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Application Brief

Maximize System Efficiency With Integrated Current
Sensing From TI GaNZach Soviero
Introduction
Switching converters generally requires some form ofsystem-level current sensing for control, protection, orboth. Many methods and devices exist to accurately
measure and translate current to some voltage
processed by a digital or analog controller. Most
designers opt for shunt resistors in low-to-mid power
converters placed in series with switching devices due
to cost savings and design simplicity.
With advancing technology, the LMG362x family ofGaN devices offers a more integrated approach tothis necessary function. The device includes builtin current sensing that continuously outputs a realtime replica of the positive-going drain to the source
device current. This new feature replaces traditional
shunt resistors without trade-offs or inaccuracies andimproves system efficiency and thermal performance.
Common Topologies
The majority of designers select some version of a
flyback converter in low-power applications, such asconsumer AC/DC converters or bias supplies. These
topologies are an excellent choice for universal AC
input voltages and wide output voltages used in USBC adapters, which range from 5 V up to 20 V, or even
48 V with extended power range (EPR). A flyback
converter operates with an analog controller using aform of current-mode control to set the correct on-timeof the switching device.
This sensing is required to accurately adjust the ontime for varying input and output voltages and loadchanges. To measure this current, a shunt resistor isplaced between the source of the switching FET andthe power ground (PGND) of the circuit. This shunt
resistor has a small resistance value and generates a
proportional voltage drop that connects to an analog
controller and is processed by an internal comparator.
In common 65-W flyback converters, the power
loss associated with the shunt resistor RCS(trad) is
calculated using the primary side RMS current. Power
loss above 350mW can cause significant buildup ofheat and lower system efficiency.

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At higher power levels, government regulations
require some stage of power factor correction(PFC) to reduce distortions on the power grid andminimize the total apparent power consumed by theapplication. Adhering to these requirements creates
new challenges to converter designs which are
difficult to accomplish in one stage. As a result, many
designs now implement an AC/DC PFC stage witha high DC output voltage, combined with a DC/DCstage, which lowers the final load voltage to an
acceptable value.
A majority of mid-power converters use boost PFC
topologies which are appropriate for universal AC
input and output and DC voltage around 400 V. In
contrast to flybacks, most transition-mode boost PFC
analog controllers use voltage-mode control to adjust
the switching time of the main FET to account forvariations in the line voltage or output load current.
Sensing the drain-to-source voltage of the FET
provides the controller with enough information toswitch properly, however, current sensing is stillrequired to use most controllers. The converter must
be able to effectively shut down and avoid thermal
runaway or permanent damage in the event of asurge condition or electrical failure is possible. Excess
current in the switching FET causes both of theseissues, which can be mitigated by sensing the drainto-source current and sending a translated voltage tothe analog controller.
Current sensing is still largely required andtraditionally uses shunt resistors, regardless of current
or voltage mode-controlled topologies. Switching
FETs with built-in current sensing can bring added
benefits while still being compatible with all existing
controllers.
Benefits of Integrated Current Sensing
Texas Instruments' LMG362x device family integrates
current sensing within the device and outputs a
dedicated signal through one of the external pins.

Maximize System Efficiency With Integrated Current Sensing From TI GaN
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Fundamentally, the device senses the positive-goingdrain-to-source current in the main GaN FET, scales
the current down, and outputs a proportional current
on the dedicated CS pin.

As previously mentioned, traditional FETs require ashunt resistor placed between the source of the FET
and system ground. The placement of this shunt
resistor causes the source to be electrically separated
from the rest of the converter and cannot connect
directly to a PGND PCB plane for thermal dissipation.
Integrated current sensing mitigates this first issue
by removing the resistor placed between source andPGND and replacing the resistor with one on a
separate path.
Figure 1 illustrates how the current sense resistor nolonger acts as a shunt and provides an adequate
input to the controller while enabling the source of theFET to connect directly to the system ground. Bothsensing methods require a resistor but the type andsize of resistor vary greatly. A typical shunt resistor
must conduct all the switch node current passing
through the FET and dissipate the current as heat, asshown in Figure 2. The distinguishable hot operating
temperature of the shunt resistor is noticeably warmer
than an integrated design and contributes to theoverall PCB heat, which must be spread.
Using a small resistance value makes thermal design
manageable but not without a large package size.
For a common 65-W quasi-resonant (QR) adapter,the shunt resistor value is approximately 100 mΩ witha 500 mW power rating in a 1206 package. If themain FET also has an on resistance of 100 mΩ, theconduction losses are twice as high due to the added
shunt resistor. An integrated current sensing resistor
only conducts a fraction of the scaled FET current andsubsequently is higher in resistance value and smallerin package size. A resistor of approximately 100 Ω,
rated for only 60 mW, is appropriate for this QR
application and available in a 0402 package (9 times
smaller than a 1206 package). This size reduction
saves precious space on a circuit board where small
form factors are becoming increasingly popular.

