LX1994

Microsemi

Integrated Products Division

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

Page 10

High Efficiency LED Driver

Copyright

© 2003

Rev. 1.0d, 2006-02-09

TM

®

APPLICATION INFORMATION

Resistors R6 and R7 of Figure 2 program the over voltage

clamp level. The value of R6 can be as high (like 1M

Ω.) to

minimize the quiescent current.

The value of R7 can be

determined using the following equation where VOVP is

OVP

OUT

OVP

V

R7=R6×

V-V

⎛⎞

⎜⎟

⎝⎠

Let R6 equal 1M and the required clamp voltage is 25V.

1.2

R7 = 1

×R

= 50.4Ω

25-1.2

M

⎛⎞

⎜⎟

⎝⎠

The output filter inductor should be a 1µF capacitor with

sufficient voltage rating for the OVP setting. Inductors in

the range of 10µH to 47µH work best. For the best

efficiency a larger value of inductor such as 47µH is

recommended; larger value inductors will reduce ripple

current

which

reduces

peak

currents

and

improves

efficiency. Smaller value inductors may be use less board

space, so a design trade off is in order.

A Schottky diode should be used with a 1 Amp current

rating and voltage rating equivalent to the OVP setting. The

transistor should be a N-channel MOSFET with a logic

level gate voltage: good candidates are the FDV303N and

the FDN337. For higher voltages, several BSS138 can be

wired in parallel.

The LX1994 requires a tight layout of the CMP pin

capacitance.

For best results, the 0.1µF CMP capacitor

should be located directly adjacent to the LX1994 package

with etch lengths as short as possible.

1

2

56

10

VIN

S/P

GND

DRV

8

OVP

9

SRC

3

BRT

FB

7

CMP

4

LS

0.1uF

LIGHT

The LX1994 has a LS input pin to simplify the interface to

an LX1970 light sensor.

Two different circuits are

described which provide slightly different response curves.

The equations for calculating the component values are

also given.

For the circuit of Figure 2, the describing equations are:

Rp×30k

R4=

Rp+30k

or

1

G4=Gp-

30k

Auto Mode:

() (

)

() (

) (

)

()

() (

) (

)

SRC

LED

CC

I+10μA × R1×R2×Rp

R1×R2 + R1×Rp + R2×Rp

DutyCycle

I=

×

R5

V× R1×Rp

+

R1×R2 + R1×Rp + R2×Rp

⎡

⎤

⎢

⎥

⎢

⎥

⎢

⎥

⎢

⎥

⎢

⎥

⎣

⎦

CC

COMPLIANCE

BRT(MAX)

SRC(MAX)

V-V

-V

I=

R3

Manual Mode:

()

() (

) (

)

()

() (

) (

)

LED

CC

10μA× R1×R2×Rp

R1×R2 + R1×Rp + R2×Rp

DutyCycle

I=

×

R5

V× R2×Rp

+

R1×R2 + R1×Rp + R2×Rp

⎡

⎤

⎢

⎥

⎢

⎥

⎢

⎥

⎢

⎥

⎢

⎥

⎣

⎦

Example:

Select R5 = 15 ohms; ILED = 20mA max; ISRC clamp at

100µA; VCC = 3.3; ILED in full darkness and 100% duty

cycle = 4mA.

With R5 = 15 ohms; ILED = 20mA max, VBRT(MAX) =

300mV.

With ISRC clamp at 100µA, Vcompliance (LX1970) =

0.68V, VCC = 3.3V, so

(

)

3.3-0.68-0.3

R3=

=23.2k

100μA