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AAT1145_08 Datasheet, PDF (15/17 Pages) Advanced Analogic Technologies – 1.5A Step-Down Converter | |||
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PRODUCT DATASHEET
AAT1145
1.5A Step-Down Converter
Step-Down Converter Design Example
Specifications
VO = 1.8V @1.5A
VIN = 2.7V to 4.2V (3.6V nominal)
fS = 1.2MHz
Transient droop = 100mV
âVO = 50mV
1.8V Output Inductor
ÎIL = 30% â
IO = 0.3 · 1.5 = 450mA
L
=
VOUT · (VIN(MAX) - VOUT)
VIN(MAX) â
ÎIL â
fOSC
=
1.8 · (4.2 - 1.8)
4.2 â
0.45 · 1.2 · 106
=1.90µH
For Cooper 2.2μH inductor (SD3118-2R2-R) with DCR 74mΩ, the âIL should be
ÎIL =
VO
L
â
ââ1
-
VO
VIN
â
â
· T = 341mA
IPKL
=
IO
+
ÎIL
2
=
1.5
+
0.341
2
=
1.67A
PL = IO2 â
DCR = 1.52 â
0.074 = 166.5mW
1.8V Output Capacitor
COUT =
2 · ÎILOAD
VDROOP · fS
=
2 · 1.2
0.1 · 1.2 · 106
= 20µF; use 22µF
ESR â¤
ÎVO
ÎIL
0.05
= 0.341 = 0.15Ω
Select a 22μF, 10mΩ ESR ceramic capacitor to meet the ripple 50mV requirement.
ÎVOUT â¤
VOUT · (VIN - VOUT)
VIN · fOSC · L
·
â
âESR +
1
â
8 · fOSC · COUT â
1.8 · (4.2 - 1.8)
â
1
â
= 4.2 · 1.2 · 106 · 2.2 · 10-6 · â0.01 + 8 · 1.2 · 106 · 22 · 10-6 â = 6mV
IRMS = ÎIL ·0.289 = 0.341 · 0.289 = 98.5mArms
PCOUT = ESR · IRMS2 = 0.01 · 0.09852 = 97μW
1145.2008.01.1.2
www.analogictech.com
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