AAT3123/23A/24
High Efficiency 1X/1.5X Fractional Charge
Pump for White LED Applications
tured display modules which are pre-wired with
is higher. If adequate voltage is available to drive
the higher voltage LED (of output D1 or D5) without
the charge pump running, the IC automatically
switches into step-down (1X) mode to maximize
efficiency.
white LED backlights connected in a parallel circuit
configuration. Any combination of outputs may be
connected in parallel to drive groups of LEDs. The
AAT3123 internal current source reference circuit
bases feedback from current sensed on the D1
output. The AAT3124 internal current source refer-
ence circuit bases feedback from current sensed
on the D1 and D5 outputs. For best operation, the
only requirement for this type of application is the
output D1 (and/or D5 for the AAT3124) should
always be connected to the load circuit.
Similarly, if a 4V photo-flash LED array is connect-
ed to outputs D1 through D4 (with the outputs
shorted together) and two 3.3V sub-LCD-display
backlight LEDs are connected to outputs D5 and
D6, then the AAT3124 can optimally drive each set
of LEDs at the programmed current level (see
application schematics). The photo flash can be
disabled by an N-channel switch connected in
series with the photo-flash LED.
The AAT3124 may be used to drive multiple LEDs
having differing forward voltages. Using feedback
techniques, the current in D1 to D4 output current
sources are referenced to the current in the LED
connected to D1. In the AAT3124 (six-output ver-
sion), the D5 and D6 output current sources are
referenced to the current in D5, not to D1. If all six
LEDs are of similar type, the diodes will be
matched in current, maintaining uniform LED
brightness despite variations in manufacturer, pro-
duction, etc.
The AAT3123/23A/24 has only one programmed
value of current and does not allow for separate
on/off or brightness control of each current source
output. This limitation can easily be circumvented
by introducing grounded source N-channel MOS-
FET switches in series with the LEDs to turn any
given LED (or bank of LEDs) on or off. The various
LEDs can be turned on and off independently,
simultaneously, or multiplexed to produce any vari-
ety of lighting effects. By reprogramming the cur-
rent source (via the serial interface) between
switching transitions, the brightness of individual
LEDs or banks of LEDs can also be controlled.
However, if the diodes are dramatically different in
type comprising a mix of high-VF and low-VF LEDs,
the AAT3124 has the capability to optimally and
simultaneously drive up to four LEDs of one type
and up to two LEDs of another type. Such a feature
can be useful for driving different color LEDs; driv-
ing both display backlight and photo-flash LEDs; or
for driving main-LCD and sub-LCD display LED
backlights from a single charge pump IC.
Charge Pump Compatibility
The four-output AAT3123 is pin compatible with the
AAT3113 in both QFN and TSOPJW packages.
The six-output AAT3124 is pin compatible with the
AAT3114 in the QFN package. Compared to the
AAT3113/14, the AAT3123/23A/24 offers an
improved overall efficiency, wider operating range,
and the ability to drive high-VF LEDs (up to 4.3V) at
full current from a 3V input condition. The
AAT3123/23A/24 is well suited for battery-powered
applications using single-cell lithium-ion (li-ion) bat-
teries (4.2V to 2.8V), lithium-polymer batteries, and
3-series connected dry cells (3.6V).
For example, when driving independent RGB LEDs,
the green and blue LEDs typically require a high VF
to operate (e.g., 3.7V), while the red LED needs a
low forward voltage (e.g., 2V). By connecting the
green and blue diodes to outputs D1 to D4 and the
red diodes to D5 and D6, good control and unifor-
mity in brightness are maintained despite the 2V dif-
ference in the diode forward voltages. The AAT3124
determines if the 1.5X charge pump circuit is need-
ed based on the voltage on D1 and D5, whichever
12
3123.2005.12.1.4
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
