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.
V f - If characteristic
160%
140%
Relative If change
120%
100%
80%
60%
40%
80%
90%
100%
110%
120%
Relative Vf change
Fig. 1
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.
Tc -Vf characteristic
Relative Vf change
110%
105%
100%
95%
90%
0
20
40
60
80
100
Case temperature Tc (degC)
Fig. 2
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.
Ref.CE-P467 04/09
CITIZEN [ CITIZEN ELECTRONICS CO., LTD. ]
