W-CDMA power supply greatly improves transmission efficiency

As the third generation (3G) era of mobile communications approaches, mobile phone designers are busy developing new solutions to solve a series of new problems brought about by high-speed data transmission. Among them, the most important problems are concentrated on software, screen technology, data processing bandwidth and battery life. In the second generation (2G) mobile phones with only voice and low-speed data functions, the problems were not so severe, allowing some simple and inexpensive solutions to make compromises. For example, the power amplifier (PA) used to transmit signals in typical 2G mobile phones is directly driven by the battery, which is simple but not optimal in efficiency. In 3G mobile phones, high-speed data transmission requires higher bandwidth and transmission power. Therefore, in order to maintain a long enough battery working time, a more efficient solution must be adopted. Now, there is a solution that is gradually becoming widely favored by cellular phone manufacturers, that is, using a highly specialized step-down DC-DC switching regulator to drive the PA.

开关型调节器改善发送效率的基本原理是，通过动态调节功放的供电电压，使其刚好能够满足功放中射频信号的幅度要求(见图1)。采用开关调节器高效率地实现这种调节，在峰值发送功率以外的任何工作条件下，都可大幅度地节省电池功率。因为峰值功率只有在手机远离基站和数据传送时需要，总体来讲，这种方案的省电效果是非常显著的。如果功放的供电电压能够在一个足够宽的范围内高效率地动态调节，就有可能采用固定增益的线性功放，省掉偏置控制(已广泛应用于目前的2G电话)。当然，仍然可以利用偏置控制来进一步增加控制能力，许多蜂窝电话制造商正在积极跟踪这种方案；然而，在W-CDMA技术领域占主导地位的一家公司坚持认为不需要偏置控制。

Figure 1. A switching regulator (MAX1820) dynamically regulates the power supply to a W-CDMA power amplifier (PA). By efficiently regulating the PA power supply, energy consumption is greatly reduced, extending the battery life of the cell phone.

Another important consideration related to system performance is what are the specific performance requirements for this special purpose step-down switching regulator. To understand this, we should first examine the load characteristics of the PA. Figure 2, provided by a major cellular phone manufacturer, shows the load curve of a fixed-gain W-CDMA PA in a bipolar process. At peak transmit power, the PA requires a 3.4V supply voltage and consumes 300mA to 600mA. At minimum transmit power, when close to the base station and transmitting only voice, the PA draws only 30mA of current with a supply voltage of 0.4V to 1V. The corresponding PA power consumption is 2040mW (maximum) and 12mW (minimum), respectively.

Figure 2. A typical load curve for a fixed-gain bipolar W-CDMA power amplifier has a significant resistive component. Supply voltage and current vary from a minimum of 0.4V/30mA (12mW) to a maximum of 3.4V/600mA (2040mW), with voice typically transmitted at 1.5V/150mA (225mW) and high-speed data at 2.5V/400mA (1000mW).

Optimizing a switching regulator for this type of power amplifier as a load is not an easy task. Maxim's MAX1820 W-CDMA cell phone step-down regulator meets this requirement. The following are the special features that set the MAX1820 apart from other types of switching regulators:

High efficiency over a wide load range—Without high efficiency, there is no point in using a switching regulator, so high efficiency and power saving are the main design ideas of the MAX1820 (see Figure 3). When transmitting data (about 500mW to 2040mW), the low on-resistance (0.15Ω) PFET power switch inside the MAX1820 can provide up to 93% efficiency. When transmitting voice (about 12mW to 500mW), the MAX1820's internal 0.2Ω NFET synchronous rectifier and low operating current of 3.3mA (forced PWM mode) enable conversion efficiency of 85%. 85% efficiency may not sound very high, but it is not easy for a converter operating at a constant switching frequency of 1MHz and very light loads. As shown in Figure 3, the converter has extremely low power losses. This is due to excellent design and the use of submicron processes, which can achieve lower gate capacitance for a given FET on-resistance.

