原创 通过改变PAVcc来减小CDMAPA中低功率下的电流消耗的理论分析与方法研究

通过改变PA Vcc来减小CDMA PA中低功率下的电流消耗的理论分析与方法研究（PassionRFSOS）

最近这一两年，随着对手持终端的功能越来越强大什么MP3，MP4，CAMERA，GPS。从而也导致整机消耗的电流也越来越大，对于耗电最大的占整机近1/3 的PA，一方面PA的设计者采用更加先进的工艺和技巧和提高PA本身效率减小电流消耗，另一方面，人们对PA电流优化外部的应用技巧也一直在进行着，我写的文章主要是把我最近半年留意到的一种技术，通过在中低功率下改变PA Vcc电压来有效减小电流消耗的技术的介绍。现在已经有很多厂家有了类似的IC，如Maxim MAX1820 ，MAX8506，National LM3208，Ana logic Technology 的AAT1171 。If you search the relative technique article from the google , you could find more information。<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

From the figure below, we can find the new method which use a changeable Voltage provides to PA Vcc according to PA out power. When PA working on High power mode, the Vcc will bypass connect to <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />Battery to ensure enough current support. While during the low power mode, the PA Vcc will decrease to a proper voltage value so that dramatically save the current. The traditional way is PA Vcc is always directly connect with Battery, thus, PA efficiency could get best status all the time.

While, I think if you really want to use this way, a challenge you have to face is to decide the proper Vcc value when PA is lower power mode. If that Vcc you choose is too higher or too lower, maybe you could not get the best efficiency as you except before.

Ok, here is beginning my brief introduction about this technique.

The figure above is relationship about transmitting power and its happened probability in urban and suburban environment. Statistical investigation shows that in the suburban area, 80% of the phone call can be covered with less than 10dBm power, while in a urban area, the necessary power for covering 80% of the calls is even lower, to 5dBm. I have done a number of PA load pull design when PA at Max power, may be we should change the idea now to mainly design how to optimize the current when PA output at around 0dBm .

Next , I will introduce some basically theory about PA application

The RF PA of CDMA, WCDMA require enough headroom in the supply voltage to keep high linearity in the transmitting signal

If the voltage supply does not have enough headroom, distortion in the transmitting signal causes the ACPR to increase, resulting in communication error. 

But most of the time, when the cell phone is close to the base station, much lower transmitting power is required.

In this case if the power is supplied from the battery, excessive headroom will cause significant power loss.

A better solution is to decrease the supply voltage to a much lower level where sufficient voltage headroom is still maintained. In this way the power dissipation is reduced considerably.

Dynamical voltage control scheme saves battery power most effectively when the necessary Pout is in a low level , as a result, talk time can be increased dramatically as well.

The figure above shows several different decided voltages in term of output power. In different changeable Vcc, the PAE will have big change.

1. 3.4v for all time,

2, tow steps, high mode is 3.4v, while 1.25v for middle and low mode. This should be common way. It’s easy to realize.

3. Four Steps 3.4V 1.5V 1.0V, 0.6v, this should be more accurate than two mode, but make you SW become complex.

4. 0.6V to 3.4C continuous

The figure below shows another changeable Vcc value according to four power step.

The theory and test before same quite make sense, then, I select one PA demo board, did several experiments according to that method, first, make the PA at lower power mode, output 5dBm, the highest probability happen for phone transmitter power.

 I fixed VCC to 3.4V, slight reduce Vref from 3.0 to 2.8V for 0.5V each step, the Current reduce 17mA overall. It’s seems that lower Vref is a way to reduce current. But 17mA is not quite enough , at same time , the PA manufacture will not recommend you to reduce the Vref for it’s importance, it’s better set that value at typical value. So, I changed the Vref back to its typical voltage 2.9V.

Because the in PA High Mode, there is no any chance to reduce current through change Vcc ,while you can use load pull skill optimize current to reduce 30 to 50mA . I set PA works on low power mode and carefully reduce the Vcc value.

one thing we have to concern is that when PA works on 16dBm ,the edge low power mode ,before jump to 3.4V Vcc high power mode, If the reduce Vcc is too low ,you will find the ACPR at that point is very worse ,can not ensure the PA necessary linearity, Finally I choose Vcc at 1.5V ,thus even the at 16dBM , the ACPR still have 4dB Margin to ensure the first stage PA has better performance. If the power is increase to 17dBm , the SW will change PA mode , turn on the both two stage internal PA to works on in high more ,and Vcc increase to 3.4V or connect to Battery directly.

Here is some my finally test data, the current reduction is not obviously as I expect. At 5dBm point only reduce 8 mA, at 0dBm point reduce about 10mA. This unsatisfied test result maybe due to the PA I selected or instrument and cable loss compensation. The main reason I think is because the PA inherent performance, when decrease Vcc at low power mode, some one could save more current, some one maybe not obviously.