BCR402 datasheet(4/8 Pages) INFINEON

4 / 8 page

AN 066 Rev D

4 / 8

16-Jan-2004

Applications Note No. 066

Silicon Discretes

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

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

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”.

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

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

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.

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

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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