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I'd like to use TPS65072 for my uC li-ion baterry design. The first DCDC will be set on 3.3V and supply uC I/O, Nand Flash and few other peripherals. The question is will it be enough to supply also GSM module. It's specs are 3.3-4.5V, 250mA avg. and 1.5A peak!. There will be two 1000uF caps next to it's pins.
Common practice for an RF PA design is to make sure the power supply is capable of providing the average RF PA current. During transmission, the load current goes higher than the power supply can deliver and the power supply operates in current limit. The output voltage is allowed to droop during this time. The amout of droop is a function of the power supply max current, the load current, the load capacitance, and the transient duration. This can be calculated starting with the basic equation i=c*dv/dt. "i" is the current out of the capacitor which is equal to the load current minus the maximum power supply current. Size your capacitor to give you the maximum voltage droop you can allow.
Unfortunately, the TPS65073 will not be stable with large output capacitors. A 47uF output capacitor may be possible if you use the smallest recommended inductor value. Ideally, you want to keep the output filter LC corner frequency within the recommended range set by the recommended output L and C values. Increasing the capacitance requires decreasing the inductance. A better option for powering the RF PA might be to use the TPS62700 or TPS62701, both of which are designed to power an RF PA.
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In reply to Matthew Day:
I though about using another DCDC converter to supply GSM module, but how to supply itself? When powered from USB there is no problem to step down from 5 to 3.3V. But when powered from battery the SYS pin is only 3.7V. Any suggestions?
In reply to Krzysztof Marcinek:
do I get your email correct that you need a fixed 3.3V for the RF-_PA?
There are step-down converters that allow a so called 100% mode which meanxs they regulate the output voltage until the high side power switch is continuously turned on. In this case there is only the voltage drop of the high side switch + the external inductor which. TPS62290 is a typical device for 1A of dc current with a 480mR maximum resistance for the high side switch. For 1A it therefore needs a minimum input voltage for regulation of (480mR x 1A) + 3.3V = 480mV +3.3V = 3.78V
In case the drop is too high for your application, we may have to find a step-down converter with a lower resistance on the high side switch or use a buck-boost converter e.g. TPS63020
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