Microsemi

Integrated Products

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

Page 5

Copyright

© 2001

Rev. 1.0d, 2005-03-14

LX1990

Dual Programmable LED Current Sink

TM

®

APPLI CATI ONS

The LX1990 is designed to drive LED’s used in display

illumination and signaling applications such as cellular

telephones and PDA’s. With a 5V supply, the two

independently regulated constant current outputs can drive

in series.

With Dropout voltage of only 300 mV at 20 mA, the

LX1990 provides regulated current for 2 green / amber

LED’s in a cell phone all the way down to the minimum

NiCd cutoff of 2.7 volts for three cells.

The LX1990 features resistor settable output current.

1.25 volts generates a current that is mirrored into each of

the output with a gain of about 100. Output current can be

termination voltage (see application schematics).

Duty cycle dimming with a fixed current amplitude is

with an open collector or open drain pulse width

modulated logic signal. Switching frequency can exceed

20 KHz, making it practical to use a PWM output channel

from popular micro controllers. The upper limit on

output. As frequency increases, these fixed delays will

cause an error in expected output current duty cycle.

All inputs and outputs are ESD and short circuit

protected making the LX1990 an exceptionally robust

component.

However

it

is

not

recommended

to

the outputs to 10V as the power dissipation under these

conditions is the greatest.

Each output has an independent current sink, however

both must be held above about 0.5 volts to maintain

specified current regulation accuracy. If only one output

is needed, the two outputs should be connected together

and the programming resistor value doubled so each

output supplies ½ the desired current. In this case, more

than 60 mA total current can be delivered from the

combined output.

Package power dissipation can be calculated from the

following equation:

() (

)

ED

LOAD

L

OUT

D

V

V

I

n

P

−

⋅

⋅

=

n

=

Number of outputs used

=

Current from each output

=

Output Supply Voltage

=

Minimum LED forward voltage

=

Power Dissipated in mW

When the ENA input is at zero volts, less than 20µA

current flows into or out of the outputs. If the disable

function is used , ENA must be driven below 0.4 volts to

open

drain

logic

PWM signal to

dim the

LED’s.

Recommended PWM frequency for dimming is between 100

Hz and 20 KHz. Below 100 Hz flicker may be observed.

Above 20 KHz duty cycle accuracy is reduced due to

drive may also be used as long as the maximum driver

delays that may limit the useful frequency and maximum

dim range.

If PWM dimming is used, use separate power and ground

lines directly from the power source point to prevent noise

generated from the LED current transients from entering

video or audio subsystems on the same supply rails.

Additional power supply filtering may be needed in PWM

dimming applications. Care should be exercised in the PCB

as this may cause the part to oscillate.

amplitude dimming. There are two ways to do this: If a

mechanical input is needed, a rheostat in series with a

output of a voltage DAC or other low impedance voltage

source. Varying the voltage between zero and 1.25 volts will

to zero:

SET

OUT

I

Ratio

Current

I

×

=

The ENA pin may be used to duty cycle dim the output

in applications where only one control line is available for

dimming and shutdown. A narrow current spike that may be

as high as 100 mA is produced when the ENA signal has

very fast rise times. This is not harmful to the LED, but will

cause dimming linearity errors when operating at very low

duty cycles.