User's Guide
LE940B6 HW User Guide
Rev. 2.02
Page 60 of 111 2020-01-10
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 2.0A. 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.
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 (typically automotive)
• 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 LE940B6 module. A 100 μF tantalum
capacitor is usually suitable on both VBATT and VBATT_PA power lines.
• 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
LE940B6 module from power polarity inversion.
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