Telit Communications S.p.A. HE910 Benutzerhandbuch
HE910 Hardware User Guide
1vv0300925 Rev.9
– 07-02-2012
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved
page 37 of 80
5.3.3 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 and a protection diode on the input to
protect the supply from spikes and polarity inversion. The placement of these
components is crucial for the correct working of the circuitry. A misplaced component
can be useless or can even decrease the power supply performances.
capacitor on the output to cut the current peaks and a protection diode on the input to
protect the supply from spikes and polarity inversion. The placement of these
components 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 HE910 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 HE910 is
wide enough to ensure a dropless connection even during the 2A current peaks.
inductor to cut the ripple provided the PCB trace from the capacitor to the HE910 is
wide enough to ensure a dropless connection even during the 2A current peaks.
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 the 2A current peaks are absorbed.
Note that this is not made in order to save power loss but especially to avoid the voltage
drops on the power line at the current peaks frequency of 216 Hz that will reflect on all
the components connected to that supply, introducing the noise floor at the burst base
frequency. For this reason while a voltage drop of 300-400 mV may be acceptable from
the power loss point of view, the same voltage drop may not be acceptable from the
noise point of view. If your application doesn't have audio interface but only uses the
data feature of the Telit HE910, then this noise is not so disturbing and power supply
layout design can be more forgiving.
Note that this is not made in order to save power loss but especially to avoid the voltage
drops on the power line at the current peaks frequency of 216 Hz that will reflect on all
the components connected to that supply, introducing the noise floor at the burst base
frequency. For this reason while a voltage drop of 300-400 mV may be acceptable from
the power loss point of view, the same voltage drop may not be acceptable from the
noise point of view. If your application doesn't have audio interface but only uses the
data feature of the Telit HE910, then this noise is not so disturbing and power supply
layout design can be more forgiving.
The PCB traces to the HE910 and the Bypass capacitor must be wide enough to ensure
no significant voltage drops occur when the 2A current peaks are absorbed. This is for
the same reason as previous point. Try to keep this trace as short as possible.
the same reason as previous point. Try to keep this trace as short as possible.
The PCB traces connecting the Switching output to the inductor and the switching diode
must be kept as short as possible by placing the inductor and the diode very close to
the power switching IC (only for switching power supply). This is done in order to
reduce the radiated field (noise) at the switching frequency (100-500 kHz usually).
the power switching IC (only for switching power supply). This is done in order to
reduce the radiated field (noise) at the switching frequency (100-500 kHz usually).
The use of a good common ground plane is suggested.
The placement of the power supply on the board should be done in such a way to
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.
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.