AAT2749 Datasheet, PDF (15/17 Pages) Skyworks Solutions Inc. – 3.0MHz Step-Down Converter and Low-V in LDO

AAT2749 Design Example
DATA SHEET
AAT2749
3.0MHz Step-Down Converter and Low-V_IN LDO
Specifications
V_O1 = 1.8V @ 600mA, Pulsed Load I_LOAD = 600mA
V_O2 = 1.2V @ 300mA
V_IN1 = 5V
F_S = 3MHz
T_AMB = 85°C in CSP-9 Package
Step-Down Converter Output Inductor
For Murata chip inductor LQM2MPN2R2NG0, 2.2μH, DCR = 0.11Ω.
ΔI =
V_OUT1
L · F_S
·
1
-
V_OUT1
V_IN1
=
1.8V
2.2μH · 3MHz
·
1
-
1.8V
5V
= 174mA
I_PK1
=
I_OUT1
+
ΔI
2
= 600mA + 87mA = 687mA
P_L1
=
I²
OUT1
·
DCR
=
600mA²
·
111mΩ
=
40mW
Step-Down Converter Output Capacitor
V_DROOP = 0.18V (10% Output Voltage)
C_OUT
=
3 · ΔI_LOAD
V_DROOP · F_S
=
3 · 600mA
0.18V · 3MHz
= 3μF; use 4.7μF
I = _RMS(MAX)
1 _· V_OUT1 · (V_IN(MAX) - V_OUT1) ₌ 1 _· 1.8V · (5.5V - 2.3V)
2· 3
L · F_S · V_IN1(MAX)
2· 3 ^{2.2μH · 3MHz · 5.5V}
= 46mA_rms
P_RMS = ESR · I_RMS2 = 5mΩ · (46mA)² = 11μW
Step-Down Converter Input Capacitor
Input Ripple V_PP = 10mV
C_IN1 =
1
=
V_PP
I_OUT1
- ESR
· 4 · F_S
1
= 7μF; use 4.7μF
10mV
600mA
- 5mΩ
· 4 · 3MHz
I_RMS =
I_{OUT1 = 300mA}
2
P = ESR · (I_RMS2) = 5mΩ · (300mA)² = 0.31mW
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
202037A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 8, 2012
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