User's Guide

UC864-E-AUTO / AWS-AUTO Hardware User Guide
1vv0300795 Rev.11 – 2010/11/18
Reproduction forbidden without Telit Communications S.p.A’s. written authorization - All Rights Reserved. Page 72 of 78
15.2. Bypass capacitor on Power supplies
When a sudden voltage is asserted to or cut from the power supplies, the steep
transition makes some reactions such as the overshoot and undershoot.
This abrupt voltage transition can affect the device not to work or make it
malfunction.
The bypass capacitors are needed to alleviate this behavior and it can be affected
differently according to the various applications. The customers have to pay special
attention to this when they design their application board..
The length and width of the power lines need to be considered carefully and the
capacitance of the capacitors need to be selected accordingly.
The capacitor will also avoid the ripple of the power supplies and the switching noise
caused in TDMA system like GSM.
Specially the suitable bypass capacitor must be mounted on the Vbatt (Pin 1,2,3,4)
and USB_VBUS (Pin 48) lines in the application board.
The recommended values can be presented as;
 100uF for Vbatt
 10uF for USB_VBUS
But the customers still have to consider that the capacitance mainly depends on the
conditions of their application board.
Generally more capacitance is required as the power line is longer.
15.3. SIM interface
The resistor value on SIMIO pulled up to SIMVCC should be defined accordingly in
order to be compliant to 3GPP specification.
6.8kohm can be recommended but it may depend on the application design..
Refer to the following document for the detail;
 Telit_SIM_interface_and ESD_protection_Application_note_r1