User's Manual

LE920 Hardware User Guide
1vv0301026 Rev.8 2015-01-03
Reproduction forbidden without written authorization from Telit Communications S.p.A. - All Rights
Reserved. Page 76 of 88
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 overshoot and undershoot.
This abrupt voltage transition can affect the device causing it to not work or make it
malfunction.
Bypass capacitors are needed to alleviate this behavior. The behavior can be affected
differently according to the various applications. Customers must 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 prevent ripple of the power supplies and the switching noise caused in
TDMA systems like GSM.
Especially, a 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:
 100uF for Vbatt
 4.7uF for USB_VBUS (including the 1uF capacitor inside the module).
Customers must still consider that the capacitance mainly depends on the conditions of their
application board.
Generally more capacitance is required when the power line is longer.