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| AAT1189 Datasheet, PDF (16/19 Pages) Advanced Analogic Technologies – High Voltage Step-Down Regulator | |||
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SwitchRegTM
PRODUCT DATASHEET
AAT1189
High Voltage Step-Down Regulator
AAT1189 Design Example
Specifications
VOUT = 5.0V @ 2.5A, Pulsed Load ÎILOAD = 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.
ÎI
=
VOUT
L1 · FS
·
1-
VOUT
VIN
=
5V
· 1 - 5V
4.7µH · 490kHz
12V
= 1.2A
ÎI
IPK = IOUT + 2 = 2.5A + 0.6A = 3.1A
PL1 = IOUT2 · DCR = 3.1A2 · 11.7mΩ = 112mW
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
·
VOUT · (VIN(MAX) - VOUT1) = 1 ·
L · FS · VIN1(MAX)
2· 3
5.0V · (24V - 5.0V)
4.7µH · 490kHz · 24V
= 496mARMS
PRMS = ESR · IRMS2 = 5mΩ · (496mA)2 = 1.2mW
Input Capacitor
Input Ripple VPP = 25mV
CIN =
1
=
VPP
IOUT
- ESR
· 4 · FS
1
= 102µF
25mV
2.5A
- 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.
IRMS =
IOUT1
2
= 1.25A
P = ESR · (IRMS)2 = 5mΩ · (1.25A)2 = 7.8mW
16
www.analogictech.com
1189.2008.06.1.0
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