A8447 Просмотр технического описания (PDF) - Allegro MicroSystems
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A8447 Datasheet PDF : 9 Pages
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A8447
High Voltage Step Down Regulator
Recommended PCB Layout
In order to minimize the effects of ground bounce and offset
issues, it is important to have a low impedance ground located
very close to the device. This grounding scheme is known as star
grounding. It is likely that a ground plane will be necessary to
meet thermal requirements. The recommended land pattern illus-
trates how to create a low impedance ground that will also assist
with removing thermal energy from the device.
The input capacitor must be placed as close as possible to the
VIN terminal because during the on cycle it is responsible for
supplying the current to the switcher. During the off cycle, the
current path is from the negative terminal of the COUT cap,
through the diode and inductor, and then to the load. As a result,
COUT and the rectifier diode must share the connection at the
negative terminal of the CIN capacitor in order to reduce ground
bounce when the diode is conducting.
The inductor should be connected as close as possible to the
switching node to minimize noise. Some applications may require
a shielded inductor due to EMI restrictions. This will depend on
the application and parameters defined by the system that will
host the regulator.
The high voltage-switching node could affect RTSET. If longer
off-times are used, the resistance on the RTSET pin can be quite
large. When designing the layout, try to keep RTSET away from
the inductor and switching node. It is also beneficial to keep the
trace as short as possible to reduce the effect of noise injection.
Because of this layout guideline, the TSET pin is located on the
other side of the device, away from the switching node.
The FB resistor network should have a lower impedance to avoid
interference from the switching node. Because the impedance
on the FB node can be controlled, it is not as critical to keep the
network isolated. It is important to keep the ground trace short so
that ground bounce cannot effect the output voltage regulation.
CBOOT
U1
RTSET
R2
GND
CIN1
VIN
GND
CIN2
GND
D1
GND
R1
L1
CBYP
VOUT
COUT
Star Ground
VIN
CBOOT
CIN2 CIN1
1 BOOT
VIN
A8447
D1
ENB
LX
PAD
RTSET
TSET
VBIAS
CBYP
GND
FB
L1
R2
R1
COUT
VOUT
Exposed copper thermal
ground area on the
unpopulated side of the PCB
The large star ground area on the populated side of the PCB, shown in the diagram as the GND nodes, supports high current throughput, and allows
the VOUT node to be located as close as practical to the A8447 (U1). Thermal conduction from the A8447 is enhanced by direct contact of its
exposed thermal pad to the smaller ground area under the A8447. This area is connected by thermal vias to the large copper ground plane on the
unpopulated side of the PCB.
Allegro MicroSystems, Inc.
8
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
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