AAT1146 Datasheet, PDF (16/20 Pages) Advanced Analogic Technologies – Fast Transient 400mA Step-Down Converter

DATA SHEET
AAT1146
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
μsec
L1 = 3 _A ⋅ V_O2 = 3 _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
⋅
⎛⎝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 _· (V_O) · (V_IN(MAX) - V_O) ₌ 1
2· 3
L1 · F · V_IN(MAX)
2·
·
3
1.8V · (4.2V - 1.8V) _{= 45mArms}
4.7μH · 1.4MHz · 4.2V
P_esr = esr · I_RMS2 = 5mΩ · (45mA)² = 10μW
Input Capacitor
Input Ripple V_PP = 25mV
1
1
C_IN = _{⎛ VPP}
⎝ I_O
- ESR^⎞⎠ · 4 · F_S
=
⎛ 25mV
⎝ 0.4A
= 3.11μF; use 4.7μF
- 5mΩ⎞⎠ · 4 · 1.4MHz
I_RMS =
I_O
2
= 0.2Arms
P = esr · I_RMS2 = 5mΩ · (0.2A)² = 0.2mW
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201985B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013