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RF DC-DC Converters for Power Amplifiers Using MIPI RFFE interface

Mon, 09/09/2013 - 3:12pm
Sasa Radovanovic, Kevin Kayser, Amanda Alfonso, David van Schravendijk, Texas Instruments

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The majority of new LTE platforms being released to the market have digital MIPI RFFE interface, which controls radio frequency front-end (RFFE) components, either from an application processor or baseband. One front-end component is a power management device for power amplifiers. There are two types of power management solutions for power amplifiers: envelope tracking, which will be implemented in high-end smartphones; and average power tracking for low- to mid-end phones. The latter will be used for size and cost reasons, as well as maturity of the solution ecosystem.

In an average power tracking system, the RF DC-DC converter is used to supply the power amplifiers, rather than the power amplifier being connected directly to a battery. By adapting the supply voltage of the RF power amplifier to the RF output amplitude, the RF power amplifier’s power consumption can be reduced considerably (Figure 1). As a result, the thermal profile in the PCB will be reduced, enabling thinner phones. The overall heat-management in the phones can be relaxed. Battery usage time is also increased as today’s power amplifier is one of the most power-hungry circuits in a smartphone.

Figure 1: RF DC-DC converter reduces power waste from the power amplifier.

RF DC-DC converters are widely implemented today in mid- and high-end smartphones and, historically, have been controlled with a single-line analog voltage from the baseband or the RFIC. New LTE platforms are supporting a larger number of LTE bands, typically on the order of 20 or even more. The large number of bands result in increased software and hardware complexity to the point where phone board layout and accurate timing is impossible with traditional analog control approaches. For that reason, the MIPI RFFE digital interface was developed to address current and future market needs. New LTE platforms designed to support average power tracking will have RF DC-DC converters with the MIPI RFFE interface. The core functionality and proven architecture of the converters remains the same. However, a new digital interface will be utilized. This digital interface does not consume significant space on the IC. As a result, RF DC-DC converters still can have a very small solution size.

There are several RF DC-DC converters with MIPI RFFE interface that can be used in LTE platforms. This article will help readers with selecting buck or buck-boost DC-DC converters, depending on the system requirements. Also a basic overview of the MIPI RFFE control interface of these converters is presented.

Buck and buck-boost RF DC-DC converters with MIPI RFFE interface

LTE platform makers can choose either a buck or buck-boost RF DC-DC converter based on these key parameters:

  • Necessity of heat and battery saving in 2G, 3G, or 4G modes.
  • Maximum transmitted power levels and a required power amplifier noise and linearity performance under lower battery conditions (Vbat<3.2 V).


3G/4G or 2G/3G/4G Transmission-Mode Support

Most of today’s smartphones have 3G-mode as a default operating mode to transmit voice and data. The 3G-network typically is deployed in densely populated geographical areas and in large cities. In areas where a LTE network is available, the smartphone switches to 4G operating mode to provide even higher data-rates. These high data-rates enable online gaming or heavy video streams to increase overall user experience. During transmission (data or voice uploading), the 3G/4G RF power amplifiers account for a significant amount of the total power consumption – up to 30 percent. Using average power tracking DC-DC converters to support 3G/4G-modes is critical to reduce total power consumption.

In less populated rural areas, a 3G/4G network might not be available due to the high costs per user. Here, 2G-networks with a much larger coverage range often are deployed. The power amplifier’s power consumption in a handset is again high, as the maximum output power levels are up to three times higher than in 3G/4G networks (28 dBm at the antenna). Many smartphone makers will use RF DC-DC converters to reduce power consumption.

The battery current saved when operating the power amplifier in 2G-mode Band8 is shown in Figure 2. For these measurements we used the LM3263, a RF DC-DC converter from Texas Instruments. This converter can be optimally used for 3G/4G modes as well.

Figure 2: Current saving in 2G-mode Band8 (897.6 MHz), VBATT = 3.8 V.

The main difference between RF DC-DC converters supporting 3G/4G versus 2G operation is in current-handling capabilities.

