Three-channel white LED driver using a simple step-down dc/dc converter-EEWORLD

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With just a few additional components, you can use a high-efficiency step-down DC/DC converter to create a constant-current multi-channel LED driver. Driving a single-channel LED is relatively simple, but driving multiple channels of parallel LEDs becomes more complicated.

In a typical dc/dc converter application, the control circuit uses a power stage consisting of an inductor and a capacitor, and generates a regulated constant voltage through the feedback of a resistor divider network, thereby obtaining a constant current through the resistor divider. By replacing the upper resistor in the feedback divider network with an LED, the LED can be driven with a regulated constant current. The current flowing through the LED is equal to the reference voltage of the dc/dc converter divided by the lower resistor value connected to ground.

While this approach works well for a single LED channel, it cannot be used to drive multiple parallel LED channels because the unmatched LED voltage drops will consume most of the current in one LED channel. As a result, only one LED channel will be lit.

Using the circuit in the figure, only one DC/DC converter can drive multiple parallel LED channels. It uses the method of adding a simple current mirror to generate the required constant current for each LED channel.

Three-channel white LED driver using a simple step-down dc/dc converter

The IC used in the figure is the TN1000, a 100mA current-mode step-down dc/dc converter from Technor Semiconductor (Reference 1).

The step-down stage includes a 12μH inductor and a 22μF capacitor. The drive current for the first LED channel of D1 is regulated at 17mA, which is equal to the IC's 0.8V reference voltage divided by R1. The voltage on C3 is regulated to a voltage value required to support the voltages on D1 and R4, and 0.8V on R1.

Emitter follower Q3 drives the bases of Q1 and Q2, mirroring D1's 17mA. The VBE drops of emitter followers Q1 and Q2 approach and compensate for Q3's VBE drop, so the voltage across R5 and R6 is also a constant 0.8V, while D2 and D3 are driven by a constant 17mA.

R4 is set so that the voltage across C3 is high enough to keep Q1 and Q2 out of saturation. The value of R4 should be set so that the voltage across C3 can support the maximum LED voltage plus about 255mV to keep Q1 and Q2 out of saturation.

Reference address：Three-channel white LED driver using a simple step-down dc/dc converter

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