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AND9685/D
eFuse Load Current
Measurementwww.onsemi.com

APPLICATION NOTE
Introduction

The On Semiconductor eFuse protection devices areprimarily used for limiting the system load current in theevent of overload or short circuit conditions. Almost all of
those eFuse protection devices provide additional features
such as soft start, output voltage clamp, reverse current
protection, adjustable load current limit with external
resistor and so on. Some of the eFuses provide additionalfeature of load current monitoring which is implemented asa voltage output through an IMON pin. The user canmeasure the voltage at the IMON pin which would be
proportional to the load current.
This application note describes the load current
measurement solution for the eFuses which do not provide
load current monitoring feature. Since almost all of theeFuses provide adjustable current limit functionality by
utilizing an external current limiting resistor between ILIMIT

Fault

11Schematics

A typical schematics of an eFuse configuration together
with the 20 W RLIMIT resistor used for load current limit
programming is shown in Figure 1. The load current
measurement circuit implementation is shown within adashed rectangle. Only current sense amplifier with adecoupling capacitor is required.

Source 10
Source 9
Source 8
Source 7
Source 6
ILIMIT 4Vcc

3 Enable/Fault

RLIMITeFuse(NIS5x2x)

Enable

dV/dt
GND
1

VOUT
2

LOAD
Cdvdt

VINand Source pins, it is possible to connect a current sense
amplifier across that resistor and measure the voltage drop
across it which would be proportional to the load current.
This method mainly requires a current sense amplifier andallows user to measure the system load current without
introducing any additional resistance in series with the loadpath.

IMONNCS210

Figure 1. 12 V eFuse Configuration with Load Current Measurement Circuit© Semiconductor Components Industries, LLC, 2017

April, 2019 − Rev. 1

1Publication Order Number:AND9685/D

AND9685/D
One of the most important parameters to consider whenselecting a current sense amplifier are: wide common mode
range, low offset voltage, low gain error, rail−to−rail input
and output and a wide input supply range which exceeds thesupply range of the eFuse input and output voltage.
Typical current sense or current shunt amplifiers also haveintegrated gain and feedback resistors. It is important toutilize such a current sense amplifier where those integratedresistors have much higher resistance than the resistance ofthe external RLIMIT current programming resistor. The
schematics in Figure. 1 shows a 12 V eFuse together with theNCS210 rail−to−rail current sense amplifier with a gain of200, ±60 mV offset voltage, 1% gain error and supply range
up to 26 V.
The Source output pin of the eFuse will be at a lower
voltage potential than the VCC input pin due to a small
voltage drop across the eFuse input and output pins. TheILIMIT pin also has a lower voltage potential than the VCC
pin, but slightly higher voltage potential than Source pins.
The higher the load current, the higher the voltage drop
across the RLIMIT resistor. This voltage drop will be
amplified by a current sense amplifier and used as a measureof load current. It is important to connect the positive inputof the current sense amplifier to the ILIMIT pin and negative
input to Source pins of the eFuse. The Reference pin of thecurrent sense amplifier should be connected to ground. It isalso recommended to use a 1% accuracy RLIMIT resistor.
The output of the current sense amplifier can easily drive
high impedance, low capacitance load such as an ADC ormeasurement equipment. If a low impedance or highcapacitance load needs to be driven it is recommended to use
additional buffer amplifier with a high slew rate after thecurrent sense amplifier. If the output of current sense
amplifier is fed to ADC for digitizing, it would be reasonable
to connect a shunt Zener diode at the output of the current
sense amplifier in order to limit its output voltage during theeFuse current overload events in order to protect ADC input.
LayoutTypical current sense amplifiers usually come in a small
packages such as SOT−363 or UQFN10. It is important toplace a current sense amplifier close to the RLIMIT resistor
and decoupling capacitor close to the supply rail of current
sense amplifier. Use value of 0.1 mF and thick traces fordecoupling capacitor. The traces connecting current sense
amplifier with RLIMIT resistor should not be close to any
traces carrying fast switching signals.
Refer to Figure 2 for an example layout configuration
when using current sense amplifier to measure the eFuse
load current. The decoupling capacitor is connected close to
the current sense amplifier supply pins. The input positive
and negative terminals of current sense amplifier areconnected to the RLIMIT resistor with minimal routing. TheRLIMIT resistor is also connected to output node and ILIMIT
pin of eFuse with minimal routing. Overall solution is nolarger than a 4x4 mm footprint of NIS5021.

NCS210RLIMIT

NIS5021

Decouplingcapacitor
Figure 2. Example PCB Layout for the Load Current Sense Solutionwww.onsemi.com
2

AND9685/D
Performance

RLIMIT resistor, this voltage drop will depend on the valueof the resistor and eFuse type, since every family of eFuse
has its own proportion between the main load current and thecurrent flowing across the RLIMIT resistor.

The typical voltage drop across RLIMIT resistor versus
load current for one of the ON Semiconductor 12V NIS5x2xeFuses is shown in Figure 3. For every 1 Amp of load current
there is approximately 7.5 mV voltage drop across 20 W

Figure 3. Voltage drop across RLIMIT versus the load current

The Figure 4 shows a schematics of two NIS5x2x eFuses
connected in parallel.
VIN

11

Source 10
Source 9
Source 8
Source 7
Source 6Vcc

3 Enable/
Fault

Fault

ILIMIT 4

RLIMITeFuse(NIS5x2x)

dV/dt

Hot-Plug2GND
1

NCS210

11eFuse #1current

3 Enable/
Fault

ILIMIT 4Total Current
20A
LOAD

Source 10
Source 9
Source 8
Source 7
Source 6

Vcc

High-Speed
Instrumentation
Current Probe (Tooscilloscope)

VOUT

RLIMITeFuse(NIS5x2x)

dV/dt

Enable2

GND
1

NCS210eFuse #2
current

Figure 4. Parallel configuration of two NIS5x2x eFuses with individual load current monitoringwww.onsemi.com
3

AND9685/D
The Figure 5 shows the measurement error versus load
current for the 12 V NIS5x2x eFuse. In this case one canachieve around 90% accuracy for most of the eFuse

continuous current capability. The measurement accuracy
would drop another 10−12% if the eFuse is operated at themaximum current rating.

Figure 5. Load current measurement error vs Load Current

shows the common connected VOUT voltage of two eFuses
and the green trace shows the overall total current measured
with instrumentation grade high−speed current probe. In thisconfiguration a current sense amplifier similar to NCS210
but with a gain of 50 was used.

The Figure 6 shows a scope screenshot of a 20 A load hot
plugged to the outputs of two NIS5x2x 12 V eFuses
connected in parallel. Each eFuse in that configuration
utilized a current sense amplifier across the RLIMIT resistor,
the output of each current sense amplifier is connected to ascope and plotted as a red and blue traces. The purple trace

Figure 6. Hot plug to 20 A load and eFuse load current measurementof around 94.3 mV x 50 = 4.7 V at the outputs of both current
sense amplifiers as shown in red and blue traces in Figure 6.
The current is also perfectly shared among eFuses duringa fast output load hot