PRODUCT DATASHEET
AAT1184
TM
SwitchReg
High Voltage Step-Down Regulator
AAT1184 Design Example
Specifications
VOUT = 5.0V @ 1.2A, Pulsed Load ΔILOAD = 1.2A
VIN = 12V
FS = 490kHz
TAMB = 85°C in TSOPJW-12 Package
Output Inductor
For TDK inductor SLF7045T-4R7M2R0-PF, 4.7μH, DCR = 30mΩ max.
VOUT
L1 · FS
VOUT
VIN
5V
490kHz
5V
12V
ΔI =
· 1 -
=
· 1 -
= 1.2A
4.7µH
·
ΔI
2
IPK
=
IOUT
+
= 1.2A + 0.6A = 1.8A
PL1 = IOUT2 · DCR = 1.8A2 · 11.7mΩ = 37.9mW
Output Capacitor
VDROOP = 0.33V (10% Output Voltage)
3 · ΔILOAD
VDROOP · FS
3 · 1.2A
0.33V · 490kHz
COUT
=
=
= 22.3µF; use 22µF
V
OUT · (VIN(MAX) - VOUT1
L · FS · VIN1(MAX)
)
1
5.0V · (24V - 5.0V)
4.7µH · 490kHz · 24V
1
·
= 496mARMS
IRMS(MAX)
=
=
·
2· 3
2· 3
PRMS = ESR · IRMS2 = 5mΩ · (496mA)2 = 1.2mW
Input Capacitor
Input Ripple VPP = 25mV
1
1
CIN =
=
= 32µF
VPP
IOUT
25mV
1.2A
- ESR · 4 · FS
- 5mW · 4 · 490kHz
For low cost applications, a 100μF/25V electrolytic capacitor in parallel with a 1μF/25V ceramic capacitor is used to
reduce the ESR.
IOUT
2
IRMS
=
= 0.6A
P = ESR · (IRMS)2 = 5mΩ · (0.6A)2 = 1.8mW
w w w . a n a l o g i c t e c h . c o m
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1184.2008.07.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
