AAT1171
600mA Voltage-Scaling Step-Down Converter
for RF Power Amplifiers with Bypass Switch
PA Step-Down Converter Design Example
Specifications
VO(BUCK) 0.6V to 3.4V with RL =10Ω
VIN
2.7V to 4.2V (3.6V nominal)
FS
2.0MHz
85°C
TAMB
Output Inductor
L1 = 2.2µH
For Copper Electronics SD3112, 2.2µH, DCR = 140mΩ.
⎛
⎞
⎠
VO
L ⋅ FS
VO
VIN
2.1
V
2.1V
4.2V
⎛
⎞
⎠
ΔIL1(MAX)
=
⋅ 1 -
⎝
=
⋅ 1 -
= 239mA
⎝
2.2µH ⋅ 2.0MHz
The maximum inductor ripple current occurs at 50% duty cycle at the maximum input voltage.
ΔI
IPKL1 = IO + L1(MAX) = 0.6A + 0.118A = 0.718A
2
PL1 = IO2 ⋅ DCR = 0.6A2 ⋅ 140mΩ = 50mW
Output Capacitor
Specify that VDROOP = 0.2V for a 600mA load pulse.
3 · ΔILOAD
VDROOP · FS
3 · 0.6A
COUT
=
=
= 4.5µF
0.2V · 2.0MHz
(VO) · (VIN(MAX) - VO)
L1 · FS · VIN(MAX)
1
3.4V · (4.2V - 3.4V)
1
·
= 69mArms
IRMS
=
·
=
4.7µH · 2.0MHz · 4.2V
2· 3
2· 3
PESR = ESR · IRMS2 = 5mΩ · (69mA)2 = 24µW
1171.2006.06.1.0
19
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