AAT2687 Datasheet, PDF (12/20 Pages) Advanced Analogic Technologies – PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters

DATA SHEET
AAT2687
PMIC Solution for 24V Systems
with 2 High Performance Step-Down Converters
Over-Current Protection
The output 1 controller provides true-load DC output cur-
rent sensing which protects the load and limits compo-
nent stresses. The output current is sensed through the
DC resistance in the output inductor (DCR). The control-
ler reduces the operating frequency when an over-current
condition is detected; limiting stresses and preventing
inductor saturation. This allows the smallest possible
inductor for a given output load. A small resistor divider
may be necessary to adjust the over-current threshold
and compensate for variation in inductor DCR.
The preset current limit threshold is triggered when the
differential voltage from RS1 to OS1 exceeds 100mV
(nominal).
V _OUT1
L1
3.3V/4.5A
LX1
4.7μH
Channel 2 Input Capacitor
Typically, a 1μF or larger capacitor is recommended for
C_IN in most applications. A C_IN capacitor is not required
for basic LDO regulator operation. However, if the
AAT2687 is physically located more than three centime-
ters from an input power source, a C_IN capacitor will be
needed for stable operation.
C_IN should be located as close to the device V_IN pin as
possible. C_IN 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 C_IN. There is no specific capacitor
ESR requirement for C_IN. However, for 150mA LDO regu-
lator output operation, ceramic capacitors are recom-
mended for C_IN due to their inherent capability over
tantalum capacitors to withstand input current surges
from low impedance sources, such as batteries in por-
table devices.
R2
2k
RS1
OS1
C4
220nF
R7
R8
Figure 2: Optional Resistor Network to Adjust the
Current Limit Less than the Pre-Set Over-Current
Threshold (Add R7 and R8).
V_OUT1
L1
3.3V/4.5A
LX1
4.7μH
R2
C4
2k
220nF
RS1
R6
R7
OS1
Figure 3: Optional Resistor Network to Adjust the
Current Limit Greater than the Pre-Set Over-
Current Level (Add R6 and R7).
Channel 2 Output Capacitor
For proper load voltage regulation and operational stabil-
ity, a capacitor is required between pins V_OUT and GND.
The C_OUT capacitor connection to the LDO regulator
ground pin should be connected as close as possible for
maximum device performance. The AAT2687 LDO has
been specifically designed to function with very low ESR
ceramic capacitors. For best performance, ceramic
capacitors are recommended.
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 C_OUT. If desired, C_OUT
may be increased without limit. In low output current
applications where output load is less than 10mA, the
minimum value for C_OUT can be as low as 0.47μF.
Channel 2 Enable Function
The AAT2687 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 V_IN to keep the LDO regulator in a con-
tinuously on state. When the LDO regulator is in shut-
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202033A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 8, 2012