AAT2120 Datasheet, PDF (15/20 Pages) Advanced Analogic Technologies – 500mA Low Noise Step-Down Converter

AAT2120
500mA Low Noise Step-Down Converter
Step-Down Converter Design Example
Specifications
V_O = 1.8V @ 250mA, Pulsed Load ΔI_LOAD = 200mA
V_IN = 2.7V to 4.2V (3.6V nominal)
F_S
= 1.8MHz
T_AMB = 85°C
1.8V Output Inductor
µsec
µsec
L1 = 1.67 _A ⋅ V_O2 = 1.67 _A ⋅ 1.8V = 3µH (use 3.0µH; see Table 1)
For Sumida inductor CDRH2D09-3R0, 3.0µH, DCR = 150mΩ.
ΔI_L1 =
V_O
L1 ⋅ F_S
⋅
⎛⎝₁
-
V_O⎞
V_IN⎠
=
1.8V
3.0µH ⋅ 1.8MHz
⋅
⎛
⎝
1
-
1.8V⎞
4.2V⎠
= 190mA
I_PKL1
=
I_O
+
ΔI_L1
2
=
250mA
+
95mA
=
345mA
P_L1 = I_O2 ⋅ DCR = 250mA² ⋅ 150mΩ = 9.375mW
1.8V Output Capacitor
V_DROOP = 0.1V
C_OUT
=
3 · ΔI_LOAD
V_DROOP · F_S
=
3 · 0.2A
0.1V · 1.8MHz
= 3.3µF (use 3.3µF)
I_RMS
=
1 _· (V_O) · (V_IN(MAX) - V_O)
2· 3
L1 · F_S · V_IN(MAX)
=¹
2·
·
3
1.8V · (4.2V - 1.8V) _{= 66mArms}
3.0µH · 1.8MHz · 4.2V
P_esr = esr · I_RMS2 = 5mΩ · (66mA)² = 21.8µW
2120.2007.10.1.1
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