AAT2603_08 Datasheet, PDF (17/26 Pages) Advanced Analogic Technologies – Total Power Solution for Portable Applications

PRODUCT DATASHEET
AAT2603
Total Power Solution for Portable Applications
It was designed to maintain over 90% efficiency at its
maximum rated output current load of 1.2A with a 3.3V
output. Peak efficiency is above 95%. Buck1 has excel-
lent transient response, load and line regulation. Transient
response time is typically less than 20μs. The peak input
current is limited to 1.7A.
DC-DC2 (Buck2) is a 600mA step-down regulator designed
to dynamically shift between two output voltages by tog-
gling the SELB2 pin. The internal reference voltage of the
buck regulator is changed based on the position of the
SELB2 pin.
Buck2 is designed to maintain over 85% efficiency at its
maximum rated output current of 600mA with a 1.2V
output. Peak efficiency is above 90%. Buck2 has excel-
lent transient response, load and line regulation. The
peak inductor current is limited to 1.3A.
The two step-down converters on the AAT2603 have
highly flexible output voltage programming capability.
The output voltages can be factory programmed to pre-
set output voltages or set by external resistors. The
“Part Number Descriptions” table lists the available volt-
age options for step-down converters Buck1 and Buck2.
Option 1 has externally adjustable output voltages for
both step-down converters. The dynamic voltage scaling
for Buck2 is still useable with external feedback resis-
tors. When SELB2 is in the low position the feedback
voltage is compared to a 600mV reference, while when
SELB2 is high the reference voltage is 775mV. For most
other options, the output voltages of Buck2 are factory
programmed.
LDO Regulators
The AAT2603 includes four LDO regulators. The regula-
tors operate from the 2.7V to 5.5V input voltage to a
regulated output voltage. The LDO regulators have
adjustable output voltages set by resistors. Each LDO
consumes 50uA of quiescent current.
The two 200mA LDO regulators are stable with a small
4.7μF ceramic output capacitor. The low 200mV dropout
voltage at 200mA load allows a regulated output voltage
approaching the input voltage. Low output noise voltage
and high power supply rejection make these regulators
ideal for powering noise sensitive circuitry.
The two 400mA LDO regulators are stable with a small
4.7μF ceramic output capacitor. The low 300mV dropout
voltage at 400mA load allows a regulated output voltage
approaching the input voltage. These LDOs offer high
power supply rejection.
Application Information
DC-DC1/DC-DC2
The step-down converter uses peak current mode con-
trol with slope compensation to maintain stability for
duty cycles greater than 50%. The output inductor value
must be selected so the inductor current down slope
meets the internal slope compensation requirements.
Table 1 displays suggested inductor values for various
output voltages.
Manufacturer’s specifications list both the inductor DC
current rating, which is a thermal limitation, and the
peak current rating, which is determined by the satura-
tion characteristics. The inductor should not show any
appreciable saturation under normal load conditions.
Some inductors may meet the peak and average current
ratings yet result in excessive losses due to a high DCR.
Always consider the losses associated with the DCR and
its effect on the total converter efficiency when selecting
an inductor.
It is recommended that the inductor current rating
exceed the current limit of the step-down converter. See
Table 2 for example inductor values/vendors.
Input Capacitor
Select a 4.7μF to 10μF X7R or X5R ceramic capacitor for
the input; see Table 3 for suggested capacitor compo-
nents. To estimate the required input capacitor size,
determine the acceptable input ripple level (V_PP) and
solve for C_IN (C_INB1/C_INB2). The calculated value varies
with input voltage and is a maximum when V_IN is double
the output voltage.
C_IN =
V_{O · 1 -} V_O
V_IN
V_IN
V_PP
I_O
- ESR
· F_S
V_{O · 1 -} V_O
V_IN
V_IN
=
1
4
for
V_IN
=
2
·
V_O
C_IN(MIN) =
1
V_PP
I_O
- ESR
· 4 · F_S
Always examine the ceramic capacitor DC voltage coef-
ficient characteristics when selecting the proper value.
For example, the capacitance of a 10μF, 6.3V, X5R ceram-
ic capacitor with 5.0V DC applied is actually about 6μF.
2603.2008.10.1.2
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