AAT1157
1MHz 1.2A Buck DC/DC Converter
Design Example
Specifications
IOUT
1.2A
IRIPPLE 330mA
VOUT 2.5V
VIN
FS
3.0V to 4.2V
1MHz
TAMB = 85°C
Maximum Input Capacitor Ripple:
V
V
O
⎛
⎞
⎠
O
IRMS = IO ·
· 1-
= 0.59Arms
⎝
V
V
IN
IN
P = esr · IRMS2 = 5mΩ · 0.592 A = 1.7mW
Inductor Selection:
VOUT
∆IPP ⋅ F
⎛
⋅ 1 -
⎝
VOUT
⎞
2.5
V
2.5V
⎞
4.2V
⎠
⎛
⎝
L =
=
⋅
1 -
= 3.07µH
⎠
VIN
0.33A ⋅ 1MHz
Select Sumida inductor CDRH5D28 3.0µH.
⎛
⎞
2.5V
⎠
4.2V
VO
L ⋅ F
VO
VIN
2.5
V
⎛
⎞
⎠
∆I =
⋅ 1 -
⎝
=
⋅ 1-
= 340mA
⎝
3.0µH ⋅ 1MHz
∆I
2
IPK = IOUT
+
= 1.2A + 0.17A = 1.37A
P = IO2 ⋅ DCR = (1.2A)2 ⋅ 31mΩ = 45mW
Output Capacitor Ripple Current:
(VOUT) · (VIN - VOUT
)
1
2.5V · (4.2V - 2.5V)
1
·
= 97.4mArms
IRMS
=
·
=
3.0µH · 1MHz · 4.2V
L·F·V
2· 3
2· 3
IN
Pesr = esr · IRMS2 = 5mΩ · (97.4mA)2 = 47.4µW
1157.2005.11.1.4
11
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
