Background

AN2754

4/20

1

Background

When designing a power supply for a white high-brightness LED, the main requirements are

efficiency, size and cost of the complete solution.

A standard buck converter is the best choice for providing a constant current because only

the buck converter among the switching topologies has an average inductor current that is

equal to the average load current. For this reason, the conversion of a constant voltage into

constant current is much easier.

LEDs are current-driven devices whose brightness is proportional to their forward current.

Forward current can be controlled in two ways: voltage mode and current mode. The first

method uses the LED V-I curve to determine what voltage has to be applied to the LED in

order to generate the desired forward current. This is typically accomplished by applying a

voltage source and using a ballast resistor as shown in Figure 1. This method has two

serious drawbacks. The first is that every change in LED forward voltage creates a change

in LED current. The second problem is the power lost across the ballast resistor which

reduces the efficiency.

Figure 1.

Constant voltage control

Equation 1

LEDs are PN junction devices with a steep I - V curve. For this reason, driving an LED with

a voltage source can lead to large swings of forward current in response to even a small

change in forward voltage. In general, to meet the needs of a driver for an HBLED, the

current output must be in the

±5% to ±20% range.

The best way to drive the LEDs is to control the forward current so that it eliminates changes

in current due to variations in forward voltage, which translates into a constant LED

feedback voltage that the buck converter requires. Multiple LEDs should be connected in a

series configuration to keep an identical current flowing in each LED.

FB

F

MAX

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F

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H

FB

OUT

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1

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V

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