AAT1155
1MHz 2.5A Step-Down DC/DC Converter
3A Surface Mount Schottky Diodes
Schottky Freewheeling Diode
The Schottky average current is the load current
multiplied by one minus the duty cycle.
Diodes Inc.
ROHM
B340LA
RB050L-40
5820SM
0.45V @ 3A
0.45 @ 3A
0.46V @ 3A
Micro Semi
VO
⎛
⎝
⎞
⎠
1-
V
IN
Input Capacitor Selection
The primary function of the input capacitor is to pro-
vide a low impedance loop for the edges of pulsed
current drawn by the AAT1155. A low ESR/ESL
ceramic capacitor is ideal for this function. To mini-
mize stray inductance, the capacitor should be
placed as closely as possible to the IC. This also
keeps the high frequency content of the input cur-
rent localized, minimizing the radiated and con-
ducted EMI while facilitating optimum performance
of the AAT1155. 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 depending on the load, out-
put voltage, and input voltage source impedance
characteristics. Typical values range from 1µF to
10µF. The input capacitor RMS current varies with
the input voltage and the output voltage. It is high-
est when the input voltage is double the output volt-
age where it is one half of the load current.
For VIN at 5V and VOUT at 3.3V, the average diode
current is:
VO
3.3V
⎛
⎝
⎞
⎛
⎞
=
IAVG = IO ·
·
1-
= 2.5A 1-
0.85A
V ⎠
⎝
5.0V⎠
IN
With a 125°C maximum junction temperature and a
120°C/W thermal resistance, the maximum aver-
age current is:
T
J(MAX)- T
125°C - 70°C
AMB
IAVG
=
=
= 1.14A
θ
J-A · VFWD 120°C/W 0.4V
·
For overload, short-circuit, and excessive ambient
temperature conditions, the AAT1155 enters over-
temperature shutdown mode protecting the
AAT1155 as well as the output Schottky. In this
mode, the output current is limited internally until
the junction temperature reaches the temperature
limit (see over-temperature characteristics graphs).
The diode reverse voltage must be rated to with-
stand the input voltage.
V
VO
⎛
⎞
⎠
O
IRMS = IO ·
· 1-
V
V
⎝
IN
IN
Vin 3.5V-5.5V
Vout 3.3V @2.5A
U1
AAT1155-3.3
R1 R2
100 100k
FB VP
C4
100µF
L1
1.5µH
GND LX
EN LX
VCC VP
C1
+
-
C2
D1
B340LA 120µF
10µF
C3
0.1µF
rtn
C1 Murata 10µF 6.3V X5R GRM42-6X5R106K6.3
C2 Vishay120µF 6.3V 594D127X96R6R3C2T
C3 0.1µF 0603ZD104M AVX
C4 Vishay Sprague 100µF 16V 595D107X0016C 100µF 16V
D1 B340LA Diodes Inc.
L1 CDRH6D28-1.5µH Sumida
Options
C2 MuRata 100µF 6.
3V (two or three in parallel)
3V GRM43-2 X5R 107M 100µF 6.
C2 TDK 100µF 6.3V C3325X5R0J107M 100µF 6.3V (two or three in parallel)
Figure 2: 3.3V, 3A Output Efficiency.
1155.2005.11.1.6
11
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