User's Manual

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User Manual 2012/06/28
3.4.2 Ripples and drops
Figure 11: GSM/GPRS Burst Current rush and VBAT drops and ripples
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The minimum voltage during the drop of VBAT must be 3.3V at 33dBm 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 tracks on connector side and shorting the other side to GND, 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.
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Noise on VBAT due to drops could result in poor audio quality.
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Serial resistor should be less than 250mΩ including the impedance of connectors.
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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 advice.
3.4.3 EXAMPLE OF POWER SUPPLIES
3.4.3.1 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 used with an 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.
3.3V Min
Ripple
VBAT drop
Current burst at 1.8A 33dBm
GSM TX Lev 5