TN0025

LED balancing circuit

3/7

Equation 2

●

For one open LED:

Equation 3

Equation 4

Equation 5

Equation 6

Equation 7

Equation 8

Equation 9

Equation 10

1.1

Power limit circuit

A power limit circuit can be added to the circuit in Figure 1 to limit the overall power when

there is a large voltage variation for parallel strings of LEDs. Figure 2, Figure 3, and Figure 4

have two LED strings and the power limit circuit has bold references. The voltage variation

can be from one string with low voltage drop LEDs and another string with a high voltage

drop or if there is a shorted LED in one string.

In Figure 2 shows the power limit circuits R11, R12, R13, R14, Q11 and Q12. If there is a

low drop in the LED string D1 and D2, the voltage at V1 (Q1 collector) is high and Q11 is

Re

V

r

V

be

–

()

I

Re

--------------------------

=

I

Rr

V

in

V

r

–

()

R

r

------------------------

=

I

Re

V

r

V

be

min

–

()

Re

--------------------------------

=

V

r

V

be

max

I

Rr

Re

•

+

=

V

r

V

be

max

V

in

V

r

–

()

Re

Rr

--------

•

+

=

V

r

V

be

max

V

in

Re

Rr

--------

•

V

r

Re

Rr

--------

•

–

+

=

V

r

V

r

Re

Rr

--------

•

V

be

max

V

in

Re

Rr

--------

•

+

=

+

V

r

V

be

max

V

in

Re

Rr

--------

•

+

1

Re

Rr

--------

+

---------------------------------------------

⎝⎠

⎜⎟

⎜⎟

⎜⎟

⎛⎞

=

I

Re

V

be

max

V

in

Re

Rr

--------

•

+

⎝⎠

⎛⎞

1

Re

Rr

--------

+

⎝⎠

⎛⎞

---------------------------------------------------

V

be

min

–

Re

-------------------------------------------------------------------------

=