-dimming range and accuracy will be diminished. For additional information on LED dimming using PWM, take a look at the PWM Dimming section on page 9 of the datasheet.

Regardless of which LED dimming method is employed, the figure below illustrates that both techniques yield seemingly excellent LED-current linearity as a function of either duty cycle for the PWM method, or pin voltage for the analog dimming technique.