PRODUCT DATASHEET
AAT2557
TM
SystemPower
500mA Battery Charger and 300mA LDO Regulator for Portable Systems
may be required to prevent the device from cycling on
and off when no battery is present.
Battery Charger Input Capacitor
In general, it is good design practice to place a decou-
pling capacitor between the ADP pin and GND. An input
capacitor in the range of 1μF to 22μF is recommended.
If the source supply is unregulated, it may be necessary
to increase the capacitance to keep the input voltage
above the under-voltage lockout threshold during device
enable and when battery charging is initiated. If the
adapter input is to be used in a system with an external
power supply source, such as a typical AC-to-DC wall
adapter, then a CADP capacitor in the range of 10μF
should be used. A larger input capacitor in this applica-
tion will minimize switching or power transient effects
when the power supply is “hot plugged” in.
Bypass Capacitor and Low Noise Applications
A bypass capacitor pin is provided to enhance the low
noise characteristics of the AAT2557 LDO regulator. The
bypass capacitor is not necessary for operation of the
AAT2557. However, for best device performance, a small
ceramic capacitor should be placed between the bypass
pin (BYP) and the device ground pin (GND). The value of
CBYP may range from 470pF to 10nF. For lowest noise and
best possible power supply ripple rejection performance,
a 10nF capacitor should be used. To practically realize
the highest power supply ripple rejection and lowest
output noise performance, it is critical that the capacitor
connection between the BYP pin and GND pin be direct
and PCB traces should be as short as possible. Refer to
the PCB Layout Recommendations section of this docu-
ment for examples.
Linear Regulator Output Capacitor
For proper load voltage regulation and operational stabil-
ity, a capacitor is required between OUT and GND. The
COUTLDO capacitor connection to the LDO regulator ground
pin should be made as directly as practically possible for
maximum device performance. Since the regulator has
been designed to function with very low ESR capacitors,
ceramic capacitors in the 1.0μF to 10μF range are rec-
ommended for best performance. Applications utilizing
the exceptionally low output noise and optimum power
supply ripple rejection should use 2.2μF or greater for
There is a relationship between the bypass capacitor
value and the LDO regulator turn-on and turnoff time. In
applications where fast device turn-on and turn-off time
are desired, the value of CBYP should be reduced.
In applications where low noise performance and/or
ripple rejection are less of a concern, the bypass capac-
itor may be omitted. The fastest device turn-on time will
be realized when no bypass capacitor is used.
COUTLDO. In low output current applications, where output
load is less than 10mA, the minimum value for COUTLDO
can be as low as 0.47μF.
Printed Circuit Board
Layout Considerations
Battery Charger Output Capacitor
The AAT2557 only requires a 1μF ceramic capacitor on
the BAT pin to maintain circuit stability. This value should
be increased to 10μF or more if the battery connection is
made any distance from the charger output. If the
AAT2557 is to be used in applications where the battery
can be removed from the charger, such as with desktop
charging cradles, an output capacitor greater than 10μF
For the best results, it is recommended to physically
place the battery pack as close as possible to the
AAT2557 BAT pin. To minimize voltage drops on the PCB,
keep the high current carrying traces adequately wide.
The input capacitors should connect as closely as possi-
ble to ADP and INLDO.
Manufacturer
Part Number
Value (μF)
Voltage Rating
Temp. Co.
Case Size
Murata
Murata
Murata
Murata
Murata
Murata
Murata
Murata
GRM21BR61A106KE19
GRM188R60J475KE19
GRM188R61A225KE34
GRM188R60J225KE19
GRM188R61A105KA61
GRM185R60J105KE26
GRM188F51H103ZA01
GRM155F51H103ZA01
10
4.7
2.2
2.2
1.0
1.0
0.01
0.01
10
6.3
10
6.3
10
6.3
50
50
X5R
X5R
X5R
X5R
X5R
X5R
Y5V
Y5V
0805
0603
0603
0603
0603
0603
0603
0402
Table 3: Surface Mount Capacitors.
w w w . a n a l o g i c t e c h . c o m
2557.2009.07.1.2
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