LP3907SQ-JXQXEV National Semiconductor, LP3907SQ-JXQXEV Datasheet - Page 15

• Manufacturer Part Number: LP3907SQ-JXQXEV
• Description: LP3907 EVALUATION BOARD
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LP3907SQ-JXQXEV

• Main Purpose: DC/DC, Step Down with LDO
• Outputs And Type: 4, Non-Isolated
• Current - Output: 1A, 600mA, 300mA, 300mA
• Regulator Topology: Buck
• Frequency - Switching: 2.1MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LP3907
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Output: -
• Voltage - Input: -
• Power - Output: -

PCB Layout Considerations
The evaluation board layers from top to bottom are:
1.
2.
3.
4.
For good performance of the circuit, it is essential to place the
input and output capacitors very close to the circuit and use
wide routing for the traces allowing high currents.
Sensitive components should be placed far from those com-
ponents with high pulsating current.
Decoupling capacitors should be close to circuit’s VIN pins.
Digital and analog ground should be routed separately and
connected together in a star connection.
It’s good practice to minimize high current and switching cur-
rent paths.
LOW DROP OUT REGULATORS
Place the filter capacitors very close to the input and output
pins. Use large trace width for high current carrying traces and
the returns to ground.
BUCK REGULATORS
Place the supply bypass, filter capacitor, and inductor close
together and keep the traces short. The traces between these
components carry relatively high switching current and act as
antennas. Following these rules reduces radiated noise.
Top, component side
Ground plane
Mid signal section
Bottom, solder side
15
Arrange the components so that the switching current loops
curl in the same direction.
Connect the buck ground and the ground of the capacitors
together using generous component-side copper fill as a
pseudo-ground plane. Then connect this back to the general
board system ground plane at a single point. Place the pseu-
do-ground plane below these components and then have it
tied to system ground of the output capacitor outside of the
current loops. This prevents the switched current from inject-
ing noise into the system ground. These components along
with the inductor and output should be placed on the same
side of the circuit board, and their connections should be
made on the same layer.
Route noise sensitive traces such as the voltage feedback
path away from the inductor. This is done by routing it on the
bottom layer or by adding a grounded copper area between
switching node and feedback path. To reduce noisy traces
between the power components, keep any digital lines away
from this section. Keep the Feedback node as small as pos-
sible so that the ground pin and ground traces will shield it
from the SW or buck output.
Use wide traces between the power components and for
power connections to the DC-DC converter circuit to reduce
voltage errors caused by resistive losses.
For the sense lines, make sure to use a Kelvin contact con-
nection.
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