AAT3175
High Efficiency 1X/1.5X/2X Charge Pump
For White LED Flash Applications
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 the AAT3175 is specifically intended for
driving white LEDs, the device can also be used to
drive most types of LEDs with forward voltage
specifications ranging from 2.0V to 4.1V. LED
applications may include display backlighting, cam-
era photo-flash applications, color (RGB) LEDs,
infrared (IR) diodes for remotes, and other loads
benefiting from a controlled output current generat-
ed from a varying input voltage. Since the D1 to D4
input current sinks are matched with negligible volt-
age dependence, the LED brightness will be
matched regardless of the specific LED forward
voltage (VF) levels. In some instances (e.g., in high
luminous output applications such as photo flash),
it may be necessary to drive high-VF type LEDs.
The low dropout current sinks in the AAT3175
make it capable of driving LEDs with forward volt-
ages as high as 4.1V at full current from an input
supply as low as 3.5V. 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 AAT3175. 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 composition,
and ambient temperature. Capacitor ESR is typi-
cally measured in milliohms for ceramic capacitors
and can range to more than several ohms for tanta-
lum or aluminum electrolytic capacitors.
Device Switching Noise Performance
The AAT3175 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 AAT3175 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 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
AAT3175 applications.
Capacitor Selection
Careful selection of the four external capacitors
CIN, C1, C2, and 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; generally, low ESR
may be defined as less than 100mΩ. A value of
1µF for all four capacitors is a good starting point
Capacitor area is another contributor to ESR.
Capacitors that are physically large will have a
lower ESR when compared to an smaller capacitor
made of equivalent material. These larger devices
can improve circuit transient response when com-
pared to an equal value capacitor in a smaller
package size.
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