Integration Manual

LISA-U2 series - System Integration Manual
UBX-13001118 - R25 Design-In Page 124 of 182
 Given the large burst current, VCC line is a source of disturbance for other signals. Therefore route
VCC through a PCB area separated from sensitive analog signals. Typically it is good practice to
interpose at least one layer of PCB ground between VCC track and other signal routing
 The VCC supply current supply flows back to main DC source through GND as ground current:
provide adequate return path with suitable uninterrupted ground plane to main DC source
 A tank bypass capacitor with low ESR is recommended to smooth current spikes. This is most
effective when placed close to the VCC pins. If the main DC source is a switching DC-DC converter,
place the large capacitor close to the DC-DC output and minimize the VCC track length. Otherwise
consider using separate capacitors for DC-DC converter and LISA-U2 module tank capacitor. Note
that the capacitor voltage rating may be adequate to withstand the charger over-voltage if
battery-pack is used. The use of very large capacitors (i.e. greater then 1000 µ F) must be carefully
evaluated, since the voltage at the VCC pins must ramp from 2.5 V to 3.2 V within 1 ms to allow a
proper switch-on of the module
 VCC is directly connected to the RF power amplifiers. It is highly recommended to place a series
ferrite bead for GHz band noise, a bypass capacitor with Self-Resonant Frequency in 800/900 MHz
range and a bypass capacitor with self-resonant frequency in 1800/1900 MHz range as close as
possible to the VCC pins, especially if the application device integrates an internal antenna. This
is described in Figure 9 and Table 10.
 Since VCC is directly connected to RF Power Amplifiers, voltage ripple at high frequency may
result in unwanted spurious modulation of transmitter RF signal. This is more likely to happen with
switching DC-DC converters, in which case it is better to select the highest operating frequency
for the switcher and add a large L-C filter before connecting to the LISA-U2 modules in the worst
case
 The large current generates a magnetic field that is not well isolated by PCB ground layers and
which may interact with other analog modules (e.g. VCO) even if placed on opposite side of PCB. In
this case route VCC away from other sensitive functional units
 The typical GSM burst has a periodic nature of approx. 217 Hz, which lies in the audible audio range.
Avoid coupling between VCC and audio lines (especially microphone inputs)
 If VCC is protected by transient voltage suppressor / reverse polarity protection diode to ensure
that the voltage maximum ratings are not exceeded, place the protecting device along the path
from the DC source toward the LISA-U2 module, preferably closer to the DC source (otherwise
functionality may be compromised)
☞ VCC line should be as wide and as short as possible.
☞ Route away from sensitive analog signals.