Improving RF PA system efficiency using DC/DC converters

From a power budget perspective, the radio frequency power amplifier (RF PA) powered directly by the battery is an important component to consider. Traditionally, RF power amplifiers used in CDMA/WCDMA cellular standards have been directly powered by batteries. This power supply method makes the system easy to design. However, the actual efficiency of the linear power amplifiers used in this standard over the entire transmit power range Very low. This article describes a solution to provide efficient RF PA system power management through DC/DC converters.

With the continuous development of cellular standards, the transmission rate has developed from 14.4kbps in the CDMA-1 standard to 2Mbps in the CDMA2000/WCDMA standard. In addition, in order to increase the average revenue obtained from each user, cellular communication operators have begun to increase 3G phone-related services. At the same time, talk time and battery life are also expected to be improved by using batteries with the same or slightly higher capacity. This makes system design more challenging. System designers must be very careful to examine the power of every component on the mobile phone circuit board. From a power budget perspective, the radio frequency power amplifier (RF PA) powered directly by the battery is an important component to consider.

The modulation circuitry used in CDMA and WCDMA results in the generation of an AM signal that exhibits a non-constant amplitude envelope. To maintain signal integrity and facilitate spectrum regeneration, a linear power amplifier is required. However, since the power amplifier can only maintain high efficiency when operating under gain compression conditions, the conversion efficiency is not high. To achieve the required linearity, the actual transmit power is compensated from the power amplifier's compression point, which results in an overall reduction in efficiency. When the mobile phone works in transmitting mode, due to the low actual efficiency of the power amplifier, the power consumption of the radio frequency part will account for 65% of the total power budget.

Figure 1: Old vs. new methods

Therefore, using a magnetic buck converter for power supply is an ideal choice for linear power amplifiers, which will greatly improve the efficiency of the system. Increased power efficiency (PAE) is the primary performance indicator of power amplifiers.

PAE(%)= (POUT-PIN)/Pdc

The main purpose of using a DC-DC converter (power amplifier power regulator) is to reduce the Pdc factor in the denominator. When the power amplifier is directly connected to the battery, Pdc=Vbatt*Ibatt, and when it is powered by the power amplifier power regulator, Pdc=V _o *I _o . Now we can see that in order to increase PAE, Vo _{and I} must be lower than Vbatt and Ibatt. This can be accomplished by reducing the transmitted RF power level and reducing the output voltage of the power amplifier supply regulator. This also reduces I _o (the current absorbed by the power amplifier), and due to the high efficiency of the DC-DC converter, the output current of the battery is also reduced.

To truly understand the power savings a power regulator brings to a power amplifier, it is important to consider power probability plots for different modulation methods (see Figure 2). Power probability maps will differ in urban and rural areas.

Figure 2: In a standard mobile phone, the power amplifier emits at low power levels most of the time. Using a power amplifier supply voltage regulator increases the probability of power savings.

As shown in Figure 3, in order to meet the adjacent channel power rejection ratio (ACPR) requirements, the output voltage of the DC-DC converter must change as the transmit power level changes. Battery current can be saved up to 50mA over the power level range of 0d Bm to 20d Bm. Figure 2 shows that the power amplifier operates within this power level range most of the time.

Figure 3: Battery current savings when the power amplifier is powered by a DC-DC converter

Figure 4: Percent power savings when the power amplifier is powered by a power regulator

So why do we have to change the voltage of the DC-DC converter when the transmit power level increases? The answer is: This change is needed to maintain the Adjacent Channel Power Rejection Ratio (ACPR). ACPR is used to indicate the distortion condition of the power amplifier and the tendency of other subsystems or systems to cause adjacent channel interference. It refers to the ratio of the power spectral density (PSD) of the main channel to the power spectral density measured at several different offset frequencies.

Figure 5: How the power amplifier’s supply voltage and POUT affect ACLR

Figure 5 shows that when P _out increases, if the supply voltage of the power amplifier does not increase, the performance index of ACLR cannot meet the requirements.

The system-level indicator (3 GPP) of WCDMA is -34d Bc. In order to maintain sufficient margin (caused by differences in temperature and devices), the value of ACLR is generally set at -38d Bc.

Key Requirements for Buck Converters Powering RF Power Amplifiers

The buck converter that powers the RF power amplifier has special functions that are very different from the buck converter that powers the digital core processor. They differ mainly in operating characteristics and parameters such as switching FET on-resistance, current limit, transient response, operating mode (such as PFM/PWM), start-up time, quiescent current and voltage drop. The following examples illustrate these differences:

High efficiency over a wide range of output voltage and load range

Example: When VIN=4.2V, V _o =3.4V, I _o =400mA (high RF power), the efficiency of LM3205 reaches 96%, when VIN=3.9V, V _o =1.5V, I _o =100mA (low RF power), the efficiency is 87%.

Fast turn-on to meet transmit on/off timing requirements

Example: When V ₀ =3.4V, when EN changes from low to high, the on-time of LM3203 is 50 microseconds.

100% duty cycle vs. bypass mode

When the buck converter is operating at 100% duty cycle, the voltage drop is:

Voltage drop = (RON,P+RL)·IO,

Here, R _ON,P is _{the RDSON} of the PFET and R _L is the DCR of the inductor. For a power amplifier power regulator with a bypass FET, the voltage drop in bypass mode is:

Voltage drop = (RON, BYP)·IO,

Here, R _{ON,BYPDSON DSON}

application circuit example

As shown in Figure 6, in this example, the baseband has a lookup table circuit that sets the output voltage based on the desired output power level.

Figure 6: Baseband directly controls Vo

In the case of Figure 7, the power detector is part of the closed loop and sets the output voltage.

Figure 7: Using the power detector to set Vo

in conclusion

DC-DC converters improve the efficiency of RF power amplifier systems in handheld mobile devices and enable additional features and functionality through increased battery life.