Microsemi
Integrated Products
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page
Copyright 2001
Rev. 1.0c, 2002-05-29
WWW.Microsemi .COM
LX1990
Dual Programmable LED Current Sink
INTEGRATED PRODUCTS
APPLICATIONS
APPLICATIONSAPPLICATIONS
APPLICATIONS
F
UNCTIONAL DESCRIPTION
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
2 white LED’s (V
F
< 4.5V) in parallel, or 4 green or
amber LED’s (V
F
< 2.25V each) arranged as 2 parallel x 2
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.
Connecting a resistor between I
SET
and a voltage of zero to
1.25 volts generates a current that is mirrored into each of
the output with a gain of about 100. Output current can be
varied in an analog fashion by varying I
SET
resistance or
termination voltage (see application schematics).
Duty cycle dimming with a fixed current amplitude is
accomplished by switching the I
SET
current on and off
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
frequency is minimum switching response from I
SET
to
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
indefinitely short the I
SET
input to ground while shorting
the outputs to 10V as the power dissipation under these
conditions is the greatest.
A
PPLICATION NOTES
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:
()( )
EDLOAD LOUTD
VVInP −⋅⋅=
n = Number of outputs used
I
OUT
= Current from each output
V
LOAD
= Output Supply Voltage
V
LED
= Minimum LED forward voltage
P
D
= 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
insure minimum current from V
DD
.
The I
SET
pin may be driven with a digital open collector or
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 switching delays from I
SET
to outputs. Totem pole output
drive may also be used as long as the maximum driver
voltage never exceeds the V
DD
supply. Since the totem pole
drive will exceed the 1.25V I
SET
voltage it can introduce
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
layout to prevent coupling from the outputs to the I
SET
pin,
as this may cause the part to oscillate.
Current out of the I
SET
pin may be varied for current
amplitude dimming. There are two ways to do this: If a
mechanical input is needed, a rheostat in series with a
resistor connected from GND to I
SET
can be used. The
second method is to connect a fixed resistor from I
SET
to the
output of a voltage DAC or other low impedance voltage
source. Varying the voltage between zero and 1.25 volts will
cause a corresponding output current change from I
OUT
max
to zero:
SETOUT
IRatioCurrent 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.
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