AAT1140 Datasheet, PDF (16/19 Pages) Advanced Analogic Technologies – Fast Transient 600mA Step-Down Converter

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PRODUCT DATASHEET
AAT1140
Fast Transient 600mA Step-Down Converter
Step-Down Converter Design Example
Specifications
V_O =
V_IN =
F_S =
T_AMB =
1.8V @ 600mA (adjustable using 0.6V version), Pulsed Load ΔI_LOAD = 300mA
2.7V to 4.2V (3.6V nominal)
1.4MHz
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 = 105mΩ.
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
=
0.6A
+
0.068A
=
0.668A
P_L1 = I_O2 ⋅ DCR = 0.6A² ⋅ 105mΩ = 38mW
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
⎝ 0.6A
= 4.87µF; use 4.7µF
- 5mΩ⎞⎠ · 4 · 1.4MHz
I_RMS =
I_O
2
= 0.3Arms
P = esr · I_RMS2 = 5mΩ · (0.3A)² = 0.45mW
16
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1140.2007.12.1.1