Traditional PC power supply way is to use fixed battery to directly power, battery voltage would change between 3V and 4.2V according to different capacity. With the service time changing, the longer the time is, the lower the voltage is.
“If we fix on some moment, for instance, the moment with 80% power, we probably have a voltage of about 3.8V. For RF signals, the battery voltage we see at this moment can be approximately equal to a constant voltage,” Pan Jing said, as the vast majority of RF PA is a linear PA, the so-called linear PC means its output is analog. The analog output can cause a problem, that is, the remaining portion of the voltage will be consumed through heat if the supply voltage is higher than the PA-desired output voltage when PA is outputting voltage. If in the condition of conventional battery powering, the entire PA power loss is quite serious, this not only will result in the low efficiency, too fast battery drain, but also lead to very serious PA heating problem.
It is revealed that as early as five or six years ago, or even earlier, before acquiring National, TI had began the study of improving traditional power supply scheme. The major approach is to insert a new DC / DC (LM3269) in the originally direct battery to VCC power supply of RF PA, and there is V Control signal controller in DC/DC to control DC/DC voltage.
There will be a low-power, middle-power or high power mode conversion when PA is at work; under these different modes, PA will change the output values on the basis of different usage scenarios, such as the strength of the station signal of PA or the emitted power size. Based on the different average voltage peak in the condition of different PF PA data power, the major function of the DC/DC is to adjust PA’s DC/DC voltage to relatively lower level, lower than the original resistive voltage. Ideally, if the proportion of input power and output power is 1:1, the output voltage is low, while input voltage is high, and then the input current of PA would be smaller. The decline of current would bring a lot of benefits, including extending battery’s service time, service life; on the other hand, improving the performance of the overall thermal dissipation, decreasing PA power consumption, which will largely enhance handset users’ experience, according to the testing of two DEMO boards. Without adopting DC/DC, the directly power-supplied DEMO board would reach its temperature of 90.5 degrees; by contract, if increasing DC/DC converter, the DEMO board can decline the temperature of 30%. In addition, when TI DC/DC resolution is included, the current can achieve current decline of 20% to 50%, as such the overall RF PA module power consumption can get even lower dropping, which is very good to promote phone product design and product reliability.
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DC/DC converter