AAT3132 Datasheet, PDF (10/14 Pages) Advanced Analogic Technologies – High Efficiency 1.5X Fractional Charge Pump for White LED Applications

AAT3132/32A
High Efficiency 1.5X Fractional Charge
Pump for White LED Applications
Applications Information
Constant Current Output Level Settings
The constant current source output amplitude for
outputs D1 to D3 and D4 are set via the serial inter-
face according to a logarithmic scale depicted in
Figure 1. Using a logarithmic scale, LED bright-
ness appears linear with each increasing code
count. Because the outputs D1 to D4 are true
independent constant current sources, the voltage
observed on any single given output will be deter-
mined by the actual forward voltage (V_F) for the
LED being driven.
Since the output current of the AAT3132/32A is
programmable through its S²Cwire interface, no
PWM (pulse width modulation) or additional control
circuitry are needed to control LED brightness. This
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."
Furthermore, with its high-speed serial interface
(1MHz data rate), the output current of the
AAT3132/32A can changed successively to bright-
en or dim LEDs in smooth transitions (e.g., to fade
out) or in abrupt steps, giving the user complete
programmability and real-time control of LED
brightness.
EN/SET Serial Interface (S²Cwire)
The current source output magnitude is controlled
by the EN/SET serial data interface. The interface
records rising edges of the EN/SET pin and
decodes them into 32 individual current level set-
tings, as summarized in Table 1. The modulo 32
interface wraps back to State 1 after the 32nd
clock, so the previous state is achieved by clocking
the EN/SET pin 31 times. The counter can be
clocked at speeds up to 1MHz, so that intermediate
states are not visible. The first rising edge of
EN/SET enables the IC and initially sets the output
LED current to 0. Once the final clock cycle is input
for the desired brightness level, the EN/SET pin is
held high to maintain the device output current at
the programmed level. The device is disabled
500µs after the EN/SET pin transitions to a logic
low state. The EN/SET timing is designed to
accommodate a wide range of data rates. After the
first rising edge of EN/SET, the charge pump is
enabled and reaches full capacity after the soft-
start time (T_SS). During the soft-start time, multiple
clock pulses may be entered on the EN/SET pin to
set the final output current level with a single burst
of clocks. Alternatively, the EN/SET clock pulses
may be entered one at a time to gradually increase
LED brightness over any desired time period. A
constant current is sourced as long as EN/SET
remains in a logic high state. The current source
outputs are switched off after EN/SET has
remained in a low state for at least the T_OFF timeout
period.
LED Selection
The AAT3132/32A is specifically intended for driving
white LEDs. However, the device design will allow
the AAT3132/32A to drive most types of LEDs with
forward voltage specifications ranging from 2.0V to
4.3V. LED applications may include main and sub-
LCD display backlighting, camera photo-flash appli-
cations, color (RGB) LEDs, infrared (IR) diodes for
remotes, and other loads benefiting from a controlled
output current generated from a varying input volt-
age. Since the D1 to D4 output current sources are
matched with negligible voltage dependence, the
LED brightness will be matched regardless of the
specific LED forward voltage (V_F) levels.
In some instances (e.g., in high luminous output
applications such as photo flash), it may be neces-
sary to drive high-V_F type LEDs. The low dropout
current sources in the AAT3132/32A make it capa-
ble of driving LEDs with forward voltages as high
as 4.3V at full current from an input supply as low
as 3.0V. Outputs can be paralleled to drive high
current LEDs without complication.
Device Switching Noise Performance
The AAT3132/32A operates at a fixed frequency of
approximately 1MHz to control noise and limit har-
monics that can interfere with the RF operation of
cellular telephone handsets or other communica-
tion 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 solu-
tions. The AAT3132/32A soft-start feature prevents
noise transient effects associated with inrush cur-
rents during start-up of the charge pump circuit.
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