AAT2557ITO-CT-T1 AnalogicTech, AAT2557ITO-CT-T1 Datasheet - Page 18

• Manufacturer Part Number: AAT2557ITO-CT-T1
• Description: 500mA Battery Charger and 300mA LDO Regulator for Portable Systems
• Manufacturer: AnalogicTech

By substitution, we can derive the maximum
charge current before reaching the thermal limit
condition (thermal cycling). The maximum charge
current is the key factor when designing battery
charger applications.
In general, the worst condition is the greatest volt-
age drop across the IC, when battery voltage is
charged up to the preconditioning voltage threshold.
Capacitor Selection
Linear Regulator Input Capacitor
An input capacitor greater than 1µF will offer supe-
rior input line transient response and maximize
power supply ripple rejection. Ceramic, tantalum,
or aluminum electrolytic capacitors may be select-
ed for C
requirement for C
ulator output operation, ceramic capacitors are rec-
ommended for C
over tantalum capacitors to withstand input current
surges from low impedance sources such as bat-
teries in portable devices.
Battery Charger Input Capacitor
In general, it is good design practice to place a
decoupling 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 C
range of 10µF should be used. A larger input
capacitor in this application will minimize switching
or power transient effects when the power supply is
"hot plugged" in.
18
IN
I
CH(MAX)
. There is no specific capacitor ESR
I
CH(MAX)
=
IN
IN
=
. However, for 300mA LDO reg-
(T
due to their inherent capability
(P
J(MAX)
D(MAX)
θ
JA
V
V
- T
IN
IN
- V
- V
A
- V
)
- V
BAT
IN
BAT
IN
· I
IN
capacitor in the
OP
· I
)
OP
LDO Regulator for Portable Systems
500mA Battery Charger and 300mA
Linear Regulator Output Capacitor
For proper load voltage regulation and operational
stability, a capacitor is required between OUT and
GND. The C
regulator ground pin should be made as directly as
practically possible for maximum device perform-
ance. Since the regulator has been designed to
function with very low ESR capacitors, ceramic
capacitors in the 1.0µF to 10µF range are recom-
mended for best performance. Applications utilizing
the exceptionally low output noise and optimum
power supply ripple rejection should use 2.2µF or
greater for C
where output load is less than 10mA, the minimum
value for C
Battery Charger Output Capacitor
The AAT2557 only requires a 1µF ceramic capaci-
tor 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 may
be required to prevent the device from cycling on
and off when no battery is present.
Bypass Capacitor and Low Noise Applications
A bypass capacitor pin is provided to enhance the
low noise characteristics of the AAT2557 LDO reg-
ulator. 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
C
noise and best possible power supply ripple rejec-
tion performance, a 10nF capacitor should be
used. To practically realize the highest power sup-
ply ripple rejection and lowest output noise per-
formance, 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 document for examples.
There is a relationship between the bypass capac-
itor value and the LDO regulator turn-on and turnoff
time. In applications where fast device turn-on and
BYP
may range from 470pF to 10nF. For lowest
OUT
OUT
OUT
can be as low as 0.47µF.
. In low output current applications,
capacitor connection to the LDO
AAT2557
2557.2007.06.1.0