AAT1151
850kHz 700mA Synchronous Buck DC/DC Converter
Design Example
Specifications
IOUT
IRIPPLE
VOUT
VIN
0.7A
40% of Full Load at Max VIN
1.5V
2.7V to 4.2V (3.6V nominal)
FS
850kHz
85°C
TAMB
Maximum Input Capacitor Ripple:
V
V
O
⎛
⎞
⎠
O
IRMS = IO ·
· 1-
= 0.35Arms, VIN = 2
×
VO
⎝
V
V
IN
IN
P = esr · IRMS2 = 5mΩ · 0.352 A = 0.6mW
Inductor Selection:
VOUT
IO ⋅ k ⋅ F
⎛
VOUT
⎞
1.5
V
1.5V
4.2V
⎛
⎞
⎠
L =
⋅ 1 -
⎝
=
⋅ 1 -
= 4.05µH
⎠
VIN 0.7A ⋅ 0.4 ⋅ 850kHz
⎝
Select Sumida inductor CDRH3D16 3.3µH 63mΩ 1.8mm height.
⎛
⎞
1.5V
⎠
4.2V
VO
L ⋅ F
VO
VIN
1.5
V
⎛
⎞
⎠
∆I =
⋅ 1 -
⎝
=
⋅ 1-
= 340mA
⎝
3.3µH ⋅ 850kHz
∆I
2
IPK = IOUT
+
= 0.7A + 0.17A = 0.87A
P = IO2 ⋅ DCR = (0.7)2 ⋅ 63mΩ = 31mW
Output Capacitor Ripple Current:
VOUT · (VIN - VOUT
)
1
1.5V · (4.2V - 1.5V)
1
·
= 99mArms
IRMS
=
·
=
3.3µH · 850kHz · 4.2V
L·F·V
2· 3
2· 3
IN
Pesr = esr · IRMS2 = 5mΩ · 992 mA = 50µW
1151.2005.11.1.4
13
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
