PRODUCT DATASHEET
AAT2120
SwitchRegTM
500mA Low Noise Step-Down Converter
Step-Down Converter Design Example
Specifications
VO = 1.8V @ 250mA, Pulsed Load ∆ILOAD = 200mA
VIN = 2.7V to 4.2V (3.6V nominal)
FS = 1.8MHz
TAMB = 85°C
1.8V Output Inductor
µs
A
µs
A
(use 3.0μH; see Table 1)
L1 = 1.67
⋅ VO2 = 1.67
⋅ 1.8V = 3µH
For Sumida inductor CDRH2D09-3R0, 3.0μH, DCR = 150mΩ.
⎛
⎞
VO
L1 ⋅ FS
VO
1.8
V
1.8V
⎛
⎞
=
ΔIL1 =
⋅ 1 -
⎝
⋅ 1 -
= 190mA
⎝
⎠
VIN
3.0µH ⋅ 1.8MHz
4.2V
⎠
ΔIL1
2
IPKL1 = IO +
= 250mA + 95mA = 345mA
2
PL1 = IO ⋅ DCR = 250mA2 ⋅ 150mΩ = 9.375mW
1.8V Output Capacitor
VDROOP = 0.1V
3 · ΔILOAD
VDROOP · FS
3 · 0.2A
COUT
=
=
= 3.3µF (use 3.3µF)
0.1V · 1.8MHz
(VO) · (VIN(MAX) - VO)
L1 · FS · VIN(MAX)
1
1.8V · (4.2V - 1.8V)
1
·
= 66mArms
IRMS
=
·
=
3.0µH · 1.8MHz · 4.2V
2· 3
2· 3
Pesr = esr · IRMS2 = 5mΩ · (66mA)2 = 21.8µW
Input Capacitor
Input Ripple VPP = 25mV
1
1
CIN =
=
= 1.16µF (use 4.7µF)
⎛ VPP
⎝ IO
⎞
⎛ 25mV
⎝ 0.2A
⎞
⎠
- ESR · 4 · FS
- 5mΩ · 4 · 1.8MHz
⎠
IO
IRMS
=
= 0.1Arms
2
P = esr · IRMS2 = 5mΩ · (0.1A)2 = 0.05mW
w w w . a n a l o g i c t e c h . c o m
2120.2008.02.1.2
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ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
