AAT2500M
400mA Step-Down Converter and 300mA LDO
Thermal Calculations
PCB Layout
There are three types of losses associated with the
AAT2500M step-down converter: switching losses,
conduction losses, and quiescent current losses.
Conduction losses are associated with the RDS(ON)
characteristics of the power output switching
devices. Switching losses are dominated by the
gate charge of the power output switching devices.
At full load, assuming continuous conduction mode
(CCM), a simplified form of the step-down convert-
er and LDO losses is given by:
The following guidelines should be used to ensure
a proper layout.
1. The input capacitor C2 should connect as
closely as possible to IN_BUCK and PGND, as
shown in Figure 5.
2. The output capacitor and inductor should be
connected as closely as possible. The connec-
tion of the inductor to the LX pin should also be
as short as possible.
3. The feedback trace should be separate from
any power trace and connect as closely as
possible to the load point. Sensing along a
high-current load trace will degrade DC load
regulation. If external feedback resistors are
used, they should be placed as closely as pos-
sible to the FB_BUCK pin. This prevents noise
from being coupled into the high impedance
feedback node.
4. The resistance of the trace from the load return
to GND 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 sig-
nal ground and the power ground.
IOBUCK2 · (RDSON(HS) · VOBUCK + RDSON(LS) · [VIN - VOBUCK])
PTOTAL
=
VIN
+ (tsw · FOSC · IOBUCK + IQBUCK + IQLDO) · VIN + IOLDO · (VIN - VOLDO
)
IQBUCK is the step-down converter quiescent cur-
rent and IQLDO is the LDO quiescent current. The
term tsw is used to estimate the full load step-down
converter switching losses.
For the condition where the buck converter is in
dropout at 100% duty cycle, the total device dissi-
pation reduces to:
PTOTAL = IOBUCK2 · RDSON(HS) + IOLDO · (VIN - VOLDO
+ (IQBUCK + IQLDO) · VIN
)
Since RDS(ON), quiescent current, and switching
losses all vary with input voltage, the total losses
should be investigated over the complete input
voltage range.
Given the total losses, the maximum junction tem-
perature can be derived from the θJA for the
TSOPJW-12 package which is 110°C/W.
TJ(MAX) = PTOTAL · ΘJA + TAMB
2500M.2007.06.1.0
17
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