AAT2120_08 Datasheet, PDF (15/19 Pages) Advanced Analogic Technologies – 500mA Low Noise Step-Down Converter

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PRODUCT DATASHEET
AAT2120
500mA Low Noise Step-Down Converter
Step-Down Converter Design Example
Specifications
V_O = 1.8V @ 250mA, Pulsed Load ∆I_LOAD = 200mA
V_IN = 2.7V to 4.2V (3.6V nominal)
F_S = 1.8MHz
T_AMB = 85°C
1.8V Output Inductor
L1
=
µs
1.67 _A
⋅ V_O2
=
µs
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) ₌ 1
2· 3
L1 · F_S · V_IN(MAX) 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
Input Capacitor
Input Ripple V_PP = 25mV
1
1
C_IN = _{⎛ VPP}
⎝ I_O
- ESR^⎞⎠ · 4 · F_S
=
⎛ 25mV
⎝ 0.2A
= 1.16µF (use 4.7µF)
- 5mΩ⎞⎠ · 4 · 1.8MHz
I_RMS =
I_O
2
= 0.1Arms
P = esr · I_RMS2 = 5mΩ · (0.1A)² = 0.05mW
2120.2008.02.1.2
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