DN422 - Tiny Universal LED Driver Can Gradate, Blink or Turn On Nine Individual LEDs with Minimal External Control

Tiny Universal LED Driver Can Gradate, Blink or Turn On Nine
Individual LEDs with Minimal External Control – Design Note 422
Marty Merchant
Introduction
LEDs are the lighting workhorse of cell phones, MP3
players and diagnostic lights in telecom systems. Their
uses are many, from utilitarian backlighting to eye-catching aesthetic effects such as slowly pulsing multicolor
indicators. As device designers strive to differentiate
their products on the shelf, the number and complexity of
lighting effects grows. It would seem that each new effect
requires significant additional hardware, and/or complex
software, right? Actually, no, there is a way to apply these
effects to a number of LEDs with only a single driver IC.
The LTC ® 3219 9-output universal LED (ULED) driver
can be programmed to individually gradate, blink or
turn on nine individual LEDs using internal logic and
circuitry to drive nine 6-bit DAC-controlled LED current
sources. Because the gradation and blinking features
are controlled internally, effects can be realized without
adding ICs, extensively tying up the I2C bus or filling
valuable memory space with complex programming
subroutines. Any feature on any 0mA to 28mA output can
be configured to activate via the external enable (ENU)
pin or I2C interface.
The LTC3219 operates from a 2.9V to 5.5V input. The
charge pump provides up to 250mA output current, and to
optimize efficiency, it automatically changes charge pump
mode to 1x, 1.5x or 2x depending on the output current
requirement. Any of these modes can also be forced.
Blinking and Gradation Modes
Each output can be set to blink each output with a 156ms
or a 625ms on time and a 1.25s or a 2.5s period. Blink
mode can be initiated and ended via the I2C interface or
using the ENU pin. Once blinking has been initiated, the
LED(s) continue to blink without any interaction from the
I2C interface or the ENU pin. This allows the controlling
interface device to shutdown and save battery power
until needed.
The LTC3219 can gradually turn on, or gradually turn off
any number of the LED channels. The gradation ramps up
08/07/422
from 0mA to the programmed LED intensity with ramp
times of 240ms, 480ms or 960ms (likewise for turn off).
Like blinking mode, gradation mode can be implemented
via minimal I2C interaction or by the ENU pin as shown
in Figure 1.
Figure 1. The LTC3219 Gradating an LED from 0mA
to 28mA in 960ms. Prior to the Gradation Ramp, the
Gradation Timer, Up Bit and ULED Registers Are Set. A
Stop Bit on the Last I2C Write Starts the Gradation Ramp.
After the Gradation Ramp Has Finished, Gradation Is
Disabled with the LED Set at Full Intensity
Single IC Drives Cell Phone Backlight, New
Message/Missed Call/Battery Charger Indicator,
and RGB Function Select Button
The circuit in Figure 2 illustrates a flip cell phone lighting
circuit with four white LEDs for backlighting the keypad,
a multicolor indicator, and a function select button illuminated by an RGB LED. The multicolor indicator consists
of a red and a green LED. The RGB LED provides full
color gamut, including white by varying its individual
LED intensities.
When the cell phone is powered on or flipped open, the
keypad and the function select button gradually illuminate
to an intensity set by the baseband controller and CPU
using the gradation feature of the LTC3219. The Function
Select button may also gradually change colors using the
gradation feature. After an idle period or during power
off, the LEDs gradually turn off using the gradation fea, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
C2
1μF
C3
1μF
BACKLIGHT
C1P C1M C2P C2M
VIN
C1
2.2μF
BASEBAND
CONTROLLER
CPU
2
I2C
VIN
CPO
C4
2.2μF
LTC3219
ULED1-4
SCL/SDA
4
MULTICOLOR
INDICATOR
KEYPAD/BUTTON
INTERFACE
ENU
ULED5-9
FUNCTION SELECT
BUTTON RGB
5
GND
DN422 F02
Figure 2. A Single IC, Multilighting Cell Phone Application. The LTC3219 Comes in a
3mm × 3mm 20-Lead QFN Package and Only Requires Five External Components
C3
2.2μF
C2
2.2μF
BACKLIGHT
C1P C1M C2P C2M
VBAT
BASEBAND
CONTROLLER
CPU
C1
2.2μF
2
I2C
DVCC
C5
0.1μF
VBAT
CPO
C4
2.2μF
LTC3219
SCL/SDA
ULED1-4
DVCC
ULED5-8
ENU
ULED9
VIBRATOR MOTOR
VCCIO
C7
0.1μF
4
4
C6
0.1μF
5
SOUND
DEVICE
GND
DN422 F03
Figure 3. Cell Phone with Backlighting, Vibrator Motor and Sound Controller
ture. When a call is missed, the baseband controller and
CPU set the multicolor indicator to blink red to indicate
a missed call or blink green if the caller left a message.
Once the multicolor indicator is blinking, the baseband
controller hands off control to the external enable pin and
shuts down to save battery power. The keypad and button
interface holds the ENU pin high until the cell phone user
takes action to turn off the blinking indicator.
Control for Cell Phone Backlight, Vibrator Motor
and Sound
Cell phones use various combinations of vibration, sound
and light to alert the users of an incoming call or message.
Figure 3 illustrates a cell phone with four backlighting
LEDs, a vibrator motor and a logic controlled sound device.
A single logic pin, ENU, turns on all simultaneously.
If the vibrator motor requires more than 100mA, simply
gang-up the ULED outputs to provide enough current. A
small ceramic capacitor may be needed across the motor
terminals and between the ULED output pins and ground
to reduce inductive spikes and to prevent false dropout.
The speed and current in the motor is proportional to the
voltage across the motor, so the voltage across the motor
must be controlled in order to control the motor speed
and current. One voltage-control method is to connect
a shunt zener diode across the motor. Use a zener diode
that provides the desired voltage across the motor with
minimal zener current for maximum efficiency.
Conclusion
The LTC3219 is a LED driver and charge pump which can
independently control nine outputs. Special features such
as gradation, blinking, and GPO modes require minimal
I2C communication. The LTC3219 is an ideal device for
many applications that use multiple lighting, logic or
other current controlled devices.
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 2364
www.linear.com
Linear Technology Corporation
dn422f LT/TP 0807 305K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2007
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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