User's Manual

LE920A4 HW User Guide Application Guide
Doc#: 1VV0301261 Bypass Capacitor on Power Supplies
Rev. 4.3 Page 104 of 123 2017-12-07
Bypass Capacitor on Power Supplies
When a sudden voltage step is asserted to the module, or a sudden cut of the power
supply occurs, the steep transition causes an overshoot or undershoot. This abrupt
voltage transition can affect the device, causing it not to operate or to malfunction. Bypass
capacitors are needed to alleviate this behavior. Customers must pay special attention to
this issue while designing the application board.
The length and width of the power lines must be considered carefully, and values of the
bypass capacitors must be selected accordingly.
The capacitors also prevent a ripple of the power supplies and the switching noise caused
in TDMA systems, such as GSM.
In particular, suitable bypass capacitors must be placed on the following lines within the
application board:
• VBATT & VBATT_PA
(Pads AP17,AP19,AR18,AR20,AS17,AS19,AT18,AU17,AU19,AT20)
• USB_VBUS (Pad A18)
Recommended values are:
• 100 uF for both VBATT and VBATT_PA together
• 4.7uF for USB_VBUS (including the 1uF capacitor inside the module)
The capacitance mainly depends on the condition of the application board. In general,
higher capacitance values are required for longer power lines.