AAT3157 Datasheet, PDF (10/11 Pages) Advanced Analogic Technologies – High Efficiency 1X/1.5X/2X Charge Pump For White LED Applications

AAT3157
High Efficiency 1X/1.5X/2X Charge Pump
For White LED Applications
Applications Information
Although the AAT3157 is designed for driving white
LEDs, the device also can be used to drive most
types of LEDs with forward voltage specifications
ranging from 2.0V to 4.7V. LED applications may
include main and sub-LCD display backlighting,
camera photo-flash applications, color (RGB)
LEDs, infrared (IR) diodes for remotes, and other
loads benefiting from a controlled output current
generated from a varying input voltage. Since the
D1 to D3 input current sinks are matched with neg-
ligible 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 necessary to drive high-V_F type LEDs.
The low dropout current sinks in the AAT3157
make it capable of driving LEDs with forward volt-
ages as high as 4.7V 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 AAT3157 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 AAT3157 soft-start feature prevents
noise transient effects associated with inrush cur-
rents during start-up of the charge pump circuit.
Capacitor Selection
Careful selection of the four external capacitors C_IN,
C₁, C₂, and C_OUT is important because they will
affect turn-on time, output ripple, and transient per-
formance. Optimum performance will be obtained
when low equivalent series resistance (ESR)
ceramic capacitors are used. In general, low ESR
may be defined as less than 100mΩ. A value of
1µF for all four capacitors is a good starting point
when choosing capacitors. If the LED current
sources are programmed only for light current lev-
els, then the capacitor size may be decreased.
Capacitor Characteristics
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT3157. Ceramic capacitors offer many
advantages over their tantalum and aluminum
electrolytic counterparts. A ceramic capacitor typi-
cally has very low ESR, is lowest cost, has a small-
er PCB footprint, and is non-polarized. Low ESR
ceramic capacitors help to maximize charge pump
transient response. Since ceramic capacitors are
non-polarized, they are not prone to incorrect con-
nection damage.
Equivalent Series Resistance
ESR is an important characteristic to consider when
selecting a capacitor. ESR is a resistance internal
to a capacitor that is caused by the leads, internal
connections, size or area, material composition,
and ambient temperature. Capacitor ESR is typi-
cally measured in milliohms for ceramic capacitors
and can range to more than several ohms for tan-
talum or aluminum electrolytic capacitors.
Ceramic Capacitor Materials
Ceramic capacitors less than 0.1µF are typically
made from NPO or C0G materials. NPO and C0G
materials generally have tight tolerance and are
very stable over temperature. Larger capacitor val-
ues are usually composed of X7R, X5R, Z5U, or
Y5V dielectric materials. Large ceramic capacitors
(i.e., greater than 2.2µF) are often available in low-
cost Y5V and Z5U dielectrics, but capacitors
greater than 1µF are not typically required for
AAT3157 applications.
Capacitor area is another contributor to ESR.
Capacitors that are physically large will have a lower
ESR when compared to an equivalent material
smaller capacitor. These larger devices can improve
circuit transient response when compared to an
equal value capacitor in a smaller package size.
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