LX1994

Microsemi

Integrated Products Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 11

High Efficiency LED Driver

Copyright

© 2003

Rev. 1.0a, 2004-08-10

TM

®

APPLICATION INFORMATION

R3 = 23.2k

The level at 100% duty cycle in full darkness is 4mA,

which is 20% of the maximum level of 20mA; this implies

the describing equation for AUTO mode gives:

()

(

) () ()

SRC

LED(MAX)

I

× R1×R2×Rp

80%×I

×R5=

R1×R2 + R1×Rp + R2×Rp

This implies:

()

(

) () ()

R1×R2×Rp

0.8×.02×15

==2.4k

R1×R2 + R1×Rp + R2×Rp

100µA

Since the left side is the three resistors in parallel, this can

be restated as:

-6

11

1

416×10 =

+

+

=G1+G2+Gp

R1 R2

Rp

The manual mode equation can be reduced to this assuming

100% duty and 20mA LED current (that is 0.3V sense

resistor voltage):

CC

R2×Rp

0.3×R1

0.3×R1

==

R2+Rp

(10µA×R1)+V -0.3

(10µA×R1)+3.0

This can be restated as:

-6

11

10

+

=33×10 +

R2

Rp

R1

or

()

-6

G2+Gp=33×10 + 10×G1

The auto mode equation can be reduced to this assuming

100% duty , 100µA ISRC current and 20mA LED current

(that is 0.3V sense resistor voltage):

()

()

()

SRC

CC

CC

R1×Rp

0.3×R2

=

R1+Rp

(I

+10µA)×R2 +V -0.3

0.3×R2

0.3×R2

==

(100µ+10µA)×R2 +V -0.3

110µA×R2 +3.0

This can be restated as:

-6

11

10

+

=367×10 +

R1 Rp

R2

or

()

-6

G1+Gp=367×10 + 10×G2

The equations above can be solved for G1, G2 and Gp:

-6

G1=34.8×10

-6

G2=4.45×10

-6

Gp=376×10

Knowing Gp we can find

-6

1

G4=Gp-

=343×10

30k

The resistance values are the reciprocal of the

conductance’s so:

R1 = 28.7k

R2 = 225k

R4 = 2.91k

The value of C1 is selected to give a time constant of ½

second and works into R3 (which is 23.2k).

0.5

C1=

23.2k

C1 = 21.5µF

The value of C2 works into Rp and the pole should be set

at 1/100 of the PWM frequency.

1

C2=

10kHz

6.28×

×2.66k

100

For a 10KHz PWM, C2 = 599nF, and a value of 1µf

works well.

Circuit of Figure 3:

The second light sensor interface is very similar to the first;

the choice is a matter of user preference. In the second

circuit, an active 325mV clamp is used to clamp the

maximum LED current in auto mode.

In this circuit,

resistor R3 is reduced to extend the

operating ambient light range of the light sensor and filter

capacitor C1 must therefore be increased.