AAT1121
1.5MHz, 250mA Step-Down Converter
Step-Down Converter Design Example
Specifications
VO
VIN
FS
= 1.8V @ 250mA, Pulsed Load ΔILOAD = 200mA
= 2.7V to 4.2V (3.6V nominal)
= 1.5MHz
TAMB = 85°C
1.8V Output Inductor
µsec
µsec
⋅ 1.8V = 3µH
A
(use 3.3μH; see Table 1)
L1 = 1.67
⋅ VO2 = 1.67
A
For Sumida inductor CDRH2D09-3R0, 3.3μH, DCR = 150mΩ.
⎛
⎞
⎠
VO
L1 ⋅ F
VO
VIN
1.8
V
1.8V
4.2V
⎛
⋅ 1 -
⎝
⎞
⎠
ΔIL1 =
=
⋅ 1 -
= 228mA
⎝
3.0µH ⋅ 1.5MHz
ΔIL1
2
IPKL1 = IO +
= 250mA + 114mA = 364mA
2
PL1 = IO ⋅ DCR = 250mA2 ⋅ 150mΩ = 9.375mW
1.8V Output Capacitor
VDROOP = 0.1V
3 · ΔILOAD
VDROOP · FS
3 · 0.2A
COUT
=
=
= 4µF; use 4.7µF
0.1V · 1.5MHz
(VO) · (VIN(MAX) - VO)
L1 · FS · VIN(MAX)
1
1.8V · (4.2V - 1.8V)
1
·
= 66mArms
IRMS
=
·
=
3.0µH · 1.5MHz · 4.2V
2· 3
2· 3
Pesr = esr · IRMS2 = 5mΩ · (66mA)2 = 21.8µW
1121.2006.04.1.0
15
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