AAT3113ITP ANALOGICTECH [Advanced Analogic Technologies], AAT3113ITP Datasheet - Page 11

• Manufacturer Part Number: AAT3113ITP
• Description: High Efficiency 1.5X Fractional Charge Pumps For White LED Applications
• Manufacturer: ANALOGICTECH [Advanced Analogic Technologies]

Capacitor Selection
Careful selection of the four external capacitors C
C
time, output ripple, and transient performance. Optimum
performance will be obtained when low equivalent series
resistance (ESR) ceramic capacitors are used. In gen-
eral, 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 Characteristics
Ceramic composition capacitors are highly recommend-
ed over all other types of capacitors for use with the
AAT3113 and AAT3114/14A products. Ceramic capaci-
tors offer many advantages over their tantalum and
aluminum electrolytic 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 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-
3113.2008.05.1.10
ChargePump
2
, and C
Figure 3: AAT3113/AAT3114 Input Power
700
600
500
400
300
200
100
0
OUT
0
is important because they will affect turn-on
V
IN
20
= 3.6V
Output (LED) Current (mA)
TM
vs. LED Current
40
High Efficiency 1.5X Fractional Charge Pumps For White LED Applications
60
80
100
120
w w w . a n a l o g i c t e c h . c o m
140
IN
, C
1
,
AAT3113, AAT3114/14A
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
generally have tight tolerance and are very stable over
temperature. Larger capacitor values are usually com-
posed 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
AAT3113 or AAT3114/14A 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.
Test Current/Channel Disable
Each channel of the output is equipped with a test cur-
rent function. The AAT3113 and AAT3114 use a small
(~2µA) current source injected into each output pin to
detect the presence of an LED. Unused channels other
than channel 1 should be left open and will be automati-
cally diasbled instead of wasting the programmed output
current. The test current in the AAT3114A is higher
(~150µA) to accommodate LEDs with lower impedance
in failure mode.
Thermal Protection
The AAT3113 and AAT3114/14A have a thermal protec-
tion circuit that will shut down the charge pump and
current outputs if the die temperature rises above the
thermal limit. However, thermal resistance of the QFN
package is so low that if, in the case of the AAT3114/14A,
all six outputs are shorted to ground at maximum 20mA
output level, the die temperature will not rise sufficient-
ly to trip the thermal protection. The thermal protection
will only trip if C
temperature is high.
OUT
is shorted to ground and the ambient
PRODUCT DATASHEET
11