AAT3120
3-Channel Charge Pump
For White LED Applications
pin and decodes them into eight individual current
level settings. Code 8 is full scale.
The counter can be clocked at speeds up to 1MHz,
so intermediate states are not visible. The first rising
edge of EN/SET enables the IC and initially sets the
output LED current to zero. The next lowest setting
is equal to 525µA. Once the final clock cycle is input
for the desired brightness level, the EN/SET pin is
held high to maintain the device output current at
the programmed level. The device is disabled
500µs after the EN/SET pin transitions to a logic low
state. The EN/SET timing is designed to accommo-
date a wide range of data rates. After the first rising
edge of EN/SET, the charge pump is enabled and
reaches full capacity after the soft-start time (T
SS
).
During the soft-start time, multiple clock pulses may
be entered on the EN/SET pin to set the final output
current level with a single burst of clocks.
Alternatively, the EN/SET clock pulses may be
entered one at a time to gradually increase the LED
brightness over any desired time period. A constant
current is sourced as long as EN/SET remains in a
logic high state. The current source outputs are
switched off after EN/SET has remained in a low
state for at least the T
OFF
timeout period.
Code
1
2
3
4
5
6
7
8
I
OUT
, Each LED (mA)
0
0.5
1.3
3.3
8.3
11.1
15
20
Table 1: Current Level Settings.
Constant Current Output Level Settings
The constant current source output amplitude for
output D1 to D3 is set via the serial interface.
Because the outputs D1 to D3 are true independent
constant current sources, the voltage observed on
any single given output will be determined by the
actual forward voltage (V
F
) for the LED being driven.
Since the output current of the AAT3120 is pro-
grammable through its simple single-wire serial
interface, no PWM (pulse width modulation) or
additional control circuitry is needed to control LED
brightness. This feature greatly reduces the burden
on a microcontroller or system IC to manage LED
or display brightness, allowing the user to "set it
and forget it."
Test Current/Channel Disable
Each channel of the output is equipped with a test
current function. A small amount of current (~2µA)
is injected into each output current source to detect
the presence of load (LED). For unused channels
that are tied to ground or load LED fail short, those
channels will be automatically disabled instead of
wasting the programmed output current.
EN/SET Serial Interface
The current source output magnitude is controlled
by the EN/SET serial data single-wire interface.
The interface records rising edges of the EN/SET
EN/SET Timing
t
HI
t
LO
t
OFF
EN/SET
Code
OFF
1
2
3
OFF
8
3120.2005.11.1.1
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
