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| AAT2687 Datasheet, PDF (17/20 Pages) Advanced Analogic Technologies – PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters | |||
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AAT2687 Design Example
DATA SHEET
AAT2687
PMIC Solution for 24V Systems
with 2 High Performance Step-Down Converters
Specifications
VO1 = 3.3V @ 4.5A, Pulsed Load ÎILOAD = 4.5A
VO2 = 1.8V @ 600mA
VIN1 = 12V
FS = 490kHz
TAMB = 85°C in TQFN45-24 Package
Channel 1 Output Inductor
For Sumida inductor RCH108NP-4R7M, 4.7μH, DCR = 11.7mΩ max.
ÎI
=
VOUT1
L1 · FS
·
1
-
VOUT1
VIN1
=
3.3V
4.7μH · 490kHz
·
1-
3.3V
12V
= 1A
ÎI
IPK1 = IOUT1 + 2 = 4.5A + 1A = 5.5A
PL1
=
I2
OUT1
·
DCR
=
5.5A2
·
11.7mΩ
=
354mW
Channel 1 Output Capacitor
VDROOP = 0.4V
COUT =
3 · ÎILOAD
VDROOP · FS
=
3 · 4.5A = 69μF; use 3x22μF
0.4V · 490kHz
IRMS(MAX)
=
1
2·
3
·
VOUT1 · (VIN(MAX) - VOUT1)
L · FS · VIN1(MAX)
=1
2·
3
·
3.3V · (24V - 3.3V)
4.7μH · 490kHz · 24V
= 357mARMS
PRMS = ESR · IRMS2 = 5mΩ · (357mA)2 = 0.6W
Channel 1 Input Capacitor
Input Ripple VPP = 33mV
1
CIN1 =
VPP - ESR
IOUT1
· 4 · FS
=
1
= 219μF
5.5mV
4.5A
- 5mΩ
· 4 · 490kHz
For low cost applications, a 220μF/25V electrolytic capacitor in parallel with a 1μF/25V ceramic capacitor is used to
reduce ESR.
IRMS =
IOUT1
2
= 2.25A
P = ESR · (IRMS)2 = 5mΩ · (2.25A)2 = 25.3mW
Skyworks Solutions, Inc. ⢠Phone [781] 376-3000 ⢠Fax [781] 376-3100 ⢠sales@skyworksinc.com ⢠www.skyworksinc.com
202033A ⢠Skyworks Proprietary Information ⢠Products and Product Information are Subject to Change Without Notice. ⢠June 8, 2012
17
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