PRODUCT DATASHEET
AAT2688
TM
SystemPower
4.5A PMIC Solution for 12V Adapter Systems with 2-Output High Performance Step-Down Converters
Over-Current Protection
Channel 2 Input Capacitor
The regulator provides true-load DC output current sens-
ing which protects the load and limits component stress-
es. The output current is sensed through the DC resis-
tance in the output inductor. The regulator reduces the
operating frequency when an over-current condition is
detected; limiting stresses and preventing inductor satu-
ration. This allows the smallest possible inductor for the
given application. A small resistor divider may be neces-
sary to adjust the over-current threshold and compen-
sate for variation in inductor DC resistance.
Typically, a 1μF or larger capacitor is recommended for
CIN in most applications. A CIN capacitor is not required
for basic LDO regulator operation. However, if the
AAT2688 is physically located more than three centime-
ters from an input power source, a CIN capacitor will be
needed for stable operation.
CIN should be located as close to the device VIN pin as
possible. CIN values greater than 1μF will offer superior
input line transient response and will assist in maximiz-
ing the highest possible power supply ripple rejection.
Ceramic, tantalum, or aluminum electrolytic capacitors
may be selected for CIN. There is no specific capacitor
ESR requirement for CIN. However, for 150mA LDO regu-
lator output operation, ceramic capacitors are recom-
mended for CIN due to their inherent capability over
tantalum capacitors to withstand input current surges
from low impedance sources, such as batteries in por-
table devices.
VOUT1
3.3V/4.5A
L1
LX1
4.7μH
C4
220nF
R2
2k
RS1
OS1
Channel 2 Output Capacitor
R7
R8
For proper load voltage regulation and operational stabil-
ity, a capacitor is required between pins VOUT and GND.
The COUT capacitor connection to the LDO regulator
ground pin should be connected as close as possible for
maximum device performance. The AAT2688 LDO has
been specifically designed to function with very low ESR
ceramic capacitors. For best performance, ceramic
capacitors are recommended.
Figure 2: Resistor Network to Adjust the Current
Limit Less than the Pre-Set Over-Current
Threshold (Add R7, R8).
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 characteristics of the chan-
nel 2 should use 2.2μF or greater for COUT. If desired, COUT
may be increased without limit. In low output current
applications where output load is less than 10mA, the
minimum value for COUT can be as low as 0.47μF.
VOUT1
3.3V/4.5A
L1
LX1
4.7μH
C4
220nF
R6
R2
2k
Channel 2 Enable Function
RS1
OS1
The AAT2688 features an LDO regulator enable/disable
function. This pin (EN) is active high and is compatible
with CMOS logic. To assure the LDO regulator will switch
on, the EN turn-on control level must be greater than
1.5V. The LDO regulator will go into the disable shutdown
mode when the voltage on the EN pin falls below 0.6V. If
the enable function is not needed in a specific application,
it may be tied to VIN to keep the LDO regulator in a con-
tinuously on state. When the LDO regulator is in shut-
R7
Figure 3: Resistor Network to Adjust the Current
Limit Greater than the Pre-Set Over-Current Level
(Add R6, R7).
w w w . a n a l o g i c t e c h . c o m
2688.2008.06.1.0
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