Dynamic Adjustment of Output Voltage—The output voltage needs to be adjusted between 3.4V and 0.4V. To achieve this, a digital-to-analog converter (DAC) is used to drive the analog control pin (REF) of the MAX1820. Since the output voltage range of the DAC does not reach 3.4V, the converter has a voltage gain of 1.76 from REF to OUT.

Fast (30?sec) Output Slew Rate and Settling—In W-CDMA system architectures, transmit power needs to be adjusted up or down by 1dB every 666?sec, depending on the base station's requirements. In addition, every 10ms, the handset enters or exits data mode, which results in large transmit power jumps. In each case, the transmit power level must change within 50?sec, but the time available for the switching regulator to change the power supply to the PA is even less, given base station, DAC, and system delays. For this reason, the MAX1820 is specifically designed to change and settle the output voltage within 30?sec, even for full voltage and current changes. Because of the requirement for fast output changes, the MAX1820 output capacitance is limited to only 4.7?F, which presents a challenge for stable operation. The added benefit of 4.7?F capacitance is that it allows the use of low-ESR ceramic capacitors, which reduces the output ripple to 5mVpp. Another problem with buck regulators occurs when the transmit power needs to be reduced quickly, such as when exiting data mode. In this case, the MAX1820 can reverse the current in the inductor and quickly pull the output voltage down to ensure a 30-second settling time. Otherwise, the linearity of the amplifier will change as the supply voltage slowly drops. In addition, this technique also returns the remaining energy in the output capacitor to the battery at the input of the MAX1820, further saving power.

Stable Operation with 9.5% to 100% PWM Duty Cycle and Low Dropout Voltage—Assuming the phone is powered by a single lithium-ion battery, the input voltage range of the switching regulator is approximately 4.2V to 2.7V. To achieve a predictable noise spectrum and low output ripple, the switching frequency should be kept constant. The MAX1820's forced-PWM operation is stable down to a 9.5% duty cycle when the battery is fully charged to 4.2V and the PA supply voltage is 0.4V. This is not difficult in itself, but the opposite extreme should be considered, when a somewhat discharged battery is operating in high-power data transmission mode, requiring a full 100% duty cycle with low dropout voltage. To achieve very low dropout voltage, the PFET inside the MAX1820 is slightly overdesigned with a very low 0.15Ω on-resistance. Assuming the inductor has a 0.1Ω series resistance, the total voltage drop at 600mA is only 150mV, and it decreases as the load decreases. According to cell phone manufacturers, a certain reduction in data transmission distance is acceptable when the battery is discharged below 3.4V. Overcoming this limitation requires the use of slightly more expensive and less efficient buck-boost regulators, which may require another entire article.

1MHz Switching Frequency and Synchronization—The MAX1820 has an internal 1MHz oscillator to control the PWM switching frequency. During the product definition phase of the MAX1820, increasing the switching frequency was a way to reduce the size of external components, but efficiency could be reduced to unacceptable levels. As mentioned earlier, using a fixed-frequency PWM method can achieve a known noise spectrum and low output ripple. The MAX1820's 1MHz internal clock has high accuracy and can guarantee a ±20% tolerance. In addition, in order to synchronize more accurately to the system clock, the MAX1820 also includes a 13-divide clock synthesizer that can be fed with a low-amplitude sine wave from 10MHz to 16MHz.

Figure 3. The MAX1820 step-down switching regulator is optimized for maximum efficiency when transmitting data, when battery drain is greatest. The 1MHz fixed switching frequency reduces output ripple and noise while maintaining relatively high efficiency and low power consumption when transmitting voice.

MAX1820 is currently widely used in 3G mobile phone designs due to its unique performance. As the power saving effect of the switching buck regulator in W-CDMA power amplifier driving has been verified, this solution can also be used for other 3G standards and more different terminal devices, making the ideal of miniaturized, personalized data phones and wireless mobile computing a reality.