PRODUCT DATASHEET
AAT2861
TM
ChargePump
Backlight/Flash LED Driver and Multiple LDO Lighting Management Unit
ceramic capacitors are used. In general, low ESR may be
defined as less than 100mΩ.
These larger size capacitors can improve circuit transient
response when compared to an equal value capacitor in
a smaller package size.
Ceramic composition capacitors are highly recommended
over all other types of capacitors for use with the
AAT2861. 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 printed circuit board (PCB)
footprint, and is non-polarized. Since ceramic capacitors
are non-polarized, they are not prone to incorrect con-
nection damage.
PCB Layout
To achieve adequate electrical and thermal performance,
careful attention must be given to the PCB layout. In the
worst-case operating condition, the chip must dissipate
considerable power at full load. Adequate heat-sinking
must be achieved to ensure intended operation.
Figures 2 and 3 illustrate an example PCB layout. The
bottom of the package features an exposed metal pad-
dle. The exposed paddle acts, thermally, to transfer heat
from the chip and, electrically, as a ground connection.
Equivalent Series Resistance (ESR)
ESR is an important characteristic to consider when
selecting a capacitor. ESR is a resistance internal to a
capacitor, which 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.
The junction-to-ambient thermal resistance (θJA) for the
connection can be significantly reduced by following a
couple of important PCB design guidelines.
The PCB area directly underneath the package should be
plated so that the exposed paddle can be mated to the
top layer PCB copper during the re-flow process. Multiple
copper plated thru-holes should be used to electrically
and thermally connect the top surface paddle area to
additional ground plane(s) and/or the bottom layer
ground pour.
Ceramic Capacitor Materials
Ceramic capacitors less than 0.1μF are typically made
from NPO or COG materials. NPO and C0G materials
typically have tight tolerance and are stable over tem-
perature. Larger capacitor values are typically composed
of X7R, X5R, Z5U or Y5V dielectric materials. Large
ceramic capacitors, typically greater than 2.2μF are
often available in low cost Y5V and Z5U dielectrics, but
capacitors greater than 1μF are typically not required for
AAT2861 applications.
The chip ground is internally connected to both the
paddle and to the AGND and PGND pins. It is good prac-
tice to connect the GND pins to the exposed paddle area
with traces as shown in the example.
The flying capacitors C1 and C2 should be connected
close to the IC. Trace length should be kept short to
minimize path resistance and potential coupling. The
input and output capacitors should also be placed as
close to the chip as possible.
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.
Manufacturer
Part Number
Value
Voltage
Temp. Co.
Case
0603ZD105K
0603ZD225K
1ꢀF
2.2ꢀF
1ꢀF
2.2ꢀF
4.7ꢀF
1ꢀF
10
10
25
16
10
16
10
10
AVX
X5R
0603
C1608X5R1E105K
C1608X5R1C225K
C1608X5R1A475K
GRM188R61C105K
GRM188R61A225K
LMK107BJ475KA
TDK
X5R
0603
Murata
X5R
X5R
0603
0603
2.2ꢀF
4.7ꢀF
Taiyo Yuden
Table 13: Surface Mount Capacitors.
w w w . a n a l o g i c t e c h . c o m
22
2861.2009.02.1.2
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
