PRODUCT DATASHEET
AAT2689
TM
SystemPower 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 ΔILOAD = 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.
VOUT1
L1 · FS
VOUT1
VIN
3.3V
4.7μH 490kHz
3.3V
12V
ΔI =
· 1 -
=
· 1 -
= 1A
·
ΔI
2
IPK1
=
IOUT1
+
= 2.5A + 1A = 3.5A
PL1 = IOUT12 · DCR = 3.5A2 · 11.7mΩ = 143mW
Channel 1 Output Capacitor
VDROOP = 0.33V (10% Output Voltage)
3 · ΔILOAD
VDROOP · FS
3 · 2.5A
0.33V · 490kHz
COUT
=
=
= 46.4μF; use 2x22μF
V
OUT1 · (VIN(MAX) - VOUT1
L · FS · VIN1(MAX)
)
1
3.3V · (24V - 3.3V)
4.7μH · 490kHz · 24V
1
·
= 357mARMS
IRMS(MAX)
=
=
·
2· 3
2· 3
PRMS = ESR · IRMS2 = 11.7mΩ · (357mA)2 = 1.5W
Channel 1 Input Capacitor
Input Ripple VPP = 25mV
1
1
CIN1
=
=
= 102μF
VPP1
IOUT1
25mV
2.5A
- ESR · 4 · FS
- 5mΩ · 4 · 490kHz
For low cost applications, 100μF/25V electrolytic capacitor in parallel with 1μF/25V ceramic capacitor are used to reduce
the ESR.
IOUT1
2
IRMS
=
= 1.25A
P = ESR · (IRMS)2 = 5mΩ · (1.25A)2 = 7.8mW
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2689.2008.06.1.0
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ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
