Texas Instruments | Improving RF Power Amplifier Efficiency in 5G Radio Systems | Application notes | Texas Instruments Improving RF Power Amplifier Efficiency in 5G Radio Systems Application notes

Texas Instruments Improving RF Power Amplifier Efficiency in 5G Radio Systems Application notes
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Improving RF power amplifier efficiency in 5G radio
systems using an adjustable DC/DC buck regulator
Timothy Hegarty
Fifth-generation (5G) wireless communications extend
the advances of today’s 4G networks by addressing
the need for increased capacity and throughput, with
improved coverage at a lower system cost. Highspeed data transmission, support for a large number of
connected devices, low latency, low power
consumption and extremely high reliability are
essential. The key to a capacity increase lies in the
densification of the network topology.
A crucial aspect of the evolution to 5G is solving
difficult base-station hardware challenges. Existing
towers must provide higher performance in order to
carry many more channels at higher data rates. One
aspect to successfully meeting expectations is the
introduction of multiple-input, multiple-output antenna
technology on a massive scale (mMIMO). Another
aspect is a large-scale integration of components,
along with higher performance and greater power
savings.
The imperative here is to operate base stations that
can flexibly adjust to traffic demand. Certainly, the
transition to and deployment of 5G communications
has an inherent requirement for adoption of smart
power management in the underlying hardware.
PAs are the main energy consumers in modern base
stations. Moreover, the inefficiency is converted into
heat, creating the need for active cooling of the
devices and further increasing total power
consumption. Consequently, high PA efficiency is
essential to reduce operating expenses for mobile
network operators, as it can lower power dissipation
and the need for cooling.
Drain Bias
Voltage
Fixed Bias Voltage
Average Power
Tracking (APT)
Envelope
Tracking (ET)
Figure 1. PA drain bias voltage modulation
Base Transceiver Station
A base station comprises multiple transceivers (TRX);
each TRX comprises a radio-frequency (RF) power
amplifier (PA), an RF small-signal section, a baseband
(BB) interface including a transmitter (downlink) and
receiver (uplink) section, a DC/DC PA power supply,
an active cooling system, and an upstream isolation
stage to convert from -48 VDC or AC mains voltage. A
popular PA circuit known as the Doherty amplifier –
using silicon LDMOS or GaN RF transistors in its
carrier and peaking cells – provides a linear and
highly-efficient RFPA, particularly when operating deep
in the output back-off (OBO) region where the
efficiency of alternative amplifier solutions drops
considerably.
Base Station Efficiency Enhancement
The proliferating frequency bands and modulation
schemes of modern cellular networks make it
increasingly important that base-station power
amplifiers offer the right combination of output power,
efficiency and multi-band support – at both peak and
average power levels.
SNVA802 – September 2018 – Revised February 2019
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The signals in modern wireless communication
systems have high peak-to-average power ratios
(PAPR). Techniques such as average power tracking
(APT) and envelope tracking (ET) increase the power
efficiency of a PA in a base-station application, as
depicted in Figure 1.
For example, APT changes the DC supply voltage to
the PA on a timeslot-by-timeslot basis to achieve high
efficiency at various loading conditions. The output of
the PA can be a function of average power but
sufficiently backed off to limit clipping RF signal peaks,
which could impact the linearity of the PA.
Meanwhile, ET is faster and more accurate, as it
adjusts the supply voltage in real time according to the
PA input signal. However, this solution is more
complicated than APT because it requires a separate
or integrated ET module.
Both APT and ET use hardware and software
elements that modulate the voltage supply to the PA in
order to reduce total power consumption.
Improving RF power amplifier efficiency in 5G radio systems using an
adjustable DC/DC buck regulator Timothy Hegarty
Copyright © 2018–2019, Texas Instruments Incorporated
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PA Power Supply
PA Voltage Adjustment
A key performance benchmark for the voltage supply
to the PA is the ability to adjust the voltage level on
the fly according to the specific use case scenario.
Typically, an MCU or FPGA provides the APT voltage
setpoint command for the PA in digital format, which
the power supply should readily interpret. For
example, a variable duty cycle PWM output from an
MCU could be RC filtered and resistor-coupled to the
feedback (FB) pin of the DC/DC regulator. Another
option is to apply a DC control voltage at the tracking
input of the controller (SS/TRK in Figure 2) for output
voltage adjustment.
