Efficient RF power management for 2g to 3G cellular mobile devices

Have you noticed the degree to which people are fascinated by mobile devices, especially smartphones? People are using smartphones to surf the Internet, check and write emails, play online games, or update social networks. All these activities, coupled with voice calls, consume battery energy, from power amplifiers (PA) to displays and kernel chipsets, which consume a lot of energy. In the end, the size of the lithium-ion battery can only be increased to ensure that the smartphone is not too heavy.

However, there are several ways to make users satisfied, so as to avoid frequent charging for smartphones or exhausting battery energy at an inappropriate time. The power amplifier consumes about 30% of the total power in smartphones. In areas with poor network coverage, this ratio is as high as 70%. Generally, smartphones and mobile phones are required to be compatible with previous generations of cellular network protocols. Most mobile phones now have the so-called HEDGE-enabled function, this means they support HSUPA, HSDPA, WCDMA, EDGE, GPRS, and GSM. In other words, 3.5G (HSUPA and HSDPA) Mobile phones support 3G (WCDMA) and 2G (EDGE, GPRS and GSM) technologies. There are also other mobile phones supporting 3G cellular technology such as the CDMA 2000 1x EV-DO and TD-SCDMA, which also consume a lot of power.

Whether it is 3G or 2G, the power amplifier consumes a lot of power, so an effective method is required to get a longer call/working time. Power Amplifiers consume both voice calls and data sent from mobile phones to base stations. In areas with poor reception performance, a higher output power level is required, which means a higher power consumption. For the EDGE section of the 3G power amplifier and GSM/EDGE Power Amplifier Module (PAM), not only the output power of a specific level is required, but also sufficient linearity is required, to ensure the fidelity of the signal sent to the base station.

Two effective methods have been developed for Power Amplifier peripheral technology: DC-DC converter and envelope tracking. The first method is a more popular solution, which is applicable to the 6 W, 3 MHz/6 MHz step-down converter FAN5904 of the 2G/3G power amplifier. This product is specially designed to support GSM/edge pam and 3G/3.5g pa. Because the space of the circuit board in the mobile phone is very precious, the designer no longer needs to use two independent step-down converters. FAN5904 supports 3G wireless standards from WCDMA to HSUPA +, as well as the world's popular CDMA 200 1x EV-DO. In addition, the device supports China's 3G standard TD-SCDMA and HSUPA for TD-SCDMA signal modulation for higher data transfer rates. FAN5904 will be an ideal solution with GSM/EDGE compatible and TD-SCDMA-supported "TEDGE" phones.

FAN5904 works with baseband processors and power amplifiers to reduce power consumption. The baseband chipset sets the output power level based on the information received from the base station and converts it to the power supply voltage so that FAN5904 can power the power amplifier. The System Application in Figure 1 shows the interface between FAN5904 and the baseband chipset, RF transceiver, and power amplifier. To generate a correct power supply voltage that corresponds to the output power, the firmware of the baseband chipset must contain another column in the Tx AGC/POUT lookup table (LUT.

Figure 1. FAN5904 system application with baseband processor, RF transceiver, 3G power amplifier, and GSM/EDGE Power Amplifier Module

Table 1 shows how to configure the query table (LUT) for the WCDMA and HEDGE ports of an HEDGE mobile phone. This table targets Avago's 3G power amplifier, which consists of three main parts: High, Medium, and low power modes. Other power amplifiers have different settings due to their different structures. In the high, medium, and low power mode, the output power and the corresponding VCON voltage are listed in the partition. These voltages are generated by the baseband processor, inform FAN5904 to provide the correct output voltage for the power amplifier to obtain the specific output power. GSM and EDGE require different query tables.

Table 1: examples of how to configure the LUT for the WCDMA and HEDGE ports of an HEDGE mobile phone

The baseband processor can dynamically adjust the power supply voltage of the power amplifier to reduce the current consumed by the power amplifier. In this way, two important results are obtained: first, the power supply voltage and current are reduced, and the heat dissipation is reduced. Second, the call time and data usage time of the smart phone are prolonged. In the case of high output power or high current load, FAN5904 will work in 6 MHz PWM mode, but when "medium" or "low" output power is required, it is automatically converted to the pulse frequency modulation (PFM) mode. In PFM mode, the converter operates at a lower switching frequency to maintain higher efficiency. Figure 2 and Figure 3 show the performance of a 3G power amplifier that uses and uses FAN5904 for WCDMA (voice and data) signal modulation. For voice calls, FAN5904 is working in the PFM mode most of the time, because according to the DG09 curve, the PFM mode takes less time than 10% of the output power over the 18dBm time. On the contrary, in order to ensure the signal-to-noise ratio of the receiver, a higher "per bit energy" is required in the data transmission mode. Therefore, in the data transmission mode, the time when the output power is 16 dBm or above occupies 33%.

