AAT1149 Datasheet, PDF (16/20 Pages) Advanced Analogic Technologies – 3MHz Fast Transient 400mA Step-Down Converter

AAT1149
3MHz Fast Transient
400mA Step-Down Converter
Step-Down Converter Design Example
Specifications
V_O = 1.8V @ 400mA (adjustable using 0.6V version), Pulsed Load ΔI_LOAD = 300mA
V_IN = 2.7V to 4.2V (3.6V nominal)
F_S
= 3.0MHz
T_AMB = 85°C
1.8V Output Inductor
μsec
μsec
L1 = 1 _A ⋅ V_O = 1 _A ⋅ 1.8V = 1.8μH _{(use 2.2μH; see Table 1)}
For Taiyo Yuden inductor CBC2518T2R2M, 2.2μH, DCR = 130mΩ.
ΔI_L1 =
V_O
L1 ⋅ F_S
⋅
⎛⎝₁
-
V_O ⎞
V_IN ⎠
=
1.8V
2.2μH ⋅ 3.0MHz
⋅
⎛
⎝^{1 -}
1.8V⎞
4.2V⎠
= 156mA
I_PKL1
=
I_O
+
ΔIL1
2
=
0.4A
+
0.078A
=
0.478A
P_L1 = I_O2 ⋅ DCR = 0.4A² ⋅ 130mΩ = 21mW
1.8V Output Capacitor
V_DROOP = 0.1V
C_OUT
=
3 · ΔI_LOAD
V_DROOP · F_S
=
3 · 0.3A
0.1V · 3.0MHz
= 3.0μF; use 4.7µF
I_RMS
=
1 _· (V_O) · (V_IN(MAX) - V_O) ₌ 1 _·
2· 3 ^{L1 · F}S ^{· V}IN(MAX)
2· 3
1.8V · (4.2V - 1.8V) _{= 45mArms}
2.2μH · 3.0MHz · 4.2V
P_esr = esr · I_RMS2 = 5mΩ · (45mA)² = 10μW
16
1149.2006.11.1.0