PRODUCT DATASHEET
AAT3105178
ChargePump
TM
Low Cost 1x/2x 4 Channel Charge Pump WLED Driver
Application Information
LED Selection
The AAT3105 is specifically designed for driving white
LEDs. However, the device design will allow the AAT3105
to drive most types of LEDs with forward voltage
specifications ranging from 2.2V to 4.7V. LED applications
may include mixed arrangements for display backlighting,
keypad display, and any other application that demands
a constant current source generated from a varying
input voltage. Since the D1 to D4 constant current
sources are matched with negligible supply voltage
dependence, the constant current channels will be
matched regardless of their respective LED forward
voltage (V
F
) levels.
The low dropout current sources in the AAT3105
maximize performance and make it capable of driving
LEDs with high forward voltages. Multiple channels can
be combined to obtain a higher LED drive current without
complication.
Any unused channel should be connected to GND; the
device will detect and shut down the channel. Do not
connect any unused channel to the OUT pin.
unchanged. By feeding the EN/PWM pin with a proper
PWM signal, the RMS current of current source outputs
is proportional to the duty ratio of the PWM signal with
very little error, which is introduced timing delay. If EN/
PWM is kept low for longer than 1ms, the device will
enter shutdown mode. In shutdown mode, the device
draws less than 1μA current and all current source out-
puts are inactive.
The relationship between PWM duty ratio and LED cur-
rent per channel is shown in Table 1. The PWM signal
should meet the timing requirements shown in the
Electrical Characteristics section of this datasheet in
order to guarantee proper operation and accuracy. For
example, if the minimum PWM duty ratio is designed as
10%, then the PWM frequency should be higher than
6Khz to meet the minimum limit of the EN/PWM input off
time, which is 150μs.
EN/PWM Duty Cycle
(F
PWM
= 25Khz)
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
D1-D4 Current (mA)
29.9
26.7
23.8
20.8
17.9
15
12
9.1
6.2
3.2
Device Switching Noise Performance
The AAT3105 operates at a fixed frequency of approxi-
mately 0.85MHz to control noise and limit harmonics
that can interfere with the RF operation of mobile com-
munication devices. Back-injected noise appearing on
the input pin of the charge pump is 20mV peak-to peak,
typically ten times less than inductor-based DC/DC boost
converter white LED backlight solutions. The AAT3105
soft-start feature prevents noise transient effects associ-
ated with inrush currents during start-up of the charge
pump circuit.
Table 1: PWM Duty Cycle vs. LED Current.
LED Current vs. PWM Duty Ratio
(F
PWM
= 25KHz)
30
Current per Channel (mA)
24
18
Shutdown
Since the current switches are the only power returns for
all loads, there is no leakage current when all source
switches are disabled. To enter shutdown operation, the
EN/PWM input for the AAT3105 should be strobed low.
After t
OFF
(500μs), the AAT3105 will be shut down and
typically draws less than 1μA from the input. Registers
are reset to 0 in shutdown.
12
6
0
10
20
30
40
50
60
70
80
90
100
Duty Ratio (%)
Figure 1: PWM Duty Cycle vs. LED Current.
3105.2008.10.1.0
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