User's Manual

page 18/64
Note d’étude / Technical document : URD1– OTL
5665.3 – 003 / 72238 Edition 01
Document Sagemcom Reproduction et divulgation interdites
Sagemcom document. Reproduction and disclosure prohibited
4.5.2 Ripples and drops
Figure 14: GSM/GPRS Burst Current rush and VBAT drops and ripples

The minimum voltage during the drop of VBAT must be 3.2V at 33dBm at pads 30 and 31 for the full
range of the required functioning temperature. To reach this aim, adapt the VBAT tracks width to minimize the
loss: the shorter and thicker is the track; the lower is the serial impedance.
To check the serial resistor, any CAD software can be used or by experiment by measuring it on the PCB by
injecting 1A into the VBAT track on connector side and shorting to GND the other side, this could be done using
a laboratory power supply set to few volts with a limitation in current to 1A. Then the measure of the drop
voltage leads to the serial resistor.

Noise on VBAT due to drops could result in poor audio quality.

Serial resistor should be less than 250mΩ including the impedance of connectors if any.

Ripple has to be minimised to have a clean RF signal. This can be improved by filtering the output of the
power supply when AC/DC or DC/DC components are used. Refer to the power converter chip supplier
application note for more information and advises.
4.6 EXAMPLE OF POWER SUPPLIES
4.6.1 DC/DC Power supply from a USB or PCMCIA port.
It the following application note from Linear Technology LTC3440, this schematic is an example of a DC/DC
power supply able to power 3.6V under 2A. This can be use with a AC/DC 5V unit or an USB or PCMCIA bus as
input power source. C6 to C9 can be followed by a serial MOS transistor to avoid a slow rise signal at VOUT.
3.2V Min
Ripple
VBAT drop
Current burst at 1.8A 33dBm
GSM TX Lev 5