LISA-U1 series - System Integration Manual
3G.G2-HW-10002-3
Preliminary
Design-In
Page 101 of 125
2.3
Thermal aspects
The operating temperature range is specified in the
LISA-U1 series
Data Sheet
The most critical condition concerning thermal performance is the uplink transmission at maximum power (data
upload or voice call in connected mode), when the baseband processor runs at full speed, radio circuits are all
active and the RF power amplifier is driven to higher output RF power. This scenario is not often encountered in
real networks; however the application should be correctly designed to cope with it.
During transmission at maximum RF power the LISA-U1 series modules generate thermal power that can exceed
2 W: this is an indicative value since the exact generated power strictly depends on operating condition such as
the number of allocated TX slot and modulation (GMSK or 8PSK) or data rate (WCDMA), transmitting frequency
band, etc. The generated thermal power must be adequately dissipated through the thermal and mechanical
design of the application.
The Module-to-Ambient thermal resistance (R
th,M-A
) of LISA-U1 series modules mounted on a 90 mm x 70 mm x
1.46 mm 4-Layers PCB with a high coverage of copper in still air conditions ranges between 9 and 12 °C/W. The
spreading of R
th,M-A
depends on the operating condition (e.g. 2G or 3G mode, transmit band): the overall
temperature distribution is influenced by the configuration of the active components during the specific mode of
operation and their different thermal resistance toward the case interface.
With this setup, the increase of the module temperature
4
referred to idle state initial condition
5
is:
around 7°C during a voice call at maximum power
19°C during GPRS data transfer with 4 TX slots
16°C during EDGE data transfer with 4 TX slots
up to 25°C in UMTS connection at max TX power
Case-to-Ambient thermal resistance value will be different for other mechanical deployments of the
module, e.g. PCB with different size and characteristics, mechanical shells enclosure, or forced air flow.
The increase of thermal dissipation, i.e. reducing the thermal resistance, will decrease the operating temperature
for internal circuitry of LISA-U1 series modules for a given operating ambient temperature. This improves the
device long-term reliability for applications operating at high ambient temperature.
A few techniques may be used to reduce the thermal resistance in the application:
Forced ventilation air-flow within mechanical enclosure
Heat sink attached to the module top side, with electrically insulated / high thermal conductivity adhesive, or
on the backside of the application board, below the wireless module
Connect each
GND
pin with solid ground layer of the application board and connect each ground area of
the multilayer application board with complete via stack down to main ground layer
For example, after the installation of a robust aluminum heat-sink with forced air ventilation on the back of the
same application board described above, the Module-to-Ambient thermal resistance is reduced to
1.5 ÷ 3.5 °C/W. The effect of lower R
th,M-A
can be seen from the module temperature which now becomes:
around 1.5°C above the ambient temperature during a voice call at maximum power
3°C during GPRS data transfer with 4 TX slots
2.5°C during EDGE data transfer with 4 TX slots
5.5°C in UMTS connection at max TX power
4
Temperature is measured by internal sensor of wireless module
5
Steady state thermal equilibrium is assumed. The module’s temperature in idle state can be considered equal to ambient temperature
