GC864 Hardware User Guide
1vv0300874 Rev.1 – 2010-03-29
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved.
Page 23 of 64
The GSM system is made in a way that the RF transmission is not continuous but 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, then a strong noise floor is generated on
the ground and the supply; this will reflect on all the audio paths producing an
audible and 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 power supply must be designed so that it is capable of a peak current output of
at least 2 A.
TIP: the supply voltage is directly measured between VBATT and GND pins. It must
stay within the Wide Supply Voltage tolerant range including any drop voltage and
overshoot voltage (during the slot tx, for example).
NOTE: The Operating Voltage Range MUST never be exceeded also in power off
condition; care must be taken in order to fulfill min/max voltage requirement
NOTE: When the power supply voltage is lower than 3.4V, to turn ON the module, the
pad ON# must be tied low for at least 3 seconds.
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
