AAT1157 AAT, AAT1157 Datasheet - Page 10

• Manufacturer Part Number: AAT1157
• Description: DC/DC Converter
• Manufacturer: AAT

Where I
Once the total losses have been determined, the
junction temperature can be derived from the θ
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
for the QFN package with no connection to the cen-
tral paddle is 50°C/W. The actual θ
the number and type of vias. The PCB board size,
number of board layers, and ground plane charac-
teristics also influence the θ
nection from the paddle to the PCB ground plane
layers can significantly reduce θ
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
10
P =
+ (t
I
O
sw
2
⋅ (R
Q
⋅ F ⋅ I
is the AAT1157 quiescent current.
DSON(HS)
V
IN
O
2.7V to 5.5V
⋅ V
T
IN
J
⋅ V
= P · Θ
+ I
22µF
C1
O
Q
V
) ⋅ V
+ R
IN
JA
DSON(LS)
IN
JA
+ T
100
R1
. A good thermal con-
C2
0.1µF
AMB
JA
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.
.
⋅ (V
JA
12
11
10
IN
will vary with
7
9
6
8
5
- V
VP
VP
VP
EN
VCC
N/C
N/C
SGND
U1
AAT1157
O
))
PGND
PGND
PGND
JA
OUT
N/C
LX
LX
LX
for
JA
4
15
14
13
16
3
2
1
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.
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.
R3 =
R3
59.0k
Voltage
Output
Table 1: Suggested Component Values.
(V)
0.8
1.0
1.2
1.5
1.8
2.5
3.3
R2
267k
3.0µH
⎛
⎝
L1
V
V
REF
O
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
-1 · R4 =
(µH)
⎞
⎠
L1
Q1
Si2302ADS
MBRM120L
D1
⎛
⎝
(C3-C4) (µF)
V
2.5V
0.6V
Capacitor
O
3.3V/600mA
Output
3x 22
2x 22
2x 22
2x 22
2x 22
2x 22
22
- 1 · 59kΩ = 187kΩ
C3,C4
2x 22µF
⎞
⎠
AAT1157
1157.2005.11.1.4
R4 = 59kΩ Ω
R3 for
(kΩ Ω )
19.6
39.2
88.7
187
267
118
59