AAT2789 Datasheet, PDF (19/22 Pages) Advanced Analogic Technologies – Low Noise, High Frequency Dual Step-Down Converter

SystemPower^TM
PRODUCT DATASHEET
AAT2789
Low Noise, High Frequency Dual Step-Down Converter
Channel 1 Output Capacitor
V_DROOP= 0.12V
C_OUT3 =
3 · ΔI_LOAD
V_DROOP · F_S
=
3 · 1.5A _{= 20.8µF; use 22µF}
0.12V · 1.8MHz
I = _RMS(MAX)
1
2·
· ^{VOUT · (VIN(MAX) - VOUT)} = ¹
3
L · F_S · V_IN(MAX)
2·
·
3
1.2V · (4.2V - 1.2V) _{= 92mA}
1.5µH · 1.8MHz · 4.2V
P_ESR = ESR · I_RMS2 = 5mΩ · 24mA² = 3µW
Channel 2 Output Capacitor
V_DROOP= 0.1V
C_OUT3 =
3 · ΔI_LOAD
V_DROOP · F_S
=
3 · 0.5A
0.1V · 2MHz
= 7.5µF; use 10µF
I = _RMS(MAX)
1 _· V_OUT · (V_IN(MAX) - V_OUT) ₌ 1 _·
2· 3
L · F_S · V_IN(MAX)
2· 3
3.3V · (4.2V - 3.3V)
3.3µH · 2MHz · 4.2V
= 30mA
P_ESR = ESR · I_RMS2 = 5mΩ · 30mA² = 4.5µW
Input Capacitor
Input Ripple V_PP1 = 50mV, V_PP2 = 25mV
C_IN1
=
⎛ V_PP1
⎝ I_O1
-
1
ESR^⎞⎠
·4
· F_S
=
⎛ 50mV
⎝ 1.5A
1
= 4.9µF
- 5mΩ⎞⎠ · 4 · 1.8MHz
C_IN2
=
⎛ V_PP2
⎝ I_O2
-
1
ESR^⎞⎠
·4
· F_S
=
⎛ 25mV
⎝ 0.5A
1
- 5mΩ⎞⎠ · 4 · 2MHz
= 3µF
C_IN = C_IN1+ C_IN2 = 4.9μF+3μF= 7.9μF; use 10μF
I = _RMS(MAX)
I_O1
+
2
I_O2
= 1A
P = ESR · I_RMS2 = ESR · (1A)² = 5mW
2789.2008.03.1.0
www.analogictech.com
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