BCR402 Datasheet, PDF (4/8 Pages) Infineon Technologies AG – Light Emitting Diode (LED) Driver IC Provides Constant LED Current Independent of Supply Voltage Variation

Applications Note No. 066
Silicon Discretes
large resistor values. Again, such large resistor
values not only reduce the number of LEDs that
can be driven, but waste additional DC power.
Figure 6. LED Current Stabilization Effect
using BCR402R versus Different Series
Resistor Values for “Resistor Method”. Note
flat current curve over supply voltage when
BCR402R is used.
3. Safe Operation of BCR402R in Systems
with Supply Voltages in Excess of Device
Maximum Ratings – e.g. Fixed or
Architectural Applications (24V).
For some LED applications, including fixed or
“architectural” displays, voltages greater than
the 18V maximum rating (pin 3) of the BCR402R
may be encountered. For example, +24V is
frequently used in so-called architectural
displays. This section describes the advantages
of using BCR402R in such systems, and how
BCR402R may be safely employed in such
higher voltage applications by using a “trick”.
These systems typically employ switch-mode
power supplies with precise voltage outputs,
eliminating the problem of supply voltage
variation encountered in automotive or portable
applications. However, unless an LED driver
like the BCR402R is used, another problem can
arise as a result of the typically large variation in
LED forward voltages (V_F). For example, one
type of amber-color LED in widespread use
today has a specified forward voltage range of
1.90 to 2.50 volts. Large displays in
architectural applications often have many
parallel branches of LEDs. In a display using
only resistors for current stabilization, if one
branch or “stack” of LEDs consists of diodes
with V_F’s in the low end of the specified range,
and another stack consists of LEDs with higher
V_F’s, the stack with the lower forward voltages
can “hog” or draw more current than the other
stack(s). This can create a situation where the
customer may readily see differences in
brightness between the adjacent stacks of
LEDs. To make matters worse, LEDs have a
negative temperature coefficient for forward
voltage as regular PN junctions do – but
frequently higher in magnitude (e.g. –4mV / °C
for an LED, versus –2.3mV / °C for a typical PN
junction). Since the stack of LEDs consisting of
diodes with lower forward voltages will draw
more current, they will tend to heat up more than
adjacent branches, which will further decrease
their forward voltage, making them draw more
current, and so on, potentially creating a thermal
runaway condition and failure mode. The key
point: if each stack of LEDs were fed with an
LED Driver device like BCR402R instead of
employing a resistor, the current through each
stack of diodes would be stabilized, and LED
stack currents would be more uniform
regardless of the normal variation in LED
forward voltages. The light outputs of adjacent
LED stacks would be equalized, and the
potential thermal runaway failure mechanism
discussed above would be eliminated.
For operation in excess of BCR402R’s specified
maximum voltage of 18V, one “trick” is to stack
a sufficient number of LEDs between the power
supply voltage +V_S and the DC input of the
BCR402R (pin 3) such that the voltage seen at
pin 3 is less than 18V. In other words, simply
use additional LEDs to drop the voltage fed to
the BCR402R below its maximum rating, and
then finish up the string of LEDs with additional
LEDs placed between pin 2 and ground, in the
usual way. Refer to Figure 7. Note that the
exact number of diodes required for the top or
“voltage dropping” stack of LEDs (D₁, D₂, … D_N)
will depend on
1) the supply voltage +V_S and
2) the voltage drops across the particular LEDs
being used. (Red, Amber, Blue and White LEDs
have varying diode forward voltages.)
AN 066 Rev D
4/ 8
16-Jan-2004