AAT3192_08 Datasheet, PDF (12/18 Pages) Advanced Analogic Technologies – 2-Channel Charge-Pump LED Driver

ChargePump^TM
Applications Information
LED Selection
The AAT3192 is specifically intended for driving white
LEDs. However, the device design will allow the AAT3192
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 sink generated from a varying input
voltage. Since the D1 to D2 constant current sinks 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 sinks in the AAT3192 maximize
performance and make it capable of driving LEDs with
high forward voltages. Multiple channels can be com-
bined to obtain a higher LED drive current without com-
plication.
Constant Current Setting
The LED current is controlled by the R_SET resistor. For
maximum accuracy, a 1% tolerance resistor is recom-
mended. Table 5 shows R_SET resistor values for
AAT3192-1/2/4 and Table 6 shows R_SET resistor values
for AAT3192-3 for various LED full-scale current levels.
I_LED (mA)
30
25
20
18
15
10
R_SET (KΩ)
9.53
11.5
14.3
15.8
19.1
28.7
Table 5: Maximum LED Current and R_SET Resistor
Values (1% Resistor Tolerance)
for the AAT3192-1/2/4.
I_LED (mA)
30
20
15
10
R_SET (KΩ)
7.68
11.5
15.4
23.2
Table 6: Maximum LED Current and R_SET Resistor
Values (1% Resistor Tolerance)
for the AAT3192-3.
PRODUCT DATASHEET
AAT3192
2-Channel Charge-Pump LED Driver
Device Switching Noise Performance
The AAT3192 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 AAT3192
soft-start feature prevents noise transient effects associ-
ated with inrush currents during start-up of the charge
pump circuit.
Shutdown
Since the current switches are the only power returns for
all loads, there is no leakage current when all sink
switches are disabled. To activate the shutdown opera-
tion, the EN/SET input for the AAT3192-1/2/3 should be
strobed low for longer than t_OFF (500μs). For the 3192-4
PWM options, shutdown operation is enabled when the
EN/PWM input is strobed low longer than 1ms. In this
state, the AAT3192 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 AAT3192, it is very difficult to measure
efficiency in terms of a percent value comparing input
power over output power.
Since the AAT3192 outputs are pure constant current
sinks 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 % efficiency.
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