Publication date: 13 July 2010
GSM modules are increasingly being integrated into different types of devices, demonstrating that having a wireless communication functionality is essential not only for consumer items such as computers, modems and PDAs, but also for industrial and automotive applications. GSM protocol requires specific current pulses, but if the capacitance and ESR of the system are not carefully designed, the voltage drop caused by the current load can result in the GSM module resetting itself.
Smart metering has emerged as a major European market for GSM modules, driven by EU legislations, and a lot of attention is being paid to energy-saving measures and energy flow control in buildings in order to reduce wastage. A GSM connection is used to transmit the measured data to a control unit, so that regulating actions can be taken if necessary. In the automotive sector, GSM tracking has been implemented in some countries to fight the criminality. Other applications include glucose meters, PC/Smart Cards and solid state drives.
The GSM protocol needs 2A current pulses of 0.577ms duration every 4.6ms to achieve a maximum RF power level of 2W. During the low current period, the current consumption of a GSM call is about 70mA. Supply voltage is 3.8V. Simulated GSM conditions demonstrate that for a 2W GSM module, a total capacitance of 6mF is sufficient to deliver the 2A pulse with acceptable voltage drop. Thus the values of capacitance and ESR of the chosen capacitor solution are the key parameters to consider.
The required capacitance depends on the maximum acceptable voltage drop and on the parameters of power supply. Application conditions and design limitations determine which capacitor type is most appropriate.
High-CV tantalum and niobium oxide capacitors are highly suitable where there are no space constraints, such as automotive and domestic energy meters. For application voltages of 3.8V a capacitor rating of 6.3V is sufficient up to 85degC operating temperature, if current surges can be avoided. Low ESR is needed to slow down the voltage drop at the same time. For designs without any significant profile limitation standard case size capacitors are sufficient, such as AVX’s TPS tantalum series which offers 1mF/6.3V (40mΩ) in the V-case (7361-35) and now newly in the E-case (7343-41) with 100mΩ ESR as a downsizing option. For automotive or other uses where higher robustness is needed, devices such as AVX’s niobium oxide NOS V Oxicap® capacitor (470µF/6.3V, 75mΩ ESR) can be chosen, connecting several parts in parallel to achieve the required capacitance. Oxicap® is suitable especially for designs requiring lower transmission power.
In the design of a handheld metering or Smart/PC Card device, there is often a limitation on capacitor profile – often due to a display module or metal cover shield, under which the capacitor must fit. For this type of product, AVX’s PulseCap tantalum solid electrolytic chip capacitor is highly suitable. TLN series capacitors offer a high CV of 2.2mF/6.3V in a flat 2mm TLN6-case (14.5 x 7.8 x 2.0mm) or 1.5mF/6.3V in the TLN4-case coming soon. A 1mF/6.3V device will soon be available in TLN Y-case, and all parts can be used up to 105degC. For higher temperature applications, the 1mF/10V TAN PulseCap, currently upon release, operates up to 125degC.
