AAT1157
1MHz 1.2A Buck DC/DC Converter
Design Example
Specifications
I
OUT
V
OUT
V
IN
F
S
T
AMB
1.2A
I
RIPPLE
330mA
2.5V
3.0V to 4.2V
1MHz
= 85°C
Maximum Input Capacitor Ripple:
I
RMS
= I
O
·
V
⎛
V
⎞
O
· 1 -
O
= 0.59Arms
V
IN
⎝
V
IN
⎠
P = esr
·
I
RMS2
= 5mΩ
·
0.59
2
A = 1.7mW
Inductor Selection:
L=
⎛
V
⎞
V
OUT
2.5V
2.5V
⎞
⎛
⋅
1 -
OUT
=
⋅
1-
= 3.07µH
∆I
PP
⋅
F
⎝
V
IN
⎠
0.33A
⋅
1MHz
⎝
4.2V
⎠
Select Sumida inductor CDRH5D28 3.0µH.
⎛
2.5V
⎞
V
O
⎛
V
⎞
2.5V
⋅
1-
O
=
⋅
⎝
1-
= 340mA
L
⋅
F
⎝
V
IN
⎠
3.0µH
⋅
1MHz
4.2V
⎠
∆I
=
I
PK
= I
OUT
+
∆I
= 1.2A + 0.17A = 1.37A
2
P = I
O2
⋅
DCR = (1.2A)
2
⋅
31mΩ = 45mW
Output Capacitor Ripple Current:
I
RMS
=
(V
OUT
) · (V
IN
- V
OUT
)
1
2.5V · (4.2V - 2.5V)
·
= 97.4mArms
=
L · F · V
IN
2 · 3 3.0µH · 1MHz · 4.2V
2· 3
1
·
Pesr = esr · I
RMS2
= 5mΩ · (97.4mA)
2
= 47.4µW
1157.2005.11.1.4
11
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