Figure 2. Call Time Analysis of 3G Power Amplifier with and without FAN5904 under WCDMA signal modulation and 1000mAh lithium-ion battery

Figure 3. Analysis of the data sending time using and without the FAN5904 3G power amplifier under WCDMA signal modulation and 1000mAh lithium-ion battery

Figure 2 and Figure 3 show the analysis of the working time used for the next 3g pa in voice and data transmission modes without fan5904. When the PA power supply voltage is set to the value of-39dBc for the adjacent channel power ratio (ACPR), the wcdma acpr can be improved to-43dBc, but the current consumption will increase, this reduces the efficiency.

Table 2: Improved call time and data usage time when FAN5904 is used for WCDMA signal modulation

Table 2 shows the results of Figure 2 and Figure 3, and the number of minutes in which the power amplifier only works in high-power mode. The current consumption and call time are improved in different signal modulation modes, such as HSUPA, CDMA 200 1x EV-DO and TD-SCDMA. The reason for the display of the call/data usage time in high-power mode is that the new technology not only gets more minutes, but also simplifies the calibration from the design point of view. When a power amplifier has multiple power modes, the firmware must adapt to when to switch from one level to the next level, because within the entire output power range, each power mode of each power amplifier has a unique VPA-POUT curve. Most mobile phones now support at least three frequencies, which means that three 3G power amplifiers may be available in the system that meets the requirements, increasing complexity. The use of FAN5904 in a Power Amplifier eliminates the complexity of mode switching and enables faster power amplifier calibration because fewer calibration points are required. This reduces testing/Calibration Time, manufacturing costs, and design processes.

Figure 4. Thermal Imaging of 3G power amplifier when POUT = 28dBm

A parameter that is often ignored but often complained about is a hot problem. When a hand-held phone is close to the head for a call, a mobile phone gets hot when surfing the internet or playing online games, so most users will be troubled. The key to the problem is that designers must take this factor into consideration when designing mobile phones. Figure 4 shows the thermal imaging of a 3G power amplifier, in which (a) is powered by FAN5904; (B) is connected directly to the battery; (c) charge the battery at V voltage. When FAN5904 is used, when the output power is full, the temperature of the power amplifier is almost impossible to reach 50? C. In the case of (B) and (c), the temperature can easily reach 50? C and 65? C. After a period of use, the phone will soon get hot. In data card applications, especially 3g usb modem cards, hot dissipation or minimizing thermal dissipation becomes very important. USB flash cards cannot be used as effectively as mobile phones for heat dissipation because they are very small. Because 3G modem is mainly used for data, the power amplifier runs in the "hot" state most of the time. The most effective way to reduce this heat is to use a step-down converter such as FAN5904.

One of the key features of FAN5904 is that it supports 6 MHz Switching frequencies of 3G power amplifiers and 3 MHz Switching frequencies of GSM/EDGE power amplifier modules, and can use small volumes of inductance and capacitance. FAN5904 use 1008 of 2.5 specifications (2.0mm x 0.47mm? H inductor, respectively use 10? F 0603 and 2.2? The F 0603 capacitor is used as the input and output capacitors to implement a PCB solution with a small footprint without affecting the efficiency of the step-down converter.

Another often forgotten parameter is the transient response time. Transient Response time is particularly important for various wireless protocols when switching between different voltages to provide an appropriate output power for the power amplifier. When the FAN5904 is switched at a 6 MHz frequency, it has a very short 10? S response time (as shown in Figure 5), which fully satisfies the time interval of 25? The WCDMA specification of s and the IOPC 6.4.2 specification.

Figure 5. Rise and Fall time of voltage hops in FAN5904 WCDMA Mode

When the output power needs to change from low level to high level, or from startup, the transient response time is 10? S. For a voltage regulator, the key is to enable the power amplifier to meet these specifications to avoid call interruptions or poor speech quality. Although the user moves in different regions and the signal coverage is good or bad, the call interruption is unacceptable to the user. For the GSM/EDGE mode, FAN5904 can be in the 5? Complete voltage switching within s to meet GSM specifications. Figure 6 shows the FAN5904 sequence for WCDMA and GSM/EDGE transmitters.

