AAT2749 Datasheet, PDF (13/17 Pages) Skyworks Solutions Inc. – 3.0MHz Step-Down Converter and Low-V in LDO

DATA SHEET
AAT2749
3.0MHz Step-Down Converter and Low-V_IN LDO
nous step-down converter and LDO losses is given by:
P_TOTAL =
I_OUT12 ·
(R_DS()ON)H ·
V_OUT1 + R_DS(ON)L ·
V_IN1
[V_IN1 -V_OUT1])
+ (t_SW · F_S · I_OUT1 + I_Q1) · V_IN1 + (V_IN2 - V_OUT2 ) · I_OUT2
I_Q1 is the step-down converter and LDO quiescent cur-
rent respectively. The term t_SW is used to estimate the
full load step-down converter switching losses.
Since R_DS(ON), quiescent current, and switching losses all
vary with input voltage, the total losses should be inves-
tigated over the complete input voltage range.
Given the total losses, the maximum junction tempera-
ture can be derived from the θ_JA for the CSP-9 package,
which is 284°C/W.
T_J(MAX) = P_TOTAL · θ_JA + T_A
Layout Considerations
The suggested PCB layout for the AAT2749 is shown in
Figures 2 and 3. The following guidelines should be used
to help ensure a proper layout.
1. The bypass capacitors (C1, C2 and C4) should con-
nect as closely as possible to input and output pin
(Pins 1, 2, and 7) and PGND (Pin 9).
2. C3 and L1 should be connected as closely as possi-
ble. The connection of L1 to the LX pin should be as
short as possible.
3. The feedback trace or OUT-BUCK pin (Pin 3) should
be separated from any power trace and connected as
closely as possible to the load point. Sensing along a
high current load trace will degrade DC load regula-
tion.
4. The resistance of the trace from the load return to
PGND (Pin 9) should be kept to a minimum. This will
help to minimize any error in DC regulation due to
differences in the potential of the internal signal
ground and the power ground.
5. The pad on the PCB for the CSP-9 package should
use NSMD (non-solder mask defined) configuration
due to its tighter control on the copper etch process.
A pad thickness of less than 1 oz. is recommended
to achieve higher stand-off. A high-density, small
footprint layout can be achieved using an inexpen-
sive, miniature, non-shielded, high DCR inductor.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
202037A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 8, 2012
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