LE910Cx
Hardware User Guide
1VV0301298 Rev. 1.05 - 2017-06-18
Reproduction forbidden without written authorization by Telit Communications S.p.A. - All Rights Reserved
Telit Confidential Information, provided under NDA
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55 of 117
NOTE:
In GSM/GPRS mode, RF transmission is not continuous, but is packed into bursts at a base
frequency of about 216 Hz with relative current peaks as high as about 2A. Therefore, the power
supply must be designed to withstand these current peaks without big voltage drops. This means
that both the electrical design and the board layout must be designed for this current flow.
If the layout of the PCB is not well designed, a strong noise floor is generated on the ground. This
will reflect on all the audio paths producing an audible annoying noise at 216 Hz.
If the voltage drops during the peaks, current absorption is too high. The device may even shut
down as a consequence of the supply voltage drop.
6.2.
General Design Rules
The principal guidelines for the Power Supply Design embrace three different design steps:
•
Electrical design
•
Thermal design
•
PCB layout
6.2.1.
Electrical Design Guidelines
The electrical design of the power supply depends strongly on the power source where this power
is drained. Power sources can be distinguished by three categories:
•
+5V input (typically PC internal regulator output)
•
+12V input
•
Battery
6.2.1.1.
+ 5V Input Source Power Supply – Design Guidelines
•
The desired output for the power supply is 3.8V. So, the difference between the input
source and the desired output is not big, and therefore a linear regulator can be used. A
switching power supply is preferred to reduce power consumption.
•
When using a linear regulator, a proper heat sink must be provided to dissipate the power
generated.
•
A bypass low ESR capacitor of adequate capacity must be provided to cut the current
absorption peaks close to the LE910Cx module. A 100
μF tantalum capacitor is usually
suitable (on both VBATT and VBATT_PA together).
•
Make sure that the low ESR capacitor on the power supply output (usually a tantalum one)
is rated at least 10V.
•
A protection diode must be inserted close to the power input to protect the LE910Cx
module from power polarity inversion.