PRODUCT DATASHEET
AAT1189
TM
SwitchReg
High Voltage Step-Down Regulator
AAT1189 Design Example
Specifications
VOUT = 5.0V @ 2.5A, Pulsed Load ΔILOAD = 2.5A
VIN = 12V
FS = 490kHz
TAMB = 85°C in TDFN34-16 Package
Output Inductor
For Sumida inductor RCH108NP-4R7M, 4.7μH, DCR = 11.7mΩ max.
VOUT
L1 · FS
VOUT
VIN
5V
490kHz
5V
12V
ΔI =
· 1 -
=
· 1 -
= 1.2A
4.7µH
·
ΔI
2
IPK
=
IOUT
+
= 2.5A + 0.6A = 3.1A
PL1 = IOUT2 · DCR = 3.1A2 · 11.7mΩ = 112mW
Output Capacitor
VDROOP = 0.33V (10% Output Voltage)
3 · ΔILOAD
VDROOP · FS
3 · 2.5A
0.33V · 490kHz
COUT
=
=
= 46.4µF; use 2x22µF
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 =
=
= 102µF
VPP
IOUT
25mV
2.5A
- ESR · 4 · FS
- 5mΩ · 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.
IOUT1
2
IRMS
=
= 1.25A
P = ESR · (IRMS)2 = 5mΩ · (1.25A)2 = 7.8mW
w w w . a n a l o g i c t e c h . c o m
16
1189.2008.06.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
