BCR402 Datasheet, PDF (6/8 Pages) Infineon Technologies AG – Light Emitting Diode (LED) Driver IC Provides Constant LED Current Independent of Supply Voltage Variation

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 base-
emitter 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)
Applications Note No. 066
Silicon Discretes
BCR402R provide more current from pin 2 (I_OUT)
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 +V_S 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
applications.
5. PCB Layout Details
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 V_BE will decrease, making the
transistor “turn on harder”, thereby making the
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.
AN 066 Rev D
6/ 8
16-Jan-2004