AAT2504_08 Datasheet, PDF (15/21 Pages) Advanced Analogic Technologies – Adjustable 3-Channel Regulator

SystemPower^TM
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 AAT2504 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 recommended
over all other types of capacitors for use with the
AAT2504. 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 resistance
associated with a capacitor that includes lead resistance,
internal connections, size and area, material composi-
tion, and ambient temperature. Typically, capacitor ESR
is measured in milliohms for ceramic capacitors and can
range to more than several ohms for tantalum or alumi-
num 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. 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 temperature
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
PRODUCT DATASHEET
AAT2504178
Adjustable 3-Channel Regulator
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.
POK Output
LDOA of the AAT2504 features an integrated Power OK
comparator which can be used as an error flag. The POK
open drain output goes low when output voltage is 10%
(typ) below its nominal regulation voltage. Additionally,
any time LDOA is in shutdown, the POK output is pulled
low. Connect a pull-up resistor from POK to OUTA.
*For the 0.6V output, R7 and R8 are open.
2504.2008.02.1.2
www.analogictech.com
15