AAT1151 Datasheet, PDF (9/16 Pages) Advanced Analogic Technologies – 850kHz 700mA Synchronous Buck DC/DC Converter

DATA SHEET
AAT1151
850kHz 700mA Synchronous Buck DC/DC Converter
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
=
I_O
V_OUT
• k • F_S
•
⎛⎝₁
-
V_OUT⎞
V_{IN âŽ} 
=
⎛
⎝
1A
•
1.5V
0.4 • 850kHz
⎞
⎠
⎛_•
⎝
1-
1.5V ^⎞
4.2V ⎠
= 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 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.
2.7V-4.2V
C1
10µF
R1
100
R2
100k
C2
0.1µF
U1
AAT1151-MSOP
5 _VP
FB ¹
4 _VCC
3 _EN
LX ⁷
LX ⁶
2 _{SGND PGND} 8
1.8V
L1
3.3µH
C3, C4
2x22µF
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
Efficiency vs. Load Current
(V_OUT = 1.8V)
100
2.7V
90
80
3.6V
4.2V
70
60
50
1
10
100
1000
Load Current (mA)
Figure 1: AAT1151 Evaluation Board.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
201991B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013
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