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Texas Instruments Current Sensing Applications in Communication Infrastructure Equipment Application notes
Current Sensing Applications in Wireless Infrastructure
Equipment
Guang Zhou
In this article, a fresh look is taken into the major
electronic end equipment for cellular wireless
infrastructure (WI) from the perspective of current
sensing (CS). Several types of CS applications in such
equipment are reviewed.
Current Sensing in Power Supply Block
As shown in Figure 1, the power supply for the WI
equipment comes from the utility grid, solar energy, or
sometimes a combination of the two. The power
supply is often backed up with battery storage for
uninterrupted service during a power outage,
especially in remote areas where solely depending on
grid electricity is not an option due to limitations from
physical accessibility or economic feasibility.
DCDC Charger
These lower voltage rails are called point of load
(POL) supplies, stemming from the fact that they
satisfy a set of specific requirements and are normally
located in the vicinity of the loads they serve.
Depending on how critical or informative the
measurements are, sometimes it is desirable to
monitor the current or voltage in one or more of these
POL supplies. The main requirements for the CSA in
this situation may include (among others) accuracy,
speed, dynamic range, and power dissipation by the
associated shunt resistor.
VDD
AC/DC
CSA
High Side I-Sense
Point of Load
Load
CSA
Figure 1. WI Power Supply Block Diagram
The power supply block can be either integrated into
the WI end equipment, or it can be stand-alone.
Regardless of the implementation, a common
requirement is an intelligent power management
system to charge batteries and ensure seamless
transitions between power sources. Current and
voltage sensing is an indispensable function in such
power management systems.
Current sensing can be implemented either on the
high side or on the low side. Dedicated high-voltage,
shunt-based, Current Sense Amplifiers (CSA) such as
the INA240 might be needed for fault to ground
prevention. Magnetic current sensors such as the
TMCS1100 family devices are another great choice for
high-voltage applications due to their inherent galvanic
isolation.
Current Sensing at Point Of Load
The typical WI electronic system is powered from a DC
bus, such as 12 V to 48 V. Lower supply rails are
derived from the bus voltage.
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Low Side I-Sense
Figure 2. Point of Load Current Sensing Options
As shown in Figure 2, current can be sensed at either
side of the load, with analog or digital CSA, and
through either external or integrated shunt resistor.
CSA comes with a matched resistor gain network that
provides value in terms of cost, board space, and
performance. Most CSAs feature fixed-gain, ranging
from 10 to 1000. Some CSAs offer configurable gain.
For example, the INA225 has configurable gain
through two digital control pins, while other CSAs have
a gain that is configurable through an external resistor,
such as INA139.
System integration is further improved when a CSA is
chosen that comes with integrated analog to digital
conversion (ADC) and a shunt resistor.
Current Sensing Applications in Wireless Infrastructure Equipment Guang Zhou
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The key considerations in selecting a CSA and
associated shunt resistor for POL measurement starts
with common-mode voltage, current range, accuracy,
and speed. In addition, if overcurrent protection (OCP)
is required, a CSA with an integrated fast-action
comparator is often an ideal choice, where system
parameters such as offset and propagation delay are
specified. Compared with discrete components, such a
CSA helps remove uncertainties and therefore
simplifies the design.
To monitor multiple POLs, a multi-channel CSA like
the INA4180 might make sense, as it offers four
channels of analog output. When a microcontroller or
FPGA is present in the equipment, an ADC channel is
normally available, as well as a digital bus such as I2C.
In this situation, either an analog or digital output CSA
may be implemented as a POL monitor. A multichannel digital monitor such as the INA3221 is another
option that frees up controller ADC channels while
taking advantage of an existing I2C bus. This device
offers a number of warning and alert signals for fast
action in case of a fault, as well as current, voltage, or
power information of three independent channels.
Current Sensing in Power Amplifiers
The bias current in power amplifiers (PA) is adjusted to
suit the need of an end application, modulation
scheme, and operation class. A typical PA with current
sensing is shown in Figure 3.
The PA is often constructed with silicon LDMOS or
GaN technology. Current sensing is important in PA
applications, both from the standpoint of the PA
operation and from the standpoint of overall energy
efficiency management. Under the same bias voltage,
the PA bias current differs due to device variation.
Further, the bias current changes with temperature.
Consequently, in order for the PA controller to
accurately control the bias current, both the current
and temperature information must be available. The
bias current information is necessary in improving
system efficiency, where around 50% of total system
power is consumed by the PA itself.
Integrated power amplifier monitor and control
systems, such as the AMC7836, can simplify PA
circuit design. As mentioned, due to natural device
variation, knowing gate voltage alone is sometimes not
sufficient in order to achieve accurate bias current
control. When current sensing is required in the control
loop, a separate high-voltage CSA, such as the
INA290, can be used.
Power amplifier monitor and control systems such as
the AMC7834 are another option with integrated
current sensing capability. Such a solution offers the
possibility of further reducing board space.
2
VDD
Current/Voltage/Power/
Energy Sense
ADC
Temperature Sense
PA Controller
(w/uC or FPGA)
DAC
d
g
s
RF Input
Figure 3. PA Biasing and Current Sensing
Alternate Device Recommendations
TI offers a complete line of CSA and magnetic sensors
that serve well in WI end equipment, from high-voltage
supply current and PA current sensing to general
purpose POL current monitoring. The high-voltage
INA202 also comes with integrated comparator and
reference output to facilitate the OCP requirement. For
applications where superior accuracy is required, the
INA190 family of devices are good choices with nA
input bias current. These devices are essential in
situations where the sensed current is very small.
Some of these devices come with Enable pins for
further power reduction; some are offered in a WCSP
package for board space optimization. For applications
with lower common mode voltage requirements, the
INA180 offers excellent speed and overall
performance value. The INA301 family of devices
features integrated fast comparators and high-speed
amplifiers. Both outputs are available that suit the
need of OCP.
Table 1. Alternate Device Recommendations
Device
Characteristics
INA202
Common mode range -16 V to 80 V;
Comparator
INA180
Low IQ; High bandwidth
INA190
Low IQ; Low IB; Enable pin
INA301
High bandwidth amplifier; Fast comparator
Table 2. Related TI Tech Notes
SLPA013
Hybrid Battery Charger With Load Control
for Telecom Equipment
SBOA165
Precision current measurements on highvoltage power supply rails
SLYA024
Common Uses for Multi-channel Current
Monitoring
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