AAT2556 Datasheet, PDF (24/29 Pages) Advanced Analogic Technologies – Battery Charger and Step-Down Converter for Portable Applications

AAT2556
Battery Charger and Step-Down
Converter for Portable Applications
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
= 1.5MHz
T_AMB = 85°C
1.8V Output Inductor
L1
=
µsec
1.67 _A
⋅ V_O2
=
µsec
1.67 _A
⋅ 1.8V
=
3µH
(use 3.0µH; see Table 3)
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 ⋅ 1.5MHz
⋅
⎛
⎝
1
-
1.8V
4.2V
⎞
⎠
= 228mA
I_PKL1
=
I_O
+
ΔIL1
2
=
250mA
+
114mA
=
364mA
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 · 1.5MHz
= 4µF; 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) _{= 66mArms}
3.0µH · 1.5MHz · 4.2V
P_esr = esr · I_RMS2 = 5mΩ · (66mA)² = 21.8µW
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2556.2006.09.1.2