AAT1145 Datasheet, PDF (14/16 Pages) Advanced Analogic Technologies – 1.2A Step-Down Converter

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PRODUCT DATASHEET
AAT1145
1.2A Step-Down Converter
Step-Down Converter Design Example
Specifications
V_O = 1.8V @1.2A
V_IN = 2.7V to 4.2V (3.6V nominal)
f_S = 1.5MHz
Transient droop = 80mV
∆V_O = 50mV
1.8V Output Inductor
ΔI_L = 30% ⋅ I_O = 0.3 · 1.2 = 360mA
L
=
V_OUT · (V_IN(MAX) - V_OUT)
V_IN(MAX) ⋅ ΔI_L ⋅ f_OSC
=
1.8 · (4.2 - 1.8)
4.2 ⋅ 0.36 · 1.5 · 10⁶
=1.90µH
For Sumida 2.2μH inductor (CDRH2D14) with DCR 75mΩ, the ∆I_L should be
ΔI_L =
V_O
L
⋅
⎛⎝₁
-
V_O
V_IN
⎞
⎠
· T = 312mA
I_PKL
=
I_O
+
ΔI_L
2
=
1.2
+
0.312
2
=
1.356A
P_L = I_O2 ⋅ DCR = 1.2² ⋅ 0.0359 = 51.7mW
1.8V Output Capacitor
C_OUT =
2 · ΔI_LOAD
V_DROOP · f_S
= ^{2 · 1.2} = 20µF; use 22µF
0.08 · 1.5 · 10⁶
ESR ≤
ΔV_O
ΔI_L
0.05
= _0.312 = 0.16Ω
Select a 22μF, 10mΩ ESR ceramic capacitor to meet the ripple 50mV requirement.
ΔV_OUT ≤
V_OUT · (V_IN - V_OUT)
V_IN · f_OSC · L
·
⎛
⎝^{ESR +}
1
⎞
8 · f_OSC · C_OUT ^⎠
1.8 · (4.2 - 1.8)
⎛
1
⎞
= _{4.2 · 1.5 · 106 · 2.2 · 10-6} · _⎝0.01 + _{8 · 1.5 · 106 · 22 · 10-6 âŽ}  = 46.8mV
I_RMS = ΔI_L ·0.289 = 0.312 · 0.289 = 90.2mArms
P_COUT = ESR · I_RMS2 = 0.01 · 0.09022 = 81.4μW
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1145.2007.11.1.1