High Efficiency 1X/1.5X/2X Charge Pump with
PWM Control for White LED Applications
Applications Information
EN/PWM Dimming Control
LED brightness is controlled with the EN/PWM pin.
By driving the pin with a PWM signal, a correspon-
ding pulse-width modulated current will be driven
through the LEDs. In this way, the duty cycle sets
the LED brightness level. The resulting average
current that flows through the LED is calculated as
follows:
AAT3143
Capacitor Selection
Careful selection of the four external capacitors
C
IN
, C
1
, C
2
, 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 only programmed for light current lev-
els, then capacitor size may be decreased.
I
LED
= DC
·
20mA
Capacitor Characteristics
The EN/PWM pin can be driven with a wide range
of PWM frequencies. Because of the short turn-on
delay during high frequency PWM, a frequency as
high as 50kHz can be used. A low PWM frequency
can also be used without complication. One should
consider that below 50Hz, the human eye can
begin to see LED flicker, so it is recommended that
users choose an adequate PWM frequency
exceeding 50Hz.
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT3143. 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.
LED Selection
Although the AAT3143 is specifically intended for
driving white LEDs, the device can also be used to
drive most types of LEDs with forward voltage spec-
ifications ranging from 2.0V to 4.7V. LED applica-
tions may include main and sub-LCD display back-
lighting, camera photo-flash applications, 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 cur-
rent 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
necessary to drive high-V
F
type LEDs. The low
dropout current sources in the AAT3143 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.
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.
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
AAT3143 applications.
3143.2006.05.1.0
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