AN 066 Rev D

5 / 8

16-Jan-2004

Applications Note No. 066

Silicon Discretes

When used in this way, the BCR402R acts as a

top stack of LEDs, and as a “current source” for

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.

D1

D

N

D2

1

2

3

4

W6s

BCR402R

R1

+V

S

D

N+1

D

N+X

D

N+2

D

Use enough LEDs in

"top LED stack" such

that voltage at pin 3 of

BCR402R is < 18V

D

Use maximum number of

LEDs in "bottom stack" that

permits sufficient voltage

between pin 2 and ground to

drive them.

Note: maximum possible number of LEDs that can be used in

entire circuit depends upon +V

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

a broad range of switching transistors suitable for Q1.

Q1

R2

1K

ON / OFF

Control

(High = ON)