Applications Note No. 066
Silicon Discretes
BCR402R: Light Emitting Diode (LED) Driver IC Provides
Constant LED Current Independent of Supply Voltage Variation
Supplies stable bias current for Light Emitting Diodes (LEDs)
Low Voltage Drop of 0.75V maximizes system DC efficiency
LED drive current adjustable via single external resistor (20 – 60 mA range)
Negative temperature coefficient protects LEDs against thermal runaway
Low Cost, Low External Parts Count, Easy-to-Use, Fast Time-To-Market
Small Surface-Mount SOT143R packaging (2.9 x 1.6 x 1.1 mm inc. leads)
Infineon’s LED driver family, consisting of
the BCR401R, BCR402R, BCR402U and
BCR405U, offers an easy, cost-effective and
reliable way to achieve LED current stabilization
while overcoming the drawbacks inherent in
using a series resistor. These LED driver IC’s
cover the most commonly used LED supply
current ranges – 10mA, 20mA, and 50mA.
Refer to Figures 1 and 2. In each device, the
nominal drive current may be easily adjusted
upward from the nominal value by adding a
single external resistor – for BCR401R / 402R,
connect this resistor between pins 3 and 4.
1. Introduction
Light-Emitting Diodes (LEDs) and LED displays
suffer from varying illumination levels as a result
of changes in power supply voltages. This is
true for displays used in automotive applications,
battery-operated handheld devices, or fixed
(architectural) installations.
To keep LED
brightness constant, it is necessary to stabilize
the LED’s current over the anticipated power
supply voltage variation range.
Often a resistor in series with the LED(s) is used
to stabilize current. However, this method has
some serious drawbacks. To achieve good LED
current stability over the varying supply voltage
range, the resistor must take on a large value
(infinite resistance, in the ideal case) to make
the power supply plus series resistor
combination approach the traits of an ideal
constant current source having infinite output
impedance. In such cases, there is a large
voltage drop across this series resistor. For a
particular supply voltage, this voltage drop 1)
reduces the number of LEDs that can be driven
by one resistor, requiring additional parallel
resistors with which to drive a given number of
LEDs, thereby increasing system current
consumption and 2) power is “burned” or wasted
as heat in these series resistors.
AN 066 Rev D
Figure 1. Infineon LED Current Driver Family
Recommended Current Ranges.
Current Range
10 – 60 mA
20 – 60 mA
330 mW *
330 mW *
20 – 65 mA
50 – 65 mA
500 mW **
500 mW **
* Soldering Point Temperature TS = 87 °C
** Soldering Point Temperature TS = 117 °C
For selecting the proper resistor value to
increase BCR402 drive current above the
nominal level of 20mA, refer to Figure 3, which
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Applications Note No. 066
Silicon Discretes
is also found in the BCR402R datasheet. Two
cases are shown: “open”, e.g. R1 is omitted (no
R1 used), showing nominal current to be 20mA,
and R1=68Ω, yielding 30mA. For current up to
60mA, R1 will take on values less than 68Ω, and
R1 can be determined experimentally. Note that
in all cases,
Figure 3.
BCR402R Output Current vs.
Supply Voltage for external resistor = OPEN
and external resistor = 68Ω.
1. BCR402R output current should be
limited to 60mA
2. Total power dissipation of BCR402R
should be limited to less than 330 mW
for device soldering point temperatures
(TS) equal to or less than 87 °C.
The power dissipation is simply calculated by
multiplying the voltage drop across the device
(e.g. voltage between pins 2 and 3) times the
device current. A good general reference for
use when determining if a given circuit design
falls within device power dissipation guidelines
may be found at [1].
Figure 2.
Nominal Output Currents of
Devices in the Infineon LED Driver Family.
Current can be adjusted from these nominal
values via use of a single external resistor.
2. Application Circuit
The application circuit is designed to
demonstrate the difference between stabilizing
LED current using an Infineon LED driver IC,
versus using a series resistance to stabilize
This particular circuit has been
initially set up to operate over a supply
voltage range of 9V to 16V. The switch may
be used to easily toggle between BCR402R
mode and “resistor” mode. Please refer to
Figure 4.
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Applications Note No. 066
Silicon Discretes
Figure 5. Comparison of Power Dissipation
in BCR402R LED Bias controller vs. Resistor
Biasing Method.
Figure 4.
