BCR402 Datasheet, PDF (5/8 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
When used in this way, the BCR402R acts as a
“current sink” for the diodes D₁, D₂, …, D_N on the
top stack of LEDs, and as a “current source” for
the diodes below, e.g. D_N+1, D_N+2, …, D_N+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
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.
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 collector-
emitter and collector-base breakdown voltages.
Figure 7. Operation of BCR402R at “High” Voltages e.g. > 18V.
+V_S
D1
D₁, D₂, ..., D_N:
D2
Use enough LEDs in
"top LED stack" such
that voltage at pin 3 of
BCR402R is < 18V
D_N
R1
Note: maximum possible number of LEDs that can be used in
entire circuit depends upon +V_S & 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 +V_S selected. Infineon produces
a broad range of switching transistors suitable for Q1.
ON / OFF
Control
(High = ON)
4
3
W6s
BCR402R
1
2
R2
1K
Q1
D_N+1
D_N+2
D_N+X
D_N+1, D_N+2, ..., D_N+X:
Use maximum number of
LEDs in "bottom stack" that
permits sufficient voltage
between pin 2 and ground to
drive them.
AN 066 Rev D
5/ 8
16-Jan-2004