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GE864 Hardware User Guide
1vv0300694 Rev.10 - 10/06/08
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved page 20 of 69
The GSM system is made in a way that the RF transmission is not continuous, else it is packed into
bursts at a base frequency of about 216 Hz, the relative current peaks can be as high as about 2A.
Therefore the power supply has to be designed in order to withstand with 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 and the
supply; this will reflect on all the audio paths producing an audible annoying noise at 216 Hz; if the
voltage drop during the peak current absorption is too much, then the device may even shutdown as a
consequence of the supply voltage drop.
TIP: The electrical design for the Power supply should be made ensuring it will be capable of a peak current output
of at least 2 A.
5.2 General Design Rules
The principal guidelines for the Power Supply Design embrace three different design steps:
• the electrical design
• the thermal design
• the PCB layout.
5.2.1 Electrical Design Guidelines
The electrical design of the power supply depends strongly from the power source where this power is
drained. We will distinguish them into three categories:
• +5V input (typically PC internal regulator output)
• +12V input (typically automotive)
• Battery
5.2.1.1 + 5V input Source Power Supply Design Guidelines
• The desired output for the power supply is 3.8V, hence there’s not a big difference between the
input source and the desired output and a linear regulator can be used. A switching power supply
will not be suited because of the low drop out requirements.
• When using a linear regulator, a proper heat sink shall be provided in order to dissipate the power
generated.
• A Bypass low ESR capacitor of adequate capacity must be provided in order to cut the current
absorption peaks close to the GE864-QUAD / PY, a 100μF tantalum capacitor is usually suited.
• Make sure the low ESR capacitor on the power supply output (usually a tantalum one) is rated at
least 10V.
• A protection diode should be inserted close to the power input, in order to save the GE864-QUAD /
PY from power polarity inversion.