AAT1157
1MHz 1.2A Buck DC/DC Converter
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.
IO2 ⋅ (RDSON(HS) ⋅ VO + RDSON(LS) ⋅ (VIN - VO))
P =
VIN
+ (tsw ⋅ F ⋅ IO ⋅ VIN + IQ) ⋅ VIN
V
2.5V
0.6V
⎛
⎝
⎞
⎛
⎝
⎞
⎠
O
R3 =
-1 · R4 =
- 1 · 59kΩ = 187kΩ
Where IQ is the AAT1157 quiescent current.
V
⎠
REF
Once the total losses have been determined, the
junction temperature can be derived from the θJA 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 θJA
for the QFN package with no connection to the cen-
tral paddle is 50°C/W. The actual θJA 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 θJA. A good thermal con-
nection from the paddle to the PCB ground plane
layers can significantly reduce θJA.
Table 1: Suggested Component Values.
Output
Voltage
(V)
Output
Capacitor
(C3-C4) (µF)
R3 for
R4 = 59kΩ
(kΩ)
L1
(µH)
0.8
1.0
1.2
1.5
1.8
2.5
3.3
1.5 - 2.6
1.5 - 3.3
2.2 - 3.3
2.2 - 4.7
3.0 - 4.7
3.0 - 4.7
2.2 - 4.7
3x 22
2x 22
2x 22
2x 22
2x 22
2x 22
22
19.6
39.2
59
88.7
118
187
267
TJ = P · ΘJA + TAMB
Buck-Boost Output
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.
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
VIN 2.7V to 5.5V
U1
VO 3.3V/600mA
R2
AAT1157
267k
12
11
10
7
4
VP
OUT
15
14
13
16
3
L1
3.0µH
R1
100
VP
LX
D1
VP
LX
MBRM120L
EN
LX
C1
22µF
9
Q1
Si2302ADS
VCC
N/C
N/C
N/C
C3,C4
2x 22µF
R3
59.0k
6
C2
0.1µF
PGND
PGND
8
2
5
1
SGND PGND
L1 SumidaCDRH5D28-3R0
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
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