AAT3142
High Efficiency 1X/1.5X/2X Charge Pump
for White LED Applications
S
2
Cwire Serial Interface
The current source output magnitude is controlled
by the EN/SET pin, using AnalogicTech's S
2
Cwire
interface. The interface records rising edges of the
EN/SET pin and decodes them into 32 individual
current level settings with Code 1 reserved for no
current. Once EN/SET has been held in the logic
high state for time T
LAT
, the programmed current is
seen at the current source outputs, and the internal
data register is reset to 0. For subsequent current
level programming, the number of rising edges cor-
responding to the desired code must be entered on
the EN/SET pin.
When EN/SET is held low for an amount of time
greater than T
OFF
, the AAT3142 enters into shut-
down mode and draws less than 1µA from V
IN
. The
internal data register is reset to 0 during shutdown.
S
2
Cwire Serial Interface Timing
T
HI
T
LO
T
LAT
T
OFF
EN/SET
1
2
n-1
n <= 32
Data Reg
0
n
0
LED Selection
The AAT3142 is specifically intended for driving
white LEDs. However, the device design will allow
the AAT3142 to drive most types of LEDs with for-
ward voltage specifications ranging from 1.0V to
4.3V. LED applications may include main and sub-
LCD display backlighting, camera photo-flash appli-
cations, color (RGB) LEDs, infrared (IR) diodes for
remotes, and other loads benefiting from a controlled
output-current generated from a varying input-volt-
age. Since the D1 to D3 output current sources are
matched with negligible voltage dependence, the
LED brightness will be matched regardless of the
specific LED forward voltage (V
F
) levels.
In some instances (e.g., in high luminous output
applications such as photo flash), it may be neces-
sary to drive high-V
F
type LEDs. The low-dropout
current sources in the AAT3142 make it capable of
driving LEDs with forward voltages as high as 4.3V
at full current from an input supply as low as 3.0V.
Outputs can be paralleled to drive high-current
LEDs without complication.
Termination of Unused Current Source
Outputs
If any outputs (D1 to D3) are not used, they should
be terminated by connecting the respective unused
output directly to ground. This is required to assure
correct charge pump mode operation. If a given
unused output is not terminated to ground, it will
appear to the respective output’s feedback control
as an infinite impedance load and the unused con-
stant current source will be set for a maximum volt-
age. Although no current flows from the unused
current source, the voltage sensed at that node
could force the charge pump control to activate the
charge pump when it is not necessary. To prevent
this effect, simply connect the unused constant
current source outputs to ground.
Device Switching Noise Performance
The AAT3142 operates at a fixed frequency of
approximately 1MHz to control noise and limit har-
monics that can interfere with the RF operation of
cellular telephone handsets or other communication
12
3142.2007.03.1.2
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
