L
LE920 Hardware User Guide
1vv0301026 Rev.0 - draft4 – 2013-05-
Reproduction forbidden without written authorization from Telit Communications S.p.A. - All Rights Reserved.
Page 69 of 76
14.2.
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 & Vbatt_PA (Pads
AP17,AP19,AR18,AS17,AS19,AT18,AU17,AU19) and USB_VBUS (Pad A18) lines in the
application board.
The recommended values can be presented as:
z
100uF for Vbatt
z
10uF for USB_VBUS
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.
14.3.
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 resistor can be recommended but it may depend on the application design.
Refer to the following document for the detail:
z
Telit_SIM_interface_and ESD_protection_Application_note_r1
