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202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013

14

AAT2138

High Current Step-Down Converter

with High-Precision Programmable Current Limited Load Switch

DATA SHEET

Considering 20% capacitance tolerance, the minimum

capacitance should be 684μF. So select a 680μF tanta-

of the step-down converter on the PCB board.

Input Capacitor Selection

Select a 1μF X7R or X5R ceramic capacitor for the sys-

tem input. To estimate the required input capacitor size,

determine the acceptable input ripple level and solve for

C

·

V

OUT

V

IN

1 -

V

OUT

V

IN

f

S

·

V

PP

I

OUT

- ESR

=

·

V

OUT

V

IN

1 -

V

OUT

V

IN

1

4

and

C

1

4 · f

S

·

V

PP

I

OUT

- ESR

The input capacitor provides a low impedance loop for

the edges of pulsed current drawn by the AAT2138. Low

ESR/ESL X7R and X5R ceramic capacitors are ideal for

this function due to their low ESR and ESL. To minimize

stray parasitic inductance, place the capacitor as close

as possible to the IC. This keeps the high frequency

content of the input current localized, minimizing EMI

and input voltage ripple.

The proper placement of the input capacitor (C1) can be

seen in the evaluation board layout in the Layout section

of this datasheet (see Figure 4).

A laboratory test set-up typically consists of two long

wires running from the bench power supply to the eval-

uation board input voltage pins. The inductance of these

wires, along with the low-ESR ceramic input capacitor,

can create a high Q network that may affect converter

performance. This problem often becomes apparent in

the form of excessive ringing in the output voltage dur-

ing load transients. Errors in the loop phase and gain

measurements can also result.

Feedback Resistor Selection

The output voltage on the AAT2138 is adjustable with

external resistors R1 and R2. To limit the bias current

required for the external feedback resistor string while

maintaining good noise immunity, the minimum sug-

gested value for R2 is 59kΩ. Although a larger value will

further reduce quiescent current, it will also increase the

impedance of the feedback node, making it more sensi-

tive to external noise and interference. The maximum

value of R1 should be below 1Mohm to keep reference

voltage normal and avoid noise coupling.

The external resistor R1, combined with an external

capacitor up to 22pF feed-forward capacitor (C4 in

Figure 3), delivers enhanced transient response for

extreme pulsed load applications and reduces ripple in

light load conditions. The addition of the feed forward

capacitor typically requires a larger output capacitor

C31-C32 for stability. The external resistors set the out-

put voltage according to the following equation:

R1 =

- 1 · R2 =

V

OUT

V

REF

- 1 · R2

V

OUT

0.6V

Table 2 shows the standard 1% metal film resistor

examples for different step-down ouput voltages

R2 = 59kΩ,

R1 (kΩ)

3.3

267

3.4

274

3.6

294

3.8

316

Table 2: Resistor Selections for Different Output

Voltage Settings.