BCR402 Datasheet, PDF (3/8 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
Figure 4. Application Circuit Schematic.
Switch SW1 permits comparison of
BCR402R vs. “Resistor Method”.
Vcc
Figure 5. Comparison of Power Dissipation
in BCR402R LED Bias controller vs. Resistor
Biasing Method.
Power Dissipation in Power Dissipation in
BCR402R
(R2+R3)
SW1
R1
22mW
235mW
For V_S=16V, I=29mA, R2║R3 = 280Ω
4
3
Q1 BC R 402R
1
2
R2
R3
D1
D2
D3
D4
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. This
brings up a key issue: overall system
efficiency. See Figure 5.
Compare results in Figure 5 for both sides of
the application circuit (e.g. BCR402R method
versus Resistor method), for the condition of
V_S=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
resistances.
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
AN 066 Rev D
3/ 8
16-Jan-2004