10

AAT1126

600mA, 1MHz Step-Down Converter

DATA SHEET

Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com

201976B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013

Applications Information

Inductor Selection

The step-down converter uses peak current mode control

with slope compensation to maintain stability for duty

cycles greater than 50%. The output inductor value must

be selected so the inductor current down slope meets the

internal slope compensation requirements. The internal

slope compensation for the adjustable and low-voltage

fixed versions of the AAT1126 is 0.24A/μsec. This

equates to a slope compensation that is 75% of the

inductor current down slope for a 1.5V output and 4.7μH

inductor.

0.75

⋅ V

O

m =

=

= 0.24

L

0.75

⋅ 1.5V

4.7µH

A

µs

This is the internal slope compensation for the adjustable

(0.6V) version or low-voltage fixed versions. When

externally programming the 0.6V version to 2.5V, the

calculated inductance is 7.5μH.

0.75

⋅ V

O

L =

=

⋅ V

O

= 3

⋅ 2.5V = 7.5µH

m

0.75

⋅ V

O

0.24A

µs

A

µs

A

A

µs

In this case, a standard 10μH value is selected.

For high-voltage fixed versions (2.5V and above), m =

0.48A/μs. Table 1 displays inductor values for the

AAT1126 fixed and adjustable options.

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 satura-

tion characteristics. The inductor should not show any

appreciable saturation under normal load conditions.

Some inductors may meet the peak and average current

ratings 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 4.7μH CDRH3D16 series inductor selected from

Sumida has a 105m DCR and a 900mA DC current rat-

ing. At full load, the inductor DC loss is 17mW which gives

a 2.8% loss in efficiency for a 400mA, 1.5V output.

Input Capacitor

Select a 4.7μF to 10μF X7R or X5R ceramic capacitor for

the input. To estimate the required input capacitor size,

solve for C. The calculated value varies with input volt-

voltage.

⎛⎞

· 1 -

⎝⎠

V

O

V

IN

C

V

O

V

IN

⎛⎞

- ESR

·

F

S

⎝⎠

V

PP

I

O

⎛⎞

· 1 -

=

for V

⎝⎠

V

O

V

IN

V

O

V

IN

1

4

C

1

⎛⎞

- ESR

·

S

⎝⎠

V

PP

I

O

Always examine the ceramic capacitor DC voltage coef-

ficient characteristics when selecting the proper value.

For example, the capacitance of a 10μF, 6.3V, X5R

ceramic capacitor with 5.0V DC applied is actually about

6μF.

Configuration

Output

Voltage Inductor

Slope

Compensation

0.6V Adjustable

With External

Resistive Divider

0.6V to

2.0V

4.7μH

0.24A/μs

2.5V to

3.3V

10μH

0.24A/μs

Fixed Output

0.6V to

2.0V

4.7μH

0.24A/μs

2.5V to

3.3V

4.7μH

0.48A/μs

Table 1: Inductor Values.

The maximum input capacitor RMS current is:

⎛⎞

I

· 1 -

⎝⎠

V

O

V

IN

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.

⎛⎞

· 1 -

=

⎝⎠

V

O

V

IN

V

O

V

IN

1

2