NL865B1 HW Design Guide
1VV0301450 Rev. 1
Page
28
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72
2017-11-20
4.3.3.
Power Supply PCB layout Guidelines
As seen on the electrical design guidelines the power supply shall have a low ESR capacitor
on the output to cut the current peaks on the input to protect the supply from spikes. The
placement of this component is crucial for the correct working of the circuitry. A misplaced
component can be useless or can even decrease the power supply performances.
•
The Bypass low ESR capacitor must be placed close to the Telit NL865B1 power
input pads or in the case the power supply is a switching type it can be placed close
to the inductor to cut the ripple provided the PCB trace from the capacitor to the
NL865B1 is wide enough to ensure a dropless connection even during an 1A current
peak.
•
The protection diode must be placed close to the input connector where the power
source is drained.
•
The PCB traces from the input connector to the power regulator IC must be wide
enough to ensure no voltage drops occur when a 500mA current peak is absorbed.
•
The PCB traces to the NL865B1 and the Bypass capacitor must be wide enough to
ensure no significant voltage drops occur. This is for the same reason as previous
point. Try to keep this trace as short as possible.
•
To reduce the EMI due to switching, it is important to keep very small the mesh
involved; thus the input capacitor, the output diode (if not embodied in the IC) and
the regulator have to form a very small loop. This is done in order to reduce the
radiated field (noise) at the switching frequency (100-500 kHz usually).
•
A dedicated ground for the Switching regulator separated by the common ground
plane is suggested.
•
The placement of the power supply on the board should be done in such a way to
guarantee that the high current return paths in the ground plane are not overlapped
to any noise sensitive circuitry as the microphone amplifier/buffer or earphone
amplifier.
•
The power supply input cables should be kept separate from noise sensitive lines
such as microphone/earphone cables.
•
VBATT and VBATT_PA can be connected together
•
The insertion of EMI filter on VBATT pins is suggested in those designs where
antenna is placed close to battery or supply lines.
A ferrite bead like Murata BLM18EG101TN1 or Taiyo Yuden P/N
FBMH1608HM101 can be used for this purpose.
The below figure shows the recommended circuit:
