AAT3104 Datasheet, PDF (10/15 Pages) Advanced Analogic Technologies – Low Cost 1x/2x 4 Channel Charge Pump WLED Driver

ChargePump^TM
PRODUCT DATASHEET
AAT3104178
Low Cost 1x/2x 4 Channel Charge Pump WLED Driver
30.0
22.5
15.0
7.5
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
S²C Wire Interface Data Code
Figure 2: AAT3104-1 Current Control Profile.
Applications Information
LED Selection
The AAT3104 is specifically designed for driving white
LEDs. However, the device design will allow the AAT3104
to drive most types of LEDs with forward voltage speci-
fications ranging from 2.2V to 4.7V. LED applications
may include mixed arrangements for display backlight-
ing, keypad display, and any other application needing 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
the specific LED forward voltage (V_F) levels.
The low dropout current sources in the AAT3104 maxi-
mize 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.
All unused LED source pins should be connected to GND
or left floating. Do not connect to the OUT pin.
Device Switching Noise Performance
The AAT3104 operates at a fixed frequency of approxi-
mately 1MHz to control noise and limit harmonics that
can interfere with the RF operation of mobile communi-
cation 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 AAT3104
soft-start feature prevents noise transient effects associ-
ated with inrush currents during start-up of the charge
pump circuit.
30
25
20
15
10
5
0 _{1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31}
S²C Wire Interface Data Code
Figure 3: AAT3104-2 Current Control Profile.
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/SET input for the AAT3104-1/2 should be strobed
low. After t_OFF (500μs), AAT3104 will be shut down and
typically draws less than 1μA from the input. Registers
are reset to 0 in shutdown.
Power Efficiency and Device Evaluation
The charge pump efficiency discussion in the following
sections accounts only for efficiency of the charge pump
section itself. Due to the unique circuit architecture and
design of the AAT3104, it is very difficult to measure
efficiency in terms of a percent value comparing input
power over output power.
Since the AAT3104 outputs are pure constant current
sources and typically drive individual loads, it is difficult
to measure the output voltage for a given output to
derive an overall output power measurement. For any
given application, white LED forward voltage levels can
differ, yet the output drive current will be maintained as
a constant.
This makes quantifying output power a difficult task
when taken in the context of comparing to other white
LED driver circuit topologies. A better way to quantify
total device efficiency is to observe the total input power
to the device for a given LED current drive level. The
best white LED driver for a given application should be
based on trade-offs of size, external component count,
reliability, operating range, and total energy usage, not
just output power over input power efficiency.
The AAT3104 efficiency may be quantified under very
specific conditions and is dependent upon the input volt-
age versus the output voltage across the loads applied
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