AAT2500B Datasheet, PDF (15/21 Pages) Advanced Analogic Technologies – Adjustable 3-Channel Regulator

SystemPower^TM
Output Capacitor
For proper load voltage regulation and operational sta-
bility, a capacitor is required between pins OUTA, OUTB,
and GND. The C_OUT capacitor connection to the LDO
regulator ground pin should be made as direct as practi-
cally possible for maximum device performance. The
AAT2500B has been specifically designed to function
with very low ESR ceramic capacitors. For best perfor-
mance, ceramic capacitors are recommended.
Typical output capacitor values for maximum output cur-
rent conditions range from 1μF to 10μF.
Applications utilizing the exceptionally low output noise
and optimum power supply ripple rejection characteris-
tics of the AAT2500B should use 2.2μF or greater for
C_OUT. If desired, C_OUT may be increased without limit. In
low output current applications where output load is less
than 10mA, the minimum value for C_OUT can be as low as
0.47μF.
Capacitor Characteristics
Ceramic composition capacitors are highly recommend-
ed over all other types of capacitors for use with the
AAT2500B. Ceramic capacitors offer many advantages
over their tantalum and aluminum electrolytic counter-
parts. A ceramic capacitor typically has very low ESR, is
lower cost, has a smaller PCB footprint, and is non-
polarized. Line and load transient response of the LDO
regulator is improved by using low ESR ceramic capaci-
tors. Since ceramic capacitors are non-polarized, they
are not prone to incorrect connection damage.
Equivalent Series Resistance
ESR is a very important characteristic to consider when
selecting a capacitor. ESR is the internal series resis-
tance associated with a capacitor that includes lead
resistance, internal connections, size and area, material
composition, and ambient temperature. Typically, capac-
itor ESR is measured in milliohms 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
PRODUCT DATASHEET
AAT2500B
Adjustable 3-Channel Regulator
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.
NPO and C0G material types are not recommended for
use with LDO regulators since the capacitor tolerance
can vary more than ±50% over the operating tempera-
ture range of the device. A 2.2μF Y5V capacitor could be
reduced to 1μF over temperature; this could cause prob-
lems for circuit operation. X7R and X5R dielectrics are
much more desirable. The temperature tolerance of X7R
dielectric is better than ±15%. Capacitor area is anoth-
er 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 material 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.
Adjustable Output Resistor Selection
The output voltage on the linear regulator is programmed
with external resistors: R4 and R7 for LDOA and R5 and
R8 for LDOB. Table 3 summarizes the resistor values for
various output voltages with R4 and R5 set to either
59kΩ for good noise immunity or 221kΩ for reduced no
load input current.
LDO
V_OUT (V)
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
R7, R8 = 59kΩ
R4, R5 (kΩ)
0
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
R7, R8 = 221kΩ
R4, R5 (kΩ)
0
75
113
150
187
221
261
301
332
442
464
523
715
1000
Table 3: LDO Linear Regulators Resistor Values
for Various Output Voltages.
* For the 0.6V output, R7 and R8 are open.
2500B.2008.03.1.0
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