AAT2845
Four-Channel Backlight Driver with Dual LDOs
cuitry is needed to control LED brightness. This
becomes active and the internal data register is
reset to zero. For subsequent current level pro-
gramming, the number of rising edges correspon-
ding to the desired code must be applied on the
EN/SET pin. When the EN/SET pin is held low for
an amount of time longer than TOFF (500µs), the
AAT2845 enters shutdown mode and draws less
than 1µA from the input. An internal data register is
reset to zero during shutdown.
feature greatly reduces the burden on a microcon-
troller or system IC to manage LED or display
brightness, allowing the user to "set it and forget it."
With its high-speed serial interface (>1MHz data
rate), the LED current drive can be changed suc-
cessively to brighten or dim LEDs in smooth transi-
tions (e.g., to fade out) or in abrupt steps, giving the
user complete programmability and real-time con-
trol of LED brightness.
The last four codes of the current level scale
engage a reduced quiescent current mode that
enhances the low-current setting efficiency. This
mode is especially useful for low-current applica-
tions where a continuous, low-current state is
maintained for a substantial length of time.
Auto-Disable Feature
The charge pump in the AAT2845 is equipped with
an auto-disable feature for each LED channel. After
the IC is enabled and started up, a test current of
100µA (typical) is forced through each sink chan-
nel. The channel will be disabled if the voltage of
that particular DX pin does not drop to a certain
threshold. This feature is convenient for disabling
an unused channel or during an LED fail-short
event.
S2Cwire Serial Interface
The current sink magnitude is controlled by
AnalogicTech's S2Cwire serial digital input. The
interface records rising edges of the EN/SET pin
and decodes them into 16 different states. The 16
current level settings available are indicated in
Table 1.
Low Dropout Regulators
The AAT2845 includes two LDO linear regulators.
The regulators run from the same 2.7V to 5.5V input
voltage as the charge pump. The regulators use a
single on/off control input, ENLDO. The LDO output
voltages are set through a resistive voltage divider
from the output (OUTA or OUTB) to the feedback
input (FBA or FBB). The ratio of resistor values
determines the LDO output voltage. The low 200mV
dropout voltage at 200mA load current allows the
regulator to maintain output voltage regulation.
Output
(mA/Ch)
Output
(mA/Ch)
Data
Data
1
2
3
4
5
6
7
8
20
9
6.0
4.2
18.9
18.0
17.0
15.7
14.8
10.2
8.0
10
11
12
13
14
15
16
2.9
2.0
1.0
0.53
0.10
0.05
Each LDO regulator can supply a continuous load
current up to 200mA. Both LDOs include current
limiting and thermal overload protection to prevent
damage to the load or to the LDOs.
Table 1: Current Level Settings.
The S2Cwire serial interface has flexible timing.
Data can be clocked-in at speeds higher than
1MHz, or much slower, such as 15kHz. After data
is applied, EN/SET is held high to latch the data.
Once EN/SET has been held in the logic high state
for time TLAT (500µs), the programmed current
Thermal Protection
The charge pump has a built-in thermal protection
circuit that will shut down the charge pump and the
LDOs if the die temperature rises above the ther-
mal limit, as is the case during a short-circuit of the
OUT pin.
2845.2006.12.1.0
11
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
