850kHz 700mA Synchronous
Buck DC/DC Converter
U1
AAT1151-QFN
12
R1
100
R2
100k
C2
0.1μF
R6
100k
11
10
9
7
6
16
5
VP
VP
VP
VCC
EN
GND
N/C
GND
OUT
LX
LX
LX
N/C
PGND
PGND
PGND
4
15
14
13
8
3
2
1
60
AAT1151
Efficiency vs. Load Current
1.8V
100
2.7V-4.2V
(V
OUT
= 1.8V)
C1
10μF
Efficiency (%)
L1
3.3μH
C3, C4
2x22μF
90
2.7V
80
3.6V
4.2V
70
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
50
1
10
100
1000
Load Current (mA)
Figure 1: AAT1151 Evaluation Board.
Current Limit and Over-Temperature
Protection
For overload conditions, the peak input current is
limited. As load impedance decreases and the
output voltage falls closer to zero, more power is
dissipated internally, raising the device tempera-
ture. Thermal protection completely 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.
peak and average current ratings yet result in exces-
sive 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.
For a 1.0A load and the ripple set to 40% at the
maximum input voltage, the maximum peak-to-
peak ripple current is 280mA. The inductance
value required is 2.84μH.
V
OUT
⎛
V
OUT
⎞
L=I •k•F • 1- V
⎝
O
S
IN
⎠
=
1.5V
⎞
⎛
⎞⎛
• 1 -
1.5V
4.2V
⎠
⎝
1A • 0.4 • 850kHz
⎠ ⎝
Inductor
The output inductor is selected to limit the ripple cur-
rent 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 limita-
tion, and the peak current rating, which is deter-
mined by the saturation characteristics. The inductor
should not show any appreciable saturation under
normal load conditions. During overload and tran-
sient conditions, the average current in the inductor
can meet or exceed the current limit point of the
AAT1151. These conditions can tolerate greater sat-
uration in the inductor without degradation in con-
verter performance. Some inductors may meet the
= 2.84μH
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 selected from the Sumida
CDRH5D28 series has a 24mΩ 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 efficiency.
1151.2007.01.1.6
9
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
