LISA-U2 series - System Integration Manual
UBX-13001118 - R19
Early Production Information
Design-In
Page 120 of 175
2.2.1.2
Main DC supply connection
The DC supply of LISA-U2 modules is very important for the overall performance and functionality of the
integrated product. For detailed description, check the design guidelines in section 1.5.2. Some main
characteristics are:
VCC
pins are internally connected, but it is recommended to use all the available pins in order to minimize
the power loss due to series resistance
VCC
connection may carry a maximum burst current in the order of 2.5 A. Therefore, it is typically
implemented as a wide PCB line with short routing from DC supply (DC-DC regulator, battery pack, etc)
The module automatically initiates an emergency shutdown if supply voltage drops below hardware
threshold. In addition, reduced supply voltage can set a worst case operation point for RF circuitry that may
behave incorrectly. It follows that each voltage drop in the DC supply track will restrict the operating margin
at the main DC source output. Therefore, the PCB connection must exhibit a minimum or zero voltage drop.
Avoid any series component with Equivalent Series Resistance (ESR) greater than a few milliohms
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 µF) 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.
