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| AAT2689 Datasheet, PDF (17/20 Pages) Advanced Analogic Technologies – PMIC Solution for 12V Adapter Systems with 2-Output High Performance Step-Down Converters | |||
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PRODUCT DATASHEET
AAT2689
SystemPowerTM PMIC Solution for 12V Adapter Systems with 2-Output High Performance Step-Down Converters
AAT2689 Design Example
Specifications
VO1 = 3.3V @ 2.5A, Pulsed Load ÎILOAD = 2.5A
VO2 = 1.8V @ 600mA
VIN1 = 12V
FS = 490kHz
TAMB = 85°C in TDFN34-16 Package
Channel 1 Output Inductor
For Sumida inductor RCH108NP-4R7M, 4.7μH, DCR = 11.7mΩ max.
ÎI
=
VOUT1
L1 · FS
·
1
-
VOUT1
VIN
=
3.3V
4.7μH · 490kHz
·
1-
3.3V
12V
= 1A
ÎI
IPK1 = IOUT1 + 2 = 2.5A + 1A = 3.5A
PL1
=
I2
OUT1
·
DCR
=
3.5A2
·
11.7mΩ
=
143mW
Channel 1 Output Capacitor
VDROOP = 0.33V (10% Output Voltage)
COUT
= 3 · ÎILOAD
VDROOP · FS
=
3 · 2.5A = 46.4μF; use 2x22μF
0.33V · 490kHz
IRMS(MAX)
=
1
2·
3
·
VOUT1 · (VIN(MAX) - VOUT1) = 1 ·
L · FS · VIN1(MAX)
2· 3
3.3V · (24V - 3.3V)
4.7μH · 490kHz · 24V
= 357mARMS
PRMS = ESR · IRMS2 = 11.7mΩ · (357mA)2 = 1.5W
Channel 1 Input Capacitor
Input Ripple VPP = 25mV
1
CIN1 =
VPP1 - ESR
IOUT1
· 4 · FS
=
1
= 102μF
25mV - 5mΩ · 4 · 490kHz
2.5A
For low cost applications, 100μF/25V electrolytic capacitor in parallel with 1μF/25V ceramic capacitor are used to reduce
the ESR.
IRMS =
IOUT1
2
= 1.25A
P = ESR · (IRMS)2 = 5mΩ · (1.25A)2 = 7.8mW
2689.2008.06.1.0
www.analogictech.com
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