PRODUCT DATASHEET
AAT3194
TM
ChargePump
High Efficiency 1.5X Fractional Charge Pump For White LED Applications
Equivalent Series Resistance
700
VIN = 3.6V
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.
600
500
400
300
200
100
0
0
20
40
60
80
100
Ceramic Capacitor Materials
Output (LED) Current (mA)
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
AAT3194 applications.
Figure 3: Input Power vs. LED Current.
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 gener-
al, 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 minimal current levels, then the
capacitor size may be decreased.
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.
Test Current/Channel Disable
Capacitor Characteristics
Each channel of the output is equipped with a test cur-
rent function. The AAT3194 uses a small (~2µA) current
source injected into each output pin to detect the pres-
ence of an LED. Unused channels other than channel 1
should be left open and will be automatically disabled
instead of wasting the programmed output current.
Ceramic composition capacitors are highly recommended
over all other types of capacitors for use with the
AAT3194. 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.
Thermal Protection
The AAT3194 has a thermal protection circuit that will
shut down the charge pump and current outputs if the
die temperature rises above the thermal limit.
w w w . a n a l o g i c t e c h . c o m
3194.2008.05.1.1
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