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AAT1154

1MHz 3A Step-Down DC/DC Converter

Analog Technology Inc 

SwitchRegTM

Schottky Freewheeling Diode

The Schottky average current is the load current multi-

plied by one minus the duty cycle. For VIN at 5V and VOUT

at 3.3V, the average diode current is:

I

AVG

=

IO

·

⎛1

⎝

-

VO ⎞ = 3A· ⎛1-

VIN ⎠

⎝

3.3V ⎞

5.0V ⎠

=

1A

With a 125°C maximum junction temperature and a

120°C/W thermal resistance, the maximum average

current is:

IAVG =

TJ(MAX)- TAMB

θJA · VFWD

= 125°C - 70°C

120°C/W · 0.4V

=

1.14A

For overload, short-circuit, and excessive ambient tem-

perature conditions, the AAT1154 enters over-tempera-

ture shutdown mode, protecting the AAT1154 and the

output Schottky. In this mode, the output current is lim-

ited internally until the junction temperature reaches the

temperature limit (see over-temperature characteristics

graphs). The diode reverse voltage must be rated to

withstand the input voltage.

Manufacturer

Diodes Inc.

ROHM

Micro Semi

Part Number

B340LA

RB050L-40

5820SM

Voltage Rating

0.45V @ 3A

0.45V @ 3A

0.46V @ 3A

Table 1: 3A Surface Mount Schottky Diodes.

Input Capacitor Selection

The primary function of the input capacitor is to provide

a low impedance loop for the edges of pulsed current

drawn by the AAT1154. A low ESR/ESL ceramic capacitor

is ideal for this function. To minimize stray inductance,

the capacitor should be placed as closely as possible to

the IC. This also keeps the high frequency content of the

input current localized, minimizing the radiated and con-

ducted EMI while facilitating optimum performance of

the AAT1154. Proper placement of the input capacitor C1

is shown in the layout in Figure 2. Ceramic X5R or X7R

capacitors are ideal. The size required will vary depend-

ing on the load, output voltage, and input voltage source

impedance characteristics. Typical values range from

PRODUCT DATASHEET

AAT1154

1MHz 3A Step-Down DC/DC Converter

1μF to 10μF. The input capacitor RMS current varies with

the input voltage and the output voltage. It is highest

when the input voltage is double the output voltage

where it is one half of the load current.

IRMS = IO ·

VO ·⎛1- VO ⎞

VIN ⎝ VIN ⎠

A high ESR tantalum capacitor with a value about 10

times the input ceramic capacitor may also be required

when using a 10μF or smaller ceramic input bypass

capacitor. This dampens any input oscillations that may

occur due to the source inductance resonating with the

converter input impedance.

Output Capacitor

With no external compensation components, the output

capacitor has a strong effect on loop stability. Larger

output capacitance will reduce the crossover frequency

with greater phase margin. A 200μF ceramic capacitor

provides sufficient bulk capacitance to stabilize the out-

put during large load transitions and has ESR and ESL

characteristics necessary for very low output ripple. The

RMS ripple current is given by:

IRMS =

2

1

·

3

· (VOUT

+

VFWD)

L · FS

·

·

(VIN

VIN

-

VOUT)

For a ceramic output capacitor, the dissipation due to the

RMS current and associated output ripple are negligible.

Tantalum capacitors with sufficiently low ESR to meet

output ripple requirements generally have an RMS cur-

rent rating much greater than that actually seen in this

application. The maximum tantalum output capacitor

ESR is:

ESR ≤ VRIPPLE

ΔI

where ΔI is the peak-to-peak inductor ripple current.

Due to the ESR zero associated with the tantalum capac-

itor, smaller values than those required with ceramic

capacitors provide more phase margin with a greater

loop crossover frequency.

1154.2007.11.1.7

www.analogictech.com

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