AAT3193 Datasheet, PDF (14/18 Pages) Advanced Analogic Technologies – 3-Channel Charge-Pump LED Driver

ChargePump^TM
VIN
EN/SET
C2
D1 D2 D3
1μF
U1
1 _D1
D2 ¹⁰
2 _{OUT D3} 9
3 _{C- GND} 8
4 _{C+ RSET} 7
5 _{IN EN/SET} 6
C3
1μF
AAT3193
R1
14.3k
Figure 7: Lower Cost 1X Mode Application.
Capacitor Selection
Careful selection of the three external capacitors C_IN, C_P,
and C_OUT 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 100mΩ. A value of
1μF for all four capacitors is a good starting point when
choosing capacitors. If the constant current sinks are
only programmed for light current levels, then the
capacitor size may be decreased.
Capacitor Characteristics
Ceramic composition capacitors are highly recommended
over all other types of capacitors for use with the
AAT3193. Ceramic capacitors offer many advantages
over their tantalum and aluminum electrolytic counter-
parts. A ceramic capacitor typically has very low ESR, is
PRODUCT DATASHEET
AAT3193
3-Channel Charge-Pump LED Driver
lowest cost, has a smaller PCB footprint, and is non-
polarized. Low ESR ceramic capacitors help maximizing
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., larger than 2.2μF) are often
available in low cost Y5V and Z5U dielectrics, but capac-
itors larger than 1μF are not typically required for
AAT3193 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.
14
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3193.2008.07.1.3