AAT2158_0810 Datasheet, PDF (15/17 Pages) Advanced Analogic Technologies – 1.5A Low Noise Step-Down Converter

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PRODUCT DATASHEET
AAT2158
1.5A Low Noise Step-Down Converter
Design Example
Specifications
V_O 3.3V @ 1.4A, Pulsed Load DI_LOAD = 1.4A
V_IN 2.7V to 4.2V (3.6V nominal)
F_S 1.2MHz
T_AMB 85°C in QFN33-16 Package
Output Inductor
L1 = V_O(µH) = 3.3µH; see Table 2.
For Sumida inductor CDRH4D28 3.3µH DCR = 49.2mW max.
∆I₁ =
V_O
L1 ⋅ F_S
⋅
_{1 -}
V_O1
V_IN 
=
3.3V
3.3µH ⋅ 1.2MHz
⋅
_{1 -}
3.3V 
4.2V 
= 179mA
I_PK1
=
I_O1
+
∆I₁
2
=
1.4A
+
0.089A
=
1.49A
P_L1 = I_O12 ⋅ DCR = 1.4A² ⋅ 49.2mΩ = 96mW
Output Capacitor
V_DROOP = 0.2V
C_OUT
=
3 · ∆I_LOAD
V_DROOP · F_S
=
3 · 1.4A
0.2V · 1.2MHz
= 17.5µF; use 22µF
I = _RMS(MAX)
1
2·
· ^{(VOUT) · (VIN(MAX) - VOUT)}
3
L · F_S · V_IN(MAX)
1
=
2·
3
·
3.3V · (4.2V - 3.3V)
3.3µH · 1.2MHz · 4.2V
= 52mArms
P_esr = esr · I_RMS2 = 5mΩ · (52mA)² = 13.3µW
Input Capacitor
Input Ripple V_PP = 50mV
1
1
C_IN =


V_{PP -}
I_O1 + I_O2
ESR^

·
4
·
F_S
=
 50mV
 1.4A
= 6.8µF; use 10µF
- 5mΩ · 4 · 1.2MHz
I = _RMS(MAX)
I_O
2
= 0.7Arms
P = esr · I_RMS2 = 5mΩ · (0.7A)² = 2.45mW
2158.2008.10.1.5
www.analogictech.com
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