Ref.CE-P467 04/09

Driving LEDs

1. Introduction

Prior to driving an LED, which is a kind of semiconductor

product, it is necessary to thoroughly comprehend its

properties. For instance, fluctuation of the forward voltage Vf

on an LED causes steep variations in the forward current If. In

the case of Fig. 1, if the temperature conditions are constant,

a 10% rise in Vf results in an increase of If by more than 40%.

Fluctuations in the forward current have a significant effect

on light emission, heat generation, and other phenomena on

LEDs. Especially regarding high-output lighting LEDs driven

by large current, strict control of the forward current is

important. In addition, the forward voltage fluctuates due to

temperature, and measures for heat release are therefore a

crucial factor.

Thus, when driving our LEDs, be sure to read the

specifications and application notes for the relevant products

and take measures according to their properties.

2. Constant current driving system (recommended)

Even under the condition that the forward voltage

fluctuates due to heat generation or other reasons, the

constant current driving system supplies a certain current

to LEDs and allows relatively stable driving under varying

environmental conditions.

In general, as shown in the example of Fig. 2, the

forward voltage to apply constant current to an LED tends

to decrease as the temperature increases.

We recommend the constant current driving system

with the object of stable light-emitting output and

reliability.

3. Constant voltage driving system

As mentioned above, the forward voltage to apply constant current to an LED tends to

decrease as the temperature increases. In the case of Fig. 2, when the case temperature Tc is

90°C, the same amount of current is achieved by the approx. 5% lower forward voltage than at Tc

= 25°C. If constant voltage driving is provided under these conditions, comparative overvoltage

and overcurrent driving status is possible.

If constant voltage driving is employed, assume the temperature in actual usage and

implement appropriate measures to limit the current.

4. Precautions on inrush current

If an LED is connected to capacitive loads, such as capacitors, an instantaneous inrush

current may occur when the system is turned on/off. (For instance, when the secondary side of a

power supply circuit in the energized state is turned on/off.) Avoid any possible occurrence of

inrush current. If it is unavoidable, implement measures so that the absolute maximum rating of

the LED will not be exceeded.

Fig. 2

Tc -Vf characteristic

90%

95%

100%

105%

110%

0

20

40

60

80

100

Case temperature Tc (degC)

Vf - If characteristic

40%

60%

80%

100%

120%

140%

160%

80%

90%

100% 110% 120%

Relative Vf change

Fig. 1