Application Circuit Schematic.
Switch SW1 permits comparison of
BCR402R vs. “Resistor Method”.
Compare results in Figure 5 for both sides of
the application circuit (e.g. BCR402R method
versus Resistor method), for the condition of
VS=16V, I=29mA. When the “resistor method” is
used to drive the same number of LEDs as the
BCR402R, more than ten times the DC power is
wasted in the resistors as is burned in the
BCR402R. The difference in power dissipation
between the two methods, 213mW, may seem
trivial, unless one considers the effect of using
large numbers of such LED circuits in a large
display. If the net series resistor value is
reduced, DC power wasted when using the
“resistor method” could be reduced, but the
already poor current regulation of the “resistor
method” gets even worse. If the net series
resistor value is increased to 410Ω (two parallel
820Ω resistors), the current stability is improved
slightly, but the larger voltage drop across the
larger net series resistance reduces the number
of LEDs that can be driven from four to just two.
For a given number of LEDs in a display, this
means that the user of the resistor method
would then have to add additional resistor + LED
branches to the display, requiring additional
current and further increasing power dissipation.
In the example given here, the current
consumption would have to double in order to
drive a total of four red LEDs with 410Ω series
BCR402R has a typical output current of 20mA
without using external resistor R1. Current may
be increased above this nominal value by using
R1, and in this case, R1 was set to 82 Ω to
achieve an output current of 29mA. Based on
the fact that the minimum supply voltage is 9V
and that there is a voltage drop of approximately
0.75V across the BCR402R at minimum supply
voltage, it is possible to drive four red LEDs. On
the right side of the circuit shown in the
schematic (“resistor mode”), two parallel
resistors (560Ω) had to be used instead of one,
so as to not exceed the maximum power
dissipation of the 1208 SMD resistors while
achieving a net resistance of 280Ω.
brings up a key issue:
overall system
efficiency. See Figure 5.
AN 066 Rev D
Power Dissipation in
For VS=16V, I=29mA, R2║R3 = 280Ω
BC R 402R
Power Dissipation in
Figure 6 shows the effect on current
stabilization using the BCR402R and different
series resistor values (280, 340 and 410 Ω,
resulting from parallel combinations of R1+R2).
Note the nice, flat curve showing nearly constant
current over the entire 9V to 16V supply range
when the BCR402R LED Bias Controller is
used, while the series resistance method shows
very limited current stabilization – even with
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Applications Note No. 066
Silicon Discretes
branch or “stack” of LEDs consists of diodes
with VF’s in the low end of the specified range,
and another stack consists of LEDs with higher
VF’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.
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”.
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 +VS 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, … DN)
will depend on
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 (VF). 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
AN 066 Rev D
1) the supply voltage +VS and
2) the voltage drops across the particular LEDs
being used. (Red, Amber, Blue and White LEDs
have varying diode forward voltages.)
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Applications Note No. 066
Silicon Discretes
and bottom stacks due to this effect. In fact,
there is far more variation in brightness between
individual LEDs of the same type, if they are all
driven with identical currents, than could be
attributed to this small “error” current due to the
BCR402R’s PNP transistor base current.
When used in this way, the BCR402R acts as a
“current sink” for the diodes D1, D2, …, DN on the
top stack of LEDs, and as a “current source” for
the diodes below, e.g. DN+1, DN+2, …, DN+X. As
there is only one current path, stabilization is
maintained for both upper and lower stacks of
LEDs. There is a vanishingly small difference or
“error” in terms of the current difference between
LEDs in the top stack and the bottom stack, this
difference being created by the base current
consumed by the internal PNP transistor in the
BCR402R. This base current, and therefore the
current difference between top and bottom LED
stacks, is only about 400 microamps at a
nominal 20mA LED current. As such, the
human eye will not be able to discern any
differences in brightness in LEDs between top
Figure 7.
In closing, for either the high voltage application,
or standard application, an external NPN
transistor (Q1) may be used as an ON / OFF
switch for the LED circuit, if desired. Infineon
Technologies markets a broad range of
switching transistors suitable for this purpose,
and the best choice will depend on the voltage
present at pin 1 of the BCR402R, e.g. select a
switching transistor with sufficient collectoremitter and collector-base breakdown voltages.
Operation of BCR402R at “High” Voltages e.g. > 18V.
