AAT2503
Adjustable 3-Channel Regulator
Applications utilizing the exceptionally low output
noise and optimum power supply ripple rejection
characteristics of the AAT2503 should use 2.2µF or
greater for COUT. If desired, COUT may be increased
without limit. In low output current applications
where output load is less than 10mA, the minimum
value for COUT can be as low as 0.47µF.
temperature; this could cause problems for circuit
operation. X7R and X5R dielectrics are much more
desirable. The temperature tolerance of X7R dielec-
tric is better than 15ꢀ. Capacitor area is another
contributor to ESR. Capacitors which are physically
large in size will have a lower ESR when compared
to a smaller sized capacitor of an equivalent materi-
al and capacitance value. These larger devices can
improve circuit transient response when compared
to an equal value capacitor in a smaller package
size. Consult capacitor vendor datasheets carefully
when selecting capacitors for LDO regulators.
Capacitor Characteristics
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT2503. Ceramic capacitors offer many
advantages over their tantalum and aluminum elec-
trolytic counterparts. A ceramic capacitor typically
has very low ESR, is lower cost, has a smaller PCB
footprint, and is non-polarized. Line and load tran-
sient response of the LDO regulator is improved by
using low ESR ceramic capacitors. Since ceramic
capacitors are non-polarized, they are not prone to
incorrect connection damage.
Adjustable Output Resistor Selection
The output voltage on the linear regulator is pro-
grammed with external resistors: R4 and R7 for
LDOA and R5 and R8 for LDOB. Table 3 summa-
rizes the resistor values for various output volt-
ages with R4 and R5 set to either 59kΩ for good
noise immunity or 221kΩ for reduced no load input
current.
Equivalent Series Resistance
R7, R8 = 59kΩ R7, R8 = 221kΩ
LDO
ESR is a very important characteristic to consider
when selecting a capacitor. ESR is the internal
series resistance associated with a capacitor that
includes lead resistance, internal connections, size
and area, material composition, and ambient tem-
perature. Typically, capacitor ESR is measured in
milliohms for ceramic capacitors and can range to
more than several ohms for tantalum or aluminum
electrolytic capacitors.
VOUT (V)
R4, R5 (kΩ)
R4, R5 (kΩ)
0.6*
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
3.3
0
0
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
75
113
150
187
221
261
301
332
442
464
523
715
1000
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 composed of X7R, X5R, Z5U, or Y5V
dielectric materials. These two material types are
not recommended for use with LDO regulators since
the capacitor tolerance can vary more than 50ꢀ
over the operating temperature range of the device.
A 2.2µF Y5V capacitor could be reduced to 1µF over
Table 3: LDO Linear Regulators Resistor
Values for Various Output Voltages.
* For the 0.6V output, R7 and R8 are open.
2503.2007.04.1.1
17
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
