AAT1157
1MHz 1.2A Buck DC/DC Converter
I
O2
⋅ (R
DSON(HS)
⋅
V
O
+ R
DSON(LS)
⋅ (V
IN
- V
O
))
P=
V
IN
+ (t
sw
⋅
F
⋅
I
O
⋅
V
IN
+ I
Q
)
⋅
V
IN
Where I
Q
is the AAT1157 quiescent current.
Once the total losses have been determined, the
junction temperature can be derived from the
θ
JA
for
the QFN package. Close attention should be paid to
the proper layout for the QFN package. Proper size
and placement of thermal routing vias below the
central paddle is necessary for good heat transfer to
other PCB layers and their ground planes. The
θ
JA
for the QFN package with no connection to the cen-
tral paddle is 50°C/W. The actual
θ
JA
will vary with
the number and type of vias. The PCB board size,
number of board layers, and ground plane charac-
teristics also influence the
θ
JA
. A good thermal con-
nection from the paddle to the PCB ground plane
layers can significantly reduce
θ
JA
.
T
J
=
P ·
Θ
JA
+ T
AMB
59kΩ. Values higher than this can cause stability
problems, while lower values can degrade light
load efficiency. For a 2.5V output with R4 set to
59kΩ, R3 is 187kΩ.
⎛
V
O
⎞
⎛
2.5V
⎞
R3 = V -1
·
R4 = 0.6V - 1
·
59kΩ = 187kΩ
⎝
REF
⎠
⎝
⎠
Table 1: Suggested Component Values.
Output
Voltage
(V)
0.8
1.0
1.2
1.5
1.8
2.5
3.3
L1
(µH)
1.5
1.5
2.2
2.2
3.0
3.0
2.2
-
-
-
-
-
-
-
2.6
3.3
3.3
4.7
4.7
4.7
4.7
Output
Capacitor
(C3-C4) (µF)
3x 22
2x 22
2x 22
2x 22
2x 22
2x 22
22
R3 for
Ω
R4 = 59kΩ
Ω
(kΩ)
19.6
39.2
59
88.7
118
187
267
Buck-Boost Output
Adjustable Output
Resistors R3 and R4, as shown in Figure 1, force
the output to regulate higher than the 0.6V refer-
ence voltage level. The optimum value for R4 is
Figure 4 shows how to configure the AAT1157 in a
buck boost configuration with an external MOSFET
and Schottky diode. The converter has a 3.3V
600mA output with an input voltage ranging from
2.7V to 5.5V.
V
IN
2.7V to 5.5V
12
U1
AAT1157
VP
VP
VP
EN
VCC
N/C
N/C
SGND
OUT
LX
LX
LX
N/C
PGND
PGND
PGND
4
R2
267k
15
V
O
3.3V/600mA
R1
100
11
L1
3.0µH
D1
MBRM120L
Q1
Si2302ADS
C3,C4
2x 22µF
10
14
7
13
C1
22µF
C2
0.1µF
9
16
6
3
R3
59.0k
8
2
5
1
L1 Sumida CDRH5D28-3R0
C1 Murata 22µF 10V X7R 1210 GRM32ER71A226KE20L
C3,C4 MuRata 22µF 6.3V X5R 0805 GRM21BR60J226ME39L
Figure 4: AAT1157 Buck Boost Converter.
10
1157.2005.11.1.4
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
