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201999B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 20, 2013

AAT1171

600mA Voltage-Scaling Step-Down Converter

for RF Power Amplifiers with Bypass Switch

DATA SHEET

3.4 ⋅ 0.6A

3.4V ⋅ 0.6A + 50mW

η =

=

= 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:

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

The maximum output capacitor RMS ripple current is:

1

23

RMS(MAX)

I

=·

·

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,

for C, as shown below. The calculated value varies with

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.

⎛⎞

· 1 -

⎝⎠

⎛⎞

- ESR

·

⎝⎠

⎛⎞

· 1 -

=

⎝⎠

1

4

1

⎛⎞

- ESR

·

⎝⎠

The maximum input capacitor RMS current is:

⎛⎞

· 1 -

⎝⎠

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.

⎛⎞

· 1 -

⎝⎠

1

2

for

RMS(MAX)

I

2

=

The term

⎛⎞

· 1 -

⎝⎠

appears in both the input voltage

ripple and input capacitor RMS current equations and is

voltage ripple and the input capacitor RMS current ripple

are a maximum at 50% duty cycle.