Background

UM0422

4/14

1

Background

In designing a power supply for white high brightness LEDs, the main requirements are

efficiency, size, and cost of the complete solution.

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 2. The output voltage

required for the circuit in Figure 1 is given in Equation 1. This method does not provide

adequate current control or the system efficiency necessary for high current LEDs.

Figure 2.

Constant voltage control

Equation 1

Controlling current accurately is a challenge when each LED has a large manufacturing

tolerance in its forward voltage. The best way to drive the LEDs is to control the forward

current in order to eliminate the brightness variations due to forward voltage spread of the

LEDs. Figure 3 shows the STCS1 application diagram. In this case the controller regulates

the voltage across a current-sense resistor rather than regulating the output voltage.

R2

Voltage Regulator

Vin

Vout

VFB

R1

Vin

Rled

-

V

OUT

V

R

1

R

2

-------

+

⎝⎠

⎛⎞ nV

FMAX

I

LED

R

LED

•

+

•

=

=