AAT1147 Datasheet, PDF (15/18 Pages) Advanced Analogic Technologies – High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter

DATA SHEET
AAT1147
High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter
Step-Down Converter Design Example
Specifications
V_O = 1.8V @ 400mA (adjustable using 0.6V version), Pulsed Load I_LOAD = 300mA
V_IN = 2.7V to 4.2V (3.6V nominal)
F_S = 1.4MHz
T_AMB = 85°C
1.8V Output Inductor
μsec
L1 = 3 _A
⋅
V_O2
=
3
μsec
A
⋅ 1.8V = 5.4μH
(use
4.7μH;
see
Table
1)
For Sumida inductor CDRH3D16, 4.7μH, DCR = 105m.
ΔI_L1 =
V_O
L1 ⋅ F_S
⋅ ^⎛⎝1 -
V_O ⎞
V_IN ⎠
=
1.8V
4.7μH ⋅ 1.4MHz
⋅
⎛⎝_{1 -}
1.8V ⎞
4.2V ⎠
= 156mA
I_PKL1
=
I_O
+
ΔI_L1
2
=
0.4A
+
0.068A
=
0.468A
P_L1 = I_O2 ⋅ DCR = 0.4A² ⋅ 105mΩ = 17mW
1.8V Output Capacitor
V_DROOP = 0.1V
C_OUT
=
3 · ΔI_LOAD
V_DROOP · F_S
=
3 · 0.3A
0.1V · 1.4MHz
= 6.4μF; use 10µF
I_RMS
=
1
2·
· ^{(VO) · (VIN(MAX) - VO)}
3 ^{L1 · F}S ^{· V}IN(MAX)
=¹
2·
·
3
1.8V · (4.2V - 1.8V) _{= 45mArms}
4.7μH · 1.4MHz · 4.2V
P_esr = esr · I_RMS2 = 5mΩ · (45mA)² = 10μW
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