For 3G/4G-modes the power amplifier’s maximum output power is between 26.5 and 28 dBm. The power amplifier’s current consumption for these power levels is less than 700 mA (steady-state). This current is presented in Figure 3 (purple line). In order to support fast transient voltage requirements, the transient currents might be 30 to 50 percent larger than the steady-state currents. The shorter the transient time, the higher the currents.

Figure 3: Example transient currents to support voltage change from 0.6 to 3.4 V. 3G, using a power amplifier and the LM3279.

Using RF DC-DC converters, the steady-state current can be reduced up to 200 mA. Taking into account the transient currents, the 3G/4G DC-DC converter typically handles 750 mA to 1 A.

To accommodate higher output power in 2G mode, maximum steady-state currents with RF DC-DC converters are around 1.2 A. To accommodate fast voltage transient requirements, the maximum current handling for these DC-DC converters is ~2.5 A. As a result, the die area of a DC-DC converter supporting 2G-mode is slightly increased. However, a big advantage of using these converters is that they can support 3G/4G modes as well. The users will get the total power savings over all modes: 2G/3G/4G with minimal increase in solution size and cost.

Input Battery and Supply Voltages for Power Amplifiers

Battery voltage in smartphones drops with the battery usage-time. The RF platform providers define a cut-off voltage, depending on the minimum voltage requirements for RF modem and power amplifiers. The cut-off voltage is the voltage below which the transmitter stops operating. The lower the cut-off voltage, the longer the usable battery time.

There are two types of DC-DC converters available, based on the cut-off voltage:

  • Buck RF DC-DC converters.
  • Buck-boost RF DC-DC converters.

Buck RF DC-DC converters dynamically lower input battery voltage to decrease power amplifier power consumption, while the power amplifier still can achieve a required linearity (ACLR). As soon as the power amplifier supply voltage is close to a battery voltage, a buck RF DC-DC converter is operated in bypass mode and does not offer any power saving benefits. The battery voltage, therefore, needs to be higher than the required power amplifier supply voltage for power savings benefits.

To enable lower cut-off voltages, RF platform makers can choose buck-boost DC-DC converters that increase the output voltage above the battery to provide a necessary power amplifier supply voltage. For battery voltages Vbat<3.2 V, the boost-mode typically is required for high load-line power amplifiers. (High load-line power amplifiers need higher supply voltage to support the same currents with respect to low load-line power amplifiers. The power amplifier choice depends on the overall efficiency benefits.) The boost functionality is provided by a boost-block in the RF DC-DC converter and one external boost inductor (Figure 3b). Note that for battery voltages higher than power amplifier supply voltages, these converters will operate in buck-mode. Therefore, for a small increase in overall solution size, buck-boost DC-DC converters provide maximum power savings – heat reduction and longer battery usage time with a minimum increase in solution size and cost.

Some typical application diagrams of buck and buck-boost RF DC-DC converters with MIPI RFFE interface are presented in Figures 4a – 4b.

Figure 4a: Typical application of buck RF DC-DC.   

Figure 4b: Typical application of buck-boost RF DC-DC.

When using proper design architecture, the boost does not necessarily erode the efficiency. For example, the LM3279 buck-boost converters with MIPI RFFE interface have excellent efficiency, even under boost conditions (Figure 5).

Figure 5: Efficiency under high-load conditions for Vout=3.

The average power tracking (APT) printed circuit board (PCB) layout is relatively straightforward by following in-advance defined design rules. It offers component-allocation flexibility on the phone. For example, the RF DC-DC converter does not need to be placed tightly next to multimode multiband power amplifiers as well as a satellite power amplifier.

Control algorithm and MIPI RFFE interface

The control algorithms and commands from RFIC when operating RF DC-DC converters should be implemented by RF platform vendors. MIPI RFFE uses a single, common control bus that has three digital lines connecting the master to multiple slaves:

  • SCLK -- serial clock.
  • SDATA -- serial data.
  • VIO -- enables slaves and resets slave registers to default values.

The serial data-line provides information about required output voltage of the DC-DC converter in proper timing sequence. The second half of this article presents two different mechanisms to set the output voltage, or to set the mode of operation (2G/3G or 4G). These mechanisms are referred to as triggered and non-triggered modes for DC-DC converters.