2Figure 3. Thermal Capture of Lossless Current
Sense Resistor
A natural reward for achieving lower power losses andbetter thermal heating is higher efficiency. Using thesame 65 W QR example, shunt resistor losses areapproximately a magnitude of hundreds of milliwatts,
while integrated current sensing resistor losses fall totens of milliwatts.
Part of this efficiency is realized in Figure 3 where theresistor temperature drops significantly. No thermal
hotspot is detected compared to the shunt resistor
for the same input and load conditions. Illustrated inFigure 4, measured power losses show the reduction
in resistor loss exceeds 90% depending on the output
power and input voltage.
180
160

Traditional current sensing with shunt
Integrated "lossless" current sensing

140Power Loss (mW)

Figure 1. Diagram of Shunt and Lossless Current
Sense Resistors

Figure 2. Thermal Capture of Traditional Current
Shunt Resistor

120
100806040200253545556575
System Input Power (W)8595

Figure 4. Integrated Lossless Current Sensing vsTraditional Current Sensing Power LossMaximize System Efficiency With Integrated Current Sensing From TI GaN
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A converter must be designed to withstand the worstcase conditions even though the loss reduction is
not constant across load. The worst-case condition isusually at the lowest input voltage and highest output
power, which produce higher currents for the main
power devices.
Proper Resistor Calculations and Design Example
With LMG362x
Each device in the LMG362x family outputs ascaled replica of the drain-to-source current, however,
the ratio between the actual drain-to-source current(gain GCSE) and outputted sense current varies foreach device. Using the integrated current sensing
feature and selecting the right resistor value is key
to compatible operations with any existing analogcontroller. Table 1 lists all devices in the LMG362x
family with corresponding current sense ratios tohelp users chose a new resistor value or replace anequivalent shunt resistor with the same behavior.
Table 1. LMG362x Family Device Table
DeviceRdson (typical)

Current Sense GainGCSE (typical)

LMG3622120 mΩ

0.691 mA/ALMG3624

170 mΩ

0.965 mA/A

LMG3626

270 mΩ

1.633 mA/A

The integrated current sense resistor RCS can be
determined multiple ways: using the FET drain current
and desired CS pin voltage or converting from a
traditional shunt resistor RCS(trad) using the integrated
current sense gain GCSE (Equation 1). With thismethod, an exact voltage range is set for an analog
controller to sense drain current or a traditional shunt
resistor design is easily replaced and swapped withintegrated current sensing.
VCS
RCS = I ×GDCSE=RCS(trad)
GCSE

(1)

An example of TI’s integrated current sensefeature is demonstrated using the LMG3624EVM-081
evaluation module for 65-W quasi-resonant flyback
converter and the LMG36XX-CALC QR power stage
design calculator. In this application, the worst-case
input and output conditions calculate the maximum
RMS current seen in the main switching FET. This
current defines the proper translated voltage sent tothe analog controller for current-mode control during
each switching cycle. Table 2 shows some important
input design specifications for this converter.

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Table 2. LMG3624EVM-081 Input Design
Specifications
SpecificationValue

Minimum input AC voltage

90 VAC

Maximum output voltage

20 VTransformer turns ratio

6:1

Bulk capacitance

120 µF

Magnetizing inductance

200 µHEntering inputs from Table 2 into the QR design
calculator returns a primary peak current IPK of
2.8 A. Solving for an integrated current sensing
resistor requires finding the appropriate traditional
shunt resistor and then converting using the inverse
current sense gain. To solve for the traditional shuntresistor RCS(trad), two other values are needed.
The first value is the controller feedback voltage VFB
which corresponds to the voltage range processed by
the analog controller for current-mode operation. Thisvalue varies for different controllers.
The second value is the feedback loop gain KFB
which is a generic multiplier to account for additional
gains in the controller feedback loop. Using Equation
2 and the previous values described, users cansolve for a traditional shunt resistor. Converting to an
integrated current sense resistor is easily calculatedwith Equation 3 by multiplying by the inverse current
sense gain GCSE for the LMG3624 device.
VFB = KFB × IPK × RCS(trad)
RCS = 1,036 × RCS(trad)

(2)
(3)

The resistor placed on the output of the LMG3624
device CS pin connects to the analog controller,
further documentation is found in Using theLMG3624EVM-081 65-W USB-C PD High-Density
Quasi-Resonant Flyback Converter. The use of adesign calculator provides easy input to adjust thecurrent sense resistor due to fluctuations in design
parameters or part selection. If a different LMG362x
device is selected, simply use the corresponding
current sense gain and recalculate. Substituting an
integrated design for a shunt is calculated by simply
dividing by the current sense gain. To set the voltageto-drain current ratio, simply divide and solve.
Implementing this feature is intended to be easy for
any topology regardless of controller or current sense
input method. Integrated sensing is an excellent
choice for bolstering efficient designs and thermal
performance and pushing the limits of what is possibleusing TI GaN.

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