PRODUCT DATASHEET
AAT2845
TM
ChargePump
Four-Channel Backlight Driver with Dual LDOs
Since the current level is programmable, no PWM (pulse
width modulation) or additional control circuitry is need-
ed to control LED brightness. This feature greatly reduc-
es the burden on a microcontroller or system IC to man-
age 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
successively 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 control of
LED brightness.
programming, the number of rising edges corresponding
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 inter-
nal data register is reset to zero during shutdown.
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 (typi-
cal) is forced through each sink channel. The channel will
be disabled if the voltage of that particular DX pin does
not drop to a certain threshold. This feature is conve-
nient for disabling an unused channel or during an LED
fail-short event.
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 applications where a continuous, low-cur-
rent state is maintained for a substantial length of time.
S2Cwire Serial Interface
Low Dropout Regulators
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 dif-
ferent states. The 16 current level settings available are
indicated in Table 1.
The AAT2845 product family includes two LDO linear
regulators. The regulators run from the same 2.7V to
5.5V input voltage as the charge pump and have a com-
mon ON/OFF control input, ENLDO. For the AAT2845-EE,
the LDO output voltages are set through a resistive volt-
age divider from the output (OUTLDOA or OUTLDOB) to
the feedback input (FBA or FBB). The ratio of resistor
values determines the LDO output voltage. For the
AAT2845-QG option, LDOA is internally set to 2.8V and
LDOB is internally set to 1.5V. For the AAT2845-QI option,
LDOA is also 2.8V and LDOB is internally set to 1.8V.
Output
(mA/Ch)
Output
(mA/Ch)
Data
Data
1
2
3
4
5
6
7
8
20
9
6.0
4.2
2.9
2.0
1.0
0.53
0.10
0.05
18.9
18.0
17.0
15.7
14.8
10.2
8.0
10
11
12
13
14
15
16
The low 200mV dropout voltage at 200mA load current
allows the regulator to maintain output voltage regula-
tion. Each LDO regulator can supply a continuous load
current up to 200mA, and both LDOs include current
limiting and thermal overload protection to prevent dam-
age 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 pro-
grammed current becomes active and the internal data
register is reset to zero. For subsequent current level
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 thermal limit, as is the
case during a short-circuit of the OUT pin.
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2845.2008.05.1.2
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
