AAT3158
High Efficiency 1X/1.5X/2X Charge Pump
for White LED Applications
1µF for all four capacitors is a good starting point
when choosing capacitors. If the LED current
sources are only programmed for light current lev-
els, then the capacitor size may be decreased.
Applications Information
LED Selection
Although AAT3158 is specifically intended for driv-
ing white LEDs, the device can also 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 dis-
play 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 D4 input current sinks are matched
with negligible voltage dependence, the LED
brightness will be matched regardless of the spe-
cific LED forward voltage (VF) levels. In some
instances (e.g., in high-luminous-output applica-
tions such as photo flash), it may be necessary to
drive high-VF type LEDs. The low dropout current
sinks in the AAT3158 make it capable of driving
LEDs with forward voltages 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.
Capacitor Characteristics
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT3158. Ceramic capacitors offer many
advantages over their tantalum and aluminum elec-
trolytic counterparts. A ceramic capacitor typically
has very low ESR, is lowest cost, has a smaller
PCB footprint, and is non-polarized. Low ESR
ceramic capacitors help 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 compo-
sition, and ambient temperature. Capacitor ESR is
typically measured in milliohms for ceramic capac-
itors and can range to more than several ohms for
tantalum or aluminum electrolytic capacitors.
Device Switching Noise Performance
The AAT3158 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 AAT3158 soft-start feature prevents
noise transient effects associated with inrush cur-
rents during start-up of the charge pump circuit.
Ceramic Capacitor Materials
Ceramic capacitors less than 0.1µF are typically
made from NPO or C0G materials. NPO and C0G
materials typically have tight tolerance and are sta-
ble over temperature. Larger capacitor values are
typically composed of X7R, X5R, Z5U, or Y5V
dielectric materials. Large ceramic capacitors, typ-
ically greater than 2.2µF, are often available in low-
cost Y5V and Z5U dielectrics, but capacitors
greater than 1µF are typically not required for
AAT3158 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 larg-
er devices can improve circuit transient response
when compared to an equal value capacitor in a
smaller package size.
Capacitor Selection
Careful selection of the four external capacitors
(CIN, C1, C2, COUT) 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
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3158.2006.02.1.0
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