Figure 6. FAN5904 Sequence Chart for WCDMA (a) and GSM/EDGE (B) Transmitters

When using a step-down converter, we are concerned about how the switching frequency affects RF performance. RF engineers are reluctant to add one that will provide RF signal fidelity (sending and receiving) A device that has potential impact. As mentioned above, FAN5904 works in PWM or PFM mode. The analysis shows that neither of them will affect RF performance. Figure 7 (a) shows the send cw of the 28.5dBm signal (turn off the modulation to display the sharp thorn near the center) and the mark 1 6 MHz from the right. The level of spurous tones produced by FAN5904 is very low, making the sharp thorn lower than the noise background. Figure 7 (B) shows the spike level when modulation is enabled. This sharp thorn is very small, far below the Tx masking level. One advantage of FAN5904's 6 MHz switching frequency is that it is far away from the signal frequency and is located outside the 5 MHz bandwidth and converted to the MHz frequency on both sides of the WCDMA signal.

Figure 7. In (a) CW modulation and (B) WCDMA modulation, FAN5904 is switched at 6 MHz frequency in PWM mode

Perhaps in the PFM mode, we are more concerned about where the sharp thorn enters the signal modulation. By looking at figure 8 (a), in CW mode, the sharp thorn is still below-40dBc, and in 8 (B), more importantly, the sharp thorn is under WCDMA modulation shielding.

Table 3: EVM measurement comparison between connected power supplies and 3G Power Amplifiers of FAN5904

Table 3 lists the measured 0 dBm and-45dBm output power EVM (VPA value in Figure 8 is close to VOUT_DCDC), and compares the use of power and FAN5904 to Power 3G power amplifiers. The difference between the two values is very small, which clearly indicates that the signal fidelity does not change much and does not affect the RF performance, far below the WCDMA limit value.

Figure 8. In (a) CW modulation and (B) WCDMA modulation, FAN5904 is switched at a frequency below 6 mHz in PFM mode.

To further understand the noise/Spike caused by the step-down converter, the noise in the receiving band is measured and compared based on the noise output of the 3G power amplifier. Figure 9 shows that, at the maximum output power of 28.5dBm, the spike level from FAN5904 is actually far below the noise level of the power amplifier, as shown in table 4, the noise from FAN5904 is the largest at-131.8dBm/Hz, but it is still far lower than the-90dBc of the Power Amplifier itself. RF engineers can rest assured that FAN5904 will not interfere with the transmission and receiving frequency bands of cellular and Other RF Frequencies when working in PWM and PFM modes.

Figure 9. sharp thorn of FAN5904 in Rx band relative to 3G power amplifier noise

Table 4: noise levels in the Rx band from FAN5904

The last important feature is that the FAN5904 controls the GSM/EDGE power amplifier module and isolates the RF power management solution from FAN5904. In GSM/EDGE mode, FAN5904 has a 3A designed peak current, so it can control the current up to 2A power amplifier module. In GSM/EDGE mode, FAN5904 always works in PWM mode because GSM and EDGE signals have a higher output power level than 3G signals, therefore, it is switched at a 3 MHz frequency. 3 MHz is an appropriate switching frequency because it is outside the bandwidth of the WCDMA channel, but can still be used with small inductance and small capacitance to handle larger current loads, and keep the efficiency up to 95%. People may ask why 6 MHz is not used in this mode, which is not feasible in design, because a large current load will lead to a higher switch loss, benefits of the miniaturization solution are not compensated in terms of efficiency. Figure 10 and Figure 11 show that the current can be reduced by 100mA using FAN5904 in GSM or EDGE mode for cellular devices.

Figure 10. FAN5904 battery current consumption within the entire output power range of the GSM/EDGE power amplifier module under GSM Modulation

Figure 11. FAN5904 battery current consumption within the entire output power range of the GSM/EDGE power amplifier module under EDGE Modulation

In short, FAN5904 is designed for 2g to 3G wireless protocol power amplifiers, and manages and meets the strict power requirements of the power amplifier. This article focuses on the WCDMA, it is worth noting that FAN5904 can support the CDMA 2000 1x EV-DO, as well as the rapid development of 3G standard TD-SCDMA in China. This device can help cellular device manufacturers significantly reduce power consumption, thus reducing system temperature and longer use time to meet user requirements such as avoiding frequent battery charging. After all, no user is willing to use a smartphone that always requires charging or heating, or that powers down when making an important phone call.