AAT2557
500mA Battery Charger and 300mA
LDO Regulator for Portable Systems
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.
Linear Regulator Output Capacitor
For proper load voltage regulation and operational
stability, a capacitor is required between OUT and
GND. The COUT capacitor connection to the LDO
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 COUT. In low output current applications,
where output load is less than 10mA, the minimum
value for COUT can be as low as 0.47µF.
(PD(MAX)
-
VIN
VIN - VBAT
· IOP)
ICH(MAX)
=
(TJ(MAX)
θJA
VIN - VBAT
- TA)
-
VIN · IOP
ICH(MAX)
=
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.
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.
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 CIN. There is no specific capacitor ESR
requirement for CIN. However, for 300mA LDO reg-
ulator output operation, ceramic capacitors are rec-
ommended for CIN due to their inherent capability
over tantalum capacitors to withstand input current
surges from low impedance sources such as bat-
teries in portable devices.
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
CBYP may range from 470pF to 10nF. For lowest
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.
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 CIN capacitor in the
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.
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
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2557.2007.06.1.0
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