Trigger-mode is essential for meeting precise timing requirements to prevent congestion using non-triggered messages. Triggers provide a tool to solve bandwidth limitations, and to achieve simultaneous precise timing at multiple destinations.

In non-triggered-mode, the trigger mask bits are written at startup. As a result, all write commands to the register take effect immediately. This mode offers less complexity, but requires higher bandwidth on RFFE MIPI interface.

Non-Triggered Operation

To change the DC-DC output voltage in correspondence with the power amplifier’s transmit power (for example from POUT = –10 dBm to POUT = 28 dBm), only one RFFE write-register should be set in the non-triggered operation as the RFFE traffic on this data bus is pretty heavy.

Figure 6 shows a single RFFE SDATA command with the information on output voltage for the next frame. In 3G-mode, there is a ~20 us window between the two data-slots in which the voltage transition can take place. Figure 6 shows an example using the LM3263 where the SDATA information needs to be available 12 us before a frame-boundary for the DC-DC output voltage to fully settle. An additional 7 us is needed for the power amplifier to deliver the required output power. Therefore, the voltage transition happens within 20 us and the 3GPP specifications are met.

Figure 6: 3G/4G APT to APT Tx POUT transitions non-triggered (trigger masked) operation.

Triggered Operation

The trigger function is not a mandatory function in every slave, but may be used if multiple registers are required to be loaded at exactly the same time or in a timing critical window. In this mode, two commands change the DC-DC output voltage. These commands happen sequentially. The value of the output voltage is first stored in the shadow register. After the trigger command is given, the output voltage is changed (Figure 7).

The timing requirements before and after frame boundaries are the same as for non-triggered operation (Figure 7).

Figure 7: 3G/4G Tx POUT transitions triggered operation.

The advantage of trigger operation is that the output voltage can be changed only after the trigger command is given at the cost of additional traffic on the RFFE bus.

Output Voltage DAC Resolution

There are two output voltage resolutions for DC-DC converters with RFFE MIPI interface. A coarse voltage resolution (30 mV typ.) might be sufficient while it reduces RFFE bus traffic timing due to lower number of bits. Typically it uses Reg0 (MIPI Spec) write, which has a lower number of clock cycles. For higher accuracies and optimized system efficiency, a lower voltage resolution can be implemented (15 mV). This resolution flexibility is implemented in the LM3263. The tradeoff is between using 30 mV versus 15 mV, which is not significant when the RFFE bus is running at maximum speed.

Average power tracking DC-DC converters with MIPI RFFE digital control interface are available on the market today. These new converters are ideal for the RF platforms being released to the market in 2013. The converters are providing proven heat-reduction and battery usage time benefits for very small total solution size and low costs. The core of the RF DC-DC converters is proven and in most cases re-used from analog-controlled RF DC-DC converters, which minimize the overall implementation risk. The digital control is expected to be done by RF ICs or baseband providers. A layout reference design typically is provided to RF engineers that follow in-advance defined routing rules similar to RF DC-DC converters with analog interfaces.
 
References

About the Authors

Saša Radovanović is a product marketing manager at Texas Instruments where he is responsible for revenue growth with existing and new RF power products and developing product roadmaps. He has nearly two decades of engineering experience and is co-author of a book published by Springer, “High-Speed Photodiodes in Standard CMOS Technology / Edition 1.” Sasa earned his PhD from the University of Twente, Department for IC design, The Netherlands. For questions about this article, contact Sasa at: ti_sasaradovanovic@list.ti.com.

Kevin Kayser is the RF Power Marketing Director for TI’s RF Power group. Kevin received his BSEE from Rose-Hulman Institute of Technology, Terre Haute, Indiana, MSEE from Walden University, and MBA from Arizona State University, Tempe, Arizona.

Amanda Alfonso is a product marketing manager for TI’s line of SuPA RF DC/DC converters. Amanda received her BSEECS from the University of California at Berkeley, Berkeley, California.

David van Schravendijk is a Technical Marketing Engineer for TI’s RF Power group. David received his BSEE from University of California at Davis, Davis, California.

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