AAT1154
1MHz 3A Step-Down DC/DC Converter
Figure 2: AAT1154 Fixed Output
Top Side Layout.
Figure 3: AAT1154 Fixed Output
Bottom Side Layout.
Layout
2
P = IRMS · RDS(ON)
ON
Figures 2 and 3 display the suggested PCB layout
for the fixed output AAT1154. The following guide-
lines should be used to help ensure a proper layout.
RDS(ON) losses
I 2
1. The connection from the input capacitor to the
Schottky anode should be as short as possible.
2. The input capacitor should connect as closely
as possible to VP (Pins 5 and 8) and GND
(Pin 2).
3. C1, L1, and CR1 should be connected as
closely as possible. The connection from
the cathode of the Schottky to the LX node
should be as short as possible.
4. The feedback trace (Pin 1) should be separate
from any power trace and connect as closely
as possible to the load point. Sensing along a
high-current load trace can degrade DC load
regulation.
5. The resistance of the trace from the load return
to GND (Pin 2) should be kept to a minimum.
This will help to minimize any error in DC regu-
lation due to differences in the potential of the
internal reference ground and the load return.
6. R1 and C3 are required in order to provide
a cleaner power source for the AAT1154 con-
trol circuitry.
Δ
12
⎛
⎝
⎞
⎠
2
IO
IRMS
=
+
·D
Internal switch RMS current
D is the duty cycle and VF is the forward drop of the
Schottky diode.
VO + VF
D =
V +VF
IN
ΔI is the peak-to-peak inductor ripple current.
A simplified form of calculating the RDS(ON) and
switching losses is given by:
2
IO
R
VO
DS(ON)
·
P =
+ tSW · FS · IO + IQ · VIN
V
IN
where IQ is the AAT1154 quiescent current.
Once the total losses have been determined, the
junction temperature can be derived. The thermal
resistance (ΘJA) for the SOP-8 package mounted
on an FR4 printed circuit board in still air is
110°C/W.
Thermal
The losses associated with the AAT1154 output
switching MOSFET are due to switching losses
and conduction losses. The conduction losses are
associated with the RDS(ON) characteristics of the
output switching device. At the full load condition,
assuming continuous conduction mode (CCM), an
accurate calculation of the RDS(ON) losses can be
derived from the following equations:
TJ = P · ΘJA + TAMB
TAMB is the maximum ambient temperature and TJ
is the resultant maximum junction temperature.
12
1154.2006.09.1.6
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
