9

AAT1151

850kHz 700mA Synchronous Buck DC/DC Converter

DATA SHEET

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

201991B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013

Power and Signal Source

Separate small signal ground and power supply pins

isolate the internal control circuitry from the noise asso-

ciated with the output MOSFET switching. The low pass

filter R1 and C2 in schematic Figure 1 filters the noise

associated with the power switching.

Current Limit and

Over-Temperature Protection

For overload conditions, the peak input current is limit-

ed. As load impedance decreases and the output voltage

falls closer to zero, more power is dissipated internally,

raising the device temperature. Thermal protection com-

pletely disables switching when internal dissipation

becomes excessive, protecting the device from damage.

The junction over-temperature threshold is 140°C with

10°C of hysteresis.

Inductor

The output inductor is selected to limit the ripple current

to some predetermined value, typically 20% to 40% of

the full load current at the maximum input voltage.

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.

During overload and transient conditions, the average

current in the inductor can meet or exceed the current

limit point of the AAT1151. These conditions can tolerate

greater saturation in the inductor without degradation in

converter performance. 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 con-

verter efficiency when selecting an inductor.

For a 1.0A load and the ripple set to 40% at the maxi-

mum input voltage, the maximum peak-to-peak ripple

current is 280mA. The inductance value required is

2.84μH.

L =

• 1 -

V

OUT

V

OUT

I

V

IN

=

• 1 -

1.5V

1.5V

1A • 0.4 • 850kHz

4.2V

⎛⎞

⎝⎠

⎛⎞ ⎛

⎞

⎝⎠ ⎝

⎠

= 2.84

μH

The factor “k” is the fraction of full load selected for the

ripple current at the maximum input voltage. For ripple

current at 40% of the full load current, the peak current

will be 120% of full load. Selecting a standard value of

3.0μH gives 38% ripple current. A 3.0μH inductor select-

ed from the Sumida CDRH5D28 series has a 24m DCR

and a 2.4A DC current rating. At full load, the inductor

DC loss is 24mW, which amounts to a 1.6% loss in effi-

ciency.

L1

3.3µH

C3, C4

2x22µF

C1

10µF

R1

100

C2

0.1µF

C1

Murata 10µF 6.3V X5R GRM42-6X5R106K6.3

C3-C4

Murata 22µF 6.3V GRM21BR60J226ME39L X5R 0805

L1

Sumida CDRH3D16-4R7NC or CDRH3D16-3R3NC

R2

100k

1.8V

2.7V-4.2V

FB

1

SGND

2

EN

3

VCC

4

LX

6

LX

7

PGND

8

VP

5

U1

AAT1151-MSOP

Efficiency vs. Load Current

50

60

70

80

90

100

1

10

100

1000

Load Current (mA)

2.7V

3.6V

4.2V

Figure 1: AAT1151 Evaluation Board.