AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Thermal
Design Example
Losses associated with the AAT1155 output switch-
ing MOSFET are due to switching losses and con-
duction losses. The conduction losses are associ-
ated with the RDS(ON) characteristics of the output
switching device. At the full load condition, assum-
ing continuous conduction mode (CCM), an accu-
rate calculation of the RDS(ON) losses can be
derived from the following equations:
(see Figures 2 and 5 for reference)
IOUT
IRIPPLE
VOUT
VIN
2.5A
40% of Full Load at Max VIN
2.5V
5V 5%
1MHz
70°C
FS
2
P = IRMS · RDS(ON)
TMAX
ON
RDS(ON) losses
Inductor Selection
VOUT
V
V
⎛
⎞
OUT
∆I 2
12
L =
· 1-
⎛
⎝
⎞
⎠
2
IO
⎝
⎠
I
k F
O · ·
IRMS
=
+
·D
IN
3.3V
3.3V
⎛
⎞
⎠
=
· 1-
= 1.23µH
⎝
Internal switch RMS current
2.5A · 0.4 · 1MHz
5.25V
D is the duty cycle and VF is the forward voltage
drop of the Schottky diode.
Use standard value of 1.5µH
Sumida inductor Series CDRH6D38.
VO + VF
D =
V +VF
IN
V
V
O
⎛
⎝
⎞
⎠
O
∆I =
1-
=
·
L F
V
IN
∆I is the peak-to-peak inductor ripple current.
3.3V
3.3V
⎛
⎝
⎞
= 0.82A
A simplified form of calculating the RDS(ON) and
switching losses is given by:
1-
⎠
5.25V
1.5µH 1MHz
·
∆I
2
2
IPK = IOUT
+
=
IO
R
V
DS(ON) o
·
P =
+ t F I + I
SW · ·
V
·
Q IN
O
V
IN
2.5A + 0.41 = 2.91A
where IQ is the AAT1155 quiescent current.
Efficiency vs. Load Current
(VIN = 5.0V; VOUT = 3.3V)
Once the total losses have been determined, the
junction temperature can be derived. The thermal
resistance (ΘJA) for the MSOP-8 package mounted
on an FR4 printed circuit board in still air is 150°C/W.
100
95
90
85
80
75
70
65
60
TJ = P QJA + TAMB
TAMB is the maximum ambient temperature and TJ
is the resultant maximum junction temperature.
0.01
0.1
1
10
Output Current (A)
Figure 5: 5V Input, 3.3V Output.
1155.2005.11.1.6
13
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