lx1991 Microsemi Corporation, lx1991 Datasheet - Page 5

• Manufacturer Part Number: lx1991
• Description: Six Output Programmable Led Current Sink
• Manufacturer: Microsemi Corporation

Copyright © 2001
Rev. 1.1, 10/27/2005
F
illumination and signaling applications such as cellular
telephones,
illumination etc. With a single 5V supply, the six independently
regulated constant current outputs can drive 6 white LED’s (V
4.0V), or 12 green or amber LED’s (V
6 parallel x 2 in series. With a separate high voltage LED supply
(up to 40V) a large matrix of LED’s (>100) can be driven.
Connecting a resistor between I
current that is mirrored into each of the outputs with a gain ratio
of about 24. This ratio remains linear for output currents from 3 to
30mA, at output current less than 3mA the ratio increases
somewhat. Output current can be varied in an analog fashion by
varying R
range of 1K to 40K.
accomplished by driving the DIG_DIM input. The switching
frequency can exceed 100 KHz, making it practical to use a PWM
output channel from popular micro controllers. The upper limit on
frequency correlates to the minimum DIG_DIM sink pulse width
which is typically about 5μS.
minimum pulse width the CSLOPE pin must be open. The
CSLOPE pin allows precise control of the sink current rise and
fall time. As the value of the CSLOPE capacitor is increased the
slope of the rise and fall time will change correspondingly along
with some delay to output. Special control circuitry maintains
symmetrical rise and fall times, preserving the LX1991’s ability to
provide accurate output current response to very narrow input
pulses on the DIG_DIM pin.
of about 1.6V with about 150mV of hysteresis. Below 0.8V the
chip is disabled and the sink outputs are off. However ENABLE
must be driven below 0.4 volts to insure the minimum sleep
current from V
making the LX1991 an exceptionally robust component. The
device also includes thermal shutdown however it is not
recommended to short the I
shorting the outputs to 40V as the power dissipation under these
conditions is the greatest.
UNCTIONAL
The LX1991 is designed to drive LED’s used in display
The LX1991 features resistor settable output current.
Duty cycle dimming with a fixed current amplitude is
To use the LX1991 at the maximum usable frequency, or
The ENABLE input is TTL compatible with a turn on threshold
All inputs and outputs are ESD and short circuit protected
TM
SET
resistor, typically this resistor shall be chosen in the
D
PDA’s
DD
ESCRIPTION
and output off current.
automotive
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
SET+
and I
SET+
THEORY OF OPERATION / APPLICATION NOTE
dashboard
and I
SET-
F
< 2.0V each) arranged as
inputs together while
SET-
pins generates a
lighting,
Integrated Products
®
Microsemi
panel
F
<
Six Output Programmable LED Current Sink
A
held above about 600-800mV (typically) to perform properly and
maintain the specified current regulation accuracy. The device is
designed however to allow one or more of the outputs to be unused
(open) without affecting the other channels Or conversely if more
than 30mA is needed the outputs can be combined for higher total
currents.
equation:
package or the maximum junction temperature of the die. Proper
mounting of the package with thermal vias (see PCB Land Guidelines
and Thermal Pad Design section) enhances power dissipation
capability. As an example to drive 24 white LED’s, 4 per channel
from a 18V V
the minimum LED forward voltage to be 3.5V with an maximum
operating ambient of 50°C.
die temperature of about 72°C, ignoring device drain current power
dissipation that only represents about a 3°C rise worst case. This is
well under the die junction temperature maximum rating of 150°C,
and within the 85°C electrical specifications.
(changing the R
DIG_DIM input) or both. Current between the I
may be varied for current amplitude dimming. If a mechanical input
is needed, a rheostat in series with a resistor connected from I
I
SET-
PPLICATION
P
Each output has an independent current sink, however each must be
Package power dissipation can be calculated from the following
n
I
V
V
Care should be exercised not to exceed the power dissipation of the
This represents a 21.6°C temperature rise (36°C/W *0.6W) for a
D
P = n×I
P = (6×25mA)×(18V-(4×3.5V)) = 15×4V = 0.6W
I
P
Dimming can be accomplished both with current amplitude control
OUT
OWER CONSIDERATIONS
OUT
D
D
IMMING
LOAD
LED
D
can be used.:
(
=Current Ratio×I
OUT
N
P
LED
OTES
SET
) (
RODUCTION
× V
=
=
=
=
=
supply @ 25mA per LED string, we will assume
resistor value) with a PWM signal (toggling the
LOAD
Number of outputs used
Current from each output
Output Supply Voltage
Minimum LED forward voltage
Power Dissipated in mW
SET
-V
LED
D
ATA
)
S
HEET
SET+
and I
LX1991
SET-
SET+
pins
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