A small form-factor power solution balancing efficiency
and size is vital. Figure 2 shows a DC/DC buck
regulator solution with PWM controller U1 driving
discrete power MOSFETs Q1 and Q2. The choice of
controller hinges initially on input voltage and switching
frequency specifications. Choosing a controller with a
wide VIN range and high line transient immunity offers
an outsized voltage rating and operating margin to
accommodate peak voltage transients.
VIN
VIN
DC/DC Buck
Regulator
PA_VDD
VOUT
FB
Selecting a wide VIN range device, such as the
LM5145, a voltage-mode synchronous buck controller
that operates from 6 V to 75 V, lets designers reduce
or eliminate the input bulk capacitor or TVS diode
clamp, saving cost and board space. The voltagemode control architecture with line feedforward
enables excellent line- and load-transient dynamics.
VIN
(optional)
RC2
RFB1
SYNC out
CC2
HO
SYNCOUT
RRT
CSS
LO
CBST
VOUT
PWM Controller SW
FB
LM5145
LO
Q2
CIN
CO
VCC
SS/TRK
CVCC
PGND
AGND
PGOOD
PG
GND
ILIM
RILIM
Figure 2. Synchronous buck DC/DC regulator with
wide input voltage range
The LM5145 controller drives external high-side and
low-side power switches with robust 7.5-V gate drivers
suitable for standard-threshold MOSFETs. Adaptivelytimed gate drivers minimize body diode conduction
during switching transitions, reducing switching losses
and improving thermal performance. If an auxiliary rail
between 8 V and 13 V is available to supply VCC bias,
the input quiescent current of the LM5145 reduces to
325 µA at an input of 50 V, maximizing light-load
efficiency and reducing the die temperature of the IC.
Moreover, the LM5145 controller offers a large degree
of flexibility in terms of platform design. The adjustable
switching frequency – as high as 1 MHz – is
synchronizable to an external clock to eliminate beat
frequencies in noise-sensitive PA applications. A 180°
out-of-phase clock output is a good fit for downstream
or multichannel power supplies to reduce input
capacitor ripple current and EMI filter size. A minimum
off-time of 140 ns enables operation at high duty cycle
and low input-to-output voltage differentials.
2
PA_GND
Figure 3. Output voltage adjustment using a digital
VID controller
Q1
BST
RT
RFB2
VID[5:0]
EN/UVLO
SYNCIN
COMP
RC1
RFB2
IOUT-DAC
CC1
CC3
LM10011
VIN
VIN
SYNC in
6-bit IDAC
EN
U1
VOUT
RFB1
The circuit in Figure 3 exploits VID control using a 6-bit
digital interface to a VID programmer that interfaces
directly to any analog power stage or controller. The
LM10011 captures the VID information presented by
the MCU and sets the output of a current DAC
connected to the FB pin of the power regulator circuit.
In 6-bit mode, 64 current settings with 940-nA
resolution and better than 1% accuracy are available.
For example, the MCU arbitrates the supply rail of a
GaN transistor PA to a voltage between 36 V and 50.8
V, with a step resolution of 235 mV. Glue logic or level
translators are not required, and a programming
resistor dictates the PA voltage at startup. The
LM10011 can interface to any voltage-mode, currentmode, or ripple-based PWM regulator with a FB input.
Table 1. Alternate controller recommendations
Device
VIN Range
Features
Package
LM25145
6 V to 42 V
Wide duty cycle
range
VQFN-20
LM5117
5.5 V to 65 V
Analog current
monitor output
HTSSOP-20,
WQFN-24
LM5141
3.8 V to 65 V
Low EMI, low IQ
VQFN-24
LM5143-Q1
3.5 V to 65 V
Multi-phase
VQFN-40
LM5146-Q1
5.5 V to 100 V
150°C operation
VQFN-20
Table 2. Related TI application notes
SNVA803
Improving EMI for free with PCB layout
SNVA806
Powering drones with a wide VIN DC/DC converter
Improving RF power amplifier efficiency in 5G radio systems using an
adjustable DC/DC buck regulator Timothy Hegarty
SNVA802 – September 2018 – Revised February 2019
Submit Documentation Feedback
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