AAT1171 Datasheet, PDF (15/22 Pages) Advanced Analogic Technologies – 600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch

AAT1171
600mA Voltage-Scaling Step-Down Converter
for RF Power Amplifiers with Bypass Switch
Applications Information
Inductor Selection
The step-down converter uses peak current mode
control with slope compensation to maintain stabil-
ity for duty cycles greater than 50%. Because the
required slope compensation varies with output
voltage, the AAT1171 varies the slope compensa-
tion to match the output voltage. This allows the
use of a single inductor value for all output voltage
levels. For the AAT1171, this value is 2.2µH.
Manufacturer's specifications list both the inductor
DC current rating, which is a thermal limitation, and
the peak current rating, which is determined by the
saturation characteristics.
The inductor should not show any appreciable sat-
uration under normal load conditions. The inductor
ripple current varies with both the input voltage and
the output voltage and peaks at the maximum input
voltage with the output at one half of the input volt-
age. For the typical AAT1171, this corresponds to a
4.2V input voltage and a 2.1V output voltage. With
the suggested 2.2µH inductor, this corresponds to
239mA peak-to-peak ripple current. For a 600mA
DC load current, the peak inductor current would
be 718mA. In order to prevent saturation under
normal load conditions, the peak inductor current
should be less than the inductor saturation current.
I_PK(MAX) = I_O +
V_IN(MAX)
8 ⋅ L ⋅ F_S
=
0.6A
+
8
⋅
4.2V
2.2µH ⋅ 2MHz
= 0.6A + 0.12A
= 0.72A
Some inductors may meet peak and average cur-
rent requirements yet result in excessive losses
due to a high DCR. Always consider the losses
associated with the DCR and its effect on the total
converter efficiency when selecting an inductor.
The inductor losses can be estimated by using the
full load output current. The output inductor losses
can then be calculated to estimate their effect on
overall device efficiency.
PL = I_O2 ⋅ DCR = 0.6A² ⋅ 0.14Ω = 50mW
ηL
=
P_O
P_O + PL
=
3.4V
3.4 ⋅ 0.6A
⋅ 0.6A + 50mW
=
97%
The 2.2µH inductor selected for the AAT1171 eval-
uation 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 is designed for use with a 4.7µF 10V
X5R ceramic output capacitor. Although a larger
output capacitor provides improved response to
large load transients, it also limits the output volt-
age rise and fall time in response to the DAC input.
For stable operation, with sufficient phase and gain
margin, the internal voltage loop compensation lim-
its 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
dominated by the output capacitor. During a step
increase in load current, the output capacitor sup-
plies the load current while the control loop
responds. Within two or three switching cycles, the
inductor current increases to match the load cur-
rent demand. The relationship of the output voltage
droop during the three switching cycles to the out-
put capacitance can be estimated by:
C_OUT
=
3 · ΔI_LOAD
V_DROOP · F_S
Once the average inductor current increases to the
DC load level, the output voltage recovers. The
above equation 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.
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