AAT3143ITP-T1 View Datasheet(PDF) - Advanced Analogic Technologies
Part Name
Description
MFG CO.
AAT3143ITP-T1
Advanced Analogic Technologies
'AAT3143ITP-T1' PDF : 10 Pages View PDF
PRODUCT DATASHEET
AAT3143
ChargePumpM 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 corresponding
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:
ILED = DC · 20mA
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 recom-
mended that users choose an adequate PWM frequency
exceeding 50Hz.
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 specifications ranging
from 2.0V to 4.7V. LED applications may include main
and sub-LCD display backlighting, camera photo-flash
applications, infrared (IR) diodes for remotes, and other
loads benefiting from a controlled output current gener-
ated from a varying input voltage. Since the D1 to D4
input current sources 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 applica-
tions such as photo flash), it may be necessary to drive
high-VF type LEDs. The low dropout current sources in
the AAT3143 make it capable of driving LEDs with for-
ward 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 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 performance. 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 100mW. 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 levels, then capacitor size
may be decreased.
Capacitor Characteristics
Ceramic composition capacitors are highly recommended
over all other types of capacitors for use with the
AAT3143. Ceramic capacitors offer many advantages
over their tantalum and aluminum electrolytic counter-
parts. 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 capaci-
tors are non-polarized, they are not prone to incorrect
connection 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 connec-
tions, size or area, material composition, and ambient
temperature. Capacitor ESR is typically measured in mil-
liohms for ceramic capacitors and can range to more
than several ohms for tantalum 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 gen-
erally have tight tolerance and are very stable over tem-
perature. Larger capacitor values 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 capac-
itors greater than 1µF are not typically required for
AAT3143 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.
Thermal Protection
The AAT3143 has a built-in thermal protection circuit
that will shut down the charge pump if the die tempera-
ture rises above the thermal limit, as is the case during
a short-circuit of the CP pin.
3143.2009.06.1.1
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