BCR402 Infineon Technologies Corporation, BCR402 Datasheet - Page 6

• Manufacturer Part Number: BCR402
• Description: Light Emitting Diode ( Led ) Driver ic Provides Constant Led Current Independent of Supply Voltage Variation
• Manufacturer: Infineon Technologies Corporation

4.
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
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.
(inside box formed by dotted line)
Ordinarily, this temperature coefficient could
create the following problem:
operating point, as we go from cold to hot, the
PNP transistor’s V
transistor “turn on harder”, thereby making the
AN 066 Rev D
Temperature
has
BCR402R Internal Schematic
a
BE
temperature
Compensation,
will decrease, making the
coefficient of
for a given
Negative
6 / 8
BCR402R provide more current from pin 2 (I
as we get hotter, creating the potential to have a
thermal runaway condition in our LED display
circuit.
diodes are placed between the +V
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
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
variations, e.g. those found in automotive
applications.
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.
that this particular PCB does not accommodate
the “High Voltage” application described in
Section 3.
To prevent this, two internal series
subjected
BCR402R
Applications Note No. 066
to
output
wide
Silicon Discretes
current
16-Jan-2004
Please note
temperature
S
node and
over
OUT
)