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03-21-2013

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

Nora Jacalan Esteves, Technor Semiconductor LLC

EDN

With only a few additional components, you can use a highly efficient step-down dc/dc converter to create a constant-current, multiple-channel LED driver. It is relatively easy to drive a single-channel LED; however, it becomes more complicated when driving multiple-channel paralleled LEDs.


n normal dc/dc-converter applications, the control circuitry uses a power stage comprising an inductor and a capacitor, along with feedback through a resistor-divider network, to generate a regulated constant voltage and thus a constant current through the resistor divider. You can use an LED in place of the upper resistor of the feedback-divider network to allow the LED to be driven with a constant regulated current. The current flowing through the LED will be equal to the reference voltage of the dc/dc converter divided by the resistance of the grounded bottom resistor.

Although this method works well with one LED channel, it cannot be used to drive multiple parallel LED channels, because the mismatch in LED voltage drops would cause one LED channel to consume most of the current. As a result, only one LED channel would be lighted.

You can use the circuit in Figure 1 to drive multiple paralleled LED channels with only one dc/dc converter by adding a simple current-mirror scheme to generate the constant current required for each LED channel. The IC used in the figure is the TN1000, a 100-mA current-mode step-down dc/dc converter from Technor Semiconductor (Reference 1).

Figure 1. Current mirrors equalize the LED currents in each branch of this multiple-LED driver.

The voltage step-down stage consists of a 12-μH inductor and a 22-μF capacitor. The first LED channel for D1 is driven by a regulated current of 17 mA, which is equal to the IC’s 0.8V reference voltage divided by R1. The voltage across C3 will be regulated to whatever voltage is required to support the voltage across D1 and R4, as well as the 0.8V across R1.

Emitter follower Q3 drives the bases of Q1 and Q2, which mirror the 17 mA of D1. The VBE drops of emitter followers Q1 and Q2 are similar to and compensate for the VBE drop of Q3, so the voltage across R5 and R6 is also a constant 0.8V, and D2 and D3 are driven with a constant 17 mA.

R4 is set so that the voltage across C3 is high enough such that Q1 and Q2 do not saturate. R4 should be set so that the voltage across C3 can support the highest LED voltage plus approximately 255 mV to keep Q1 and Q2 from saturating.

Reference

  1. 1. “TN1000 100-mA Monolithic,Synchronous Step-Down DC-DCConverter,” Technor Semiconductor, July 2011.

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