PRODUCT DATASHEET
AAT1154
SwitchRegTM
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.
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:
V
VO
⎛
⎞
⎠
O
IRMS = IO ·
· 1-
VO
3.3V
⎛
⎞
⎛
⎞
=
V
V
·
IAVG= IO 1-
·
= 3A 1-
1A
⎝
IN
IN
⎝
V ⎠
⎝
5.0V⎠
IN
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.
With a 125°C maximum junction temperature and a
120°C/W thermal resistance, the maximum average
current is:
T
J(MAX)- T
125°C - 70°C
AMB
IAVG
=
=
= 1.14A
θJA · VFWD 120°C/W 0.4V
·
Output Capacitor
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.
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:
Manufacturer
Part Number
Voltage Rating
1
(VOUT + VFWD) · (VIN - VOUT
)
Diodes Inc.
ROHM
Micro Semi
B340LA
RB050L-40
5820SM
0.45V @ 3A
0.45V @ 3A
0.46V @ 3A
IRMS
=
·
L · FS · VIN
2 · 3
For a ceramic output capacitor, the dissipation due to the
RMS current and associated output ripple are negligible.
Table 1: 3A Surface Mount Schottky Diodes.
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:
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
VRIPPLE
ESR ≤
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.
w w w . a n a l o g i c t e c h . c o m
1154.2007.11.1.7
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