AAT1171 Datasheet, PDF (14/22 Pages) Advanced Analogic Technologies – 600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch

DATA SHEET
AAT1171
600mA Voltage-Scaling Step-Down Converter
for RF Power Amplifiers with Bypass Switch
PL = I_O2 ⋅ DCR = 0.6A² ⋅ 0.14Ω = 50mW
η
=
P_O
P_O + PL
=
3.4V
3.4 ⋅ 0.6A
⋅ 0.6A + 50mW
=
97%
The 2.2μH inductor selected for the AAT1171 evaluation
board has a 140mΩ DCR and a 0.91A DC current rating.
At 600mA load current, the inductor loss is 50mW which
gives 2.4% loss in efficiency for a 600mA 3.4V output
voltage with an inductor that measures 3.2x3.2x1.2mm.
Output Capacitor Selection
The AAT1171-1/AAT1171-4 are designed for use with
4.7μF 10V X5R ceramic output capacitors, while the
AAT1171-5 is designed for use with 10μF 10V X5R
ceramic output capacitors. Although a larger output
capacitor provides improved response to large load tran-
sients, it also limits the output voltage rise and fall time
in response to the DAC input. For stable operation, with
sufficient phase and gain margin, the internal voltage
loop compensation limits the minimum output capacitor
value to 4.7μF. Increased output capacitance will reduce
the crossover frequency with greater phase margin.
The output voltage droop due to load transients is dom-
inated by the output capacitor. During a step increase in
load current, the output capacitor supplies the load cur-
rent while the control loop responds. Within two or three
switching cycles, the inductor current increases to match
the load current demand. The relationship of the output
voltage droop during the three switching cycles to the
output capacitance can be estimated by:
C_OUT
=
3 · ∆I_LOAD
V_DROOP · F_S
Once the average inductor current increases to the DC
load level, the output voltage recovers. The above equa-
tion establishes a limit on the minimum output capacitor
value necessary to meet a given output voltage droop
requirement (V_DROOP) for a given load transient.
The maximum output capacitor RMS ripple current is:
I = _RMS(MAX)
1
2·
·
3
V_OUT · (V_IN(MAX) - V_OUT)
L · FS · V_IN(MAX)
Dissipation due to the RMS current in the ceramic output
capacitor ESR is typically minimal, resulting in less than
a few degrees rise in hot-spot temperature.
Input Capacitor Selection
A 10V X5R or X7R ceramic capacitor is suggested for the
input capacitor with typical values ranging from 4.7μF to
10μF. To estimate the required input capacitance size,
determine the acceptable input ripple level (V_PP) and solve
for C, as shown below. The calculated value varies with
input voltage and is a maximum when V_IN is double the
output voltage. Always examine the ceramic capacitor DC
voltage coefficient characteristics when selecting the
proper value. For example, due to the voltage coefficient
of a 10μF 6.3V X5R ceramic capacitor, with an applied
voltage of 5V DC the capacitance decreases to 6μF.
C_IN =
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
⎛ V_PP
⎝ I_O
- ESR^⎞⎠ · F_S
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
=
1
4
1
C_IN(MIN) = _{⎛ VPP}
⎝ I_O
- ESR^⎞⎠ · 4 · F_S
The maximum input capacitor RMS current is:
I_RMS = I_O ·
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
The input capacitor RMS ripple current varies with the
input and output voltage and will always be less than or
equal to half of the total DC load current.
V_O
V_IN
· ^⎛⎝1 -
V_O ⎞
V_IN ⎠
=
D · (1 - D) =
0.5²
=
1
2
for V_IN = 2 · V_O
I = _RMS(MAX)
I_O
2
The term
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
appears in both the input voltage
ripple and input capacitor RMS current equations and is
a maximum when V_IN is twice V_o; therefore, the input
voltage ripple and the input capacitor RMS current ripple
are a maximum at 50% duty cycle.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
14
201999B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 20, 2013