D1, D2, ..., DN:
Use enough LEDs in
"top LED stack" such
that voltage at pin 3 of
BCR402R is < 18V
Note: maximum possible number of LEDs that can be used in
entire circuit depends upon +VS & voltage drop across particular
diode types. Q1 and R2 are optional, if it is desired to switch LED
stack on and off at this point. If switch is not needed, ground pin
1 of BCR402R. Be sure Q1 can handle voltage present at pin 1
of BCR402R given the value of +VS selected. Infineon produces
a broad range of switching transistors suitable for Q1.
(High = ON)
AN 066 Rev D
DN+1, DN+2, ..., DN+X:
Use maximum number of
LEDs in "bottom stack" that
permits sufficient voltage
between pin 2 and ground to
drive them.
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Applications Note No. 066
Silicon Discretes
BCR402R provide more current from pin 2 (IOUT)
as we get hotter, creating the potential to have a
thermal runaway condition in our LED display
circuit. To prevent this, two internal series
diodes are placed between the +VS node and
the internal PNP transistor’s base. Since each
of these two internal diodes also has a
temperature coefficient of the same value (2.3mV / °C), as we go hotter, the PNP
transistor’s base voltage will rise, tending to
“throttle back” the PNP transistor. If only one
diode junction were used, we would more or less
evenly temperature compensate for the PNP
transistor’s B-E junction and maintain a fairly
constant BCR402R output current over
temperature. The addition of the second diode
“over-compensates” or actually causes the
BCR402R to “source” slightly less current as
one goes hot – providing negative feedback and
thereby preventing a potentially dangerous
thermal runaway condition for the LEDs in the
display circuit. This is especially useful for LED
displays subjected to wide temperature
variations, e.g. those found in automotive
4. Temperature Compensation, Negative
Temperature Coefficient of BCR402R and
Protection of LEDs from Thermal Runaway
For this section, please refer to the internal
schematic diagram of the BCR402R shown in
Figure 8 below.
Note that, as a rough rule-of-thumb, the baseemitter forward junction voltage of a Silicon PN
junction has a temperature coefficient of
something like –2.3 mV / °C. In other words, as
temperature increases, the emitter-base voltage
of the internal PNP transistor in BCR402R
decreases at a rate of about 2 millivolts per
degree C.
Figure 8.
BCR402R Internal Schematic
(inside box formed by dotted line)
5. PCB Layout Details
The top view of the Application Board is given in
Figure 9 on the next page. An enlarged picture
including component placement is provided in
Figure 11 on page 7. Note that there are two
positions in which to place R1, in order to
accommodate either 0402 or 1208 case size
resistors. Open connections to ground on the
left of the LED stack make it easy to modify the
board for use with less than four LEDs. Figure
10 on the next page shows an enlarged view of
the PCB section where the LED driver is placed.
The top and bottom metal layers are shown.
This reveals the heat sink (metal area) for the
BCR402R on the backplane or bottom metal
layer of the PC board. The PC board footprint
was designed to fit the SOT143R package used
by BCR401R and BCR402R, but it will also
accommodate the SC74 package as used by the
BCR405U without modification. Please note
that this particular PCB does not accommodate
the “High Voltage” application described in
Section 3.
Ordinarily, this temperature coefficient could
create the following problem:
for a given
operating point, as we go from cold to hot, the
PNP transistor’s VBE will decrease, making the
transistor “turn on harder”, thereby making the
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Applications Note No. 066
Silicon Discretes
Figure 10. Close-In View of Top Metal Layer
with “Heat Sink” on Backplane.
Figure 9. Top View of Application PC Board.
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Applications Note No. 066
Silicon Discretes
Figure 11. Enlarged Top View of PCB Showing Parts Placement.
Board size: 35mm x 65mm
Board material: FR4
Board thickness: 1mm
6. Conclusions
7. References
Infineon Technologies’ LED Driver family offers
the end user an easy, cost-effective way to
stabilize LED current in fixed, portable or
automotive LED display applications. DC power
consumption is minimized, as compared to LED
driver circuits employing resistors for current
The negative temperature
coefficient of these LED Drivers helps to prevent
thermal runaway conditions in LED displays.
[1] Application Note 077, “Thermal Reisistance
Calculations”, Infineon Technologies AG, Silicon
Discretes Group. (App note gives overview of
thermal overstress issues.
Available at
ents/039/975/Appli077.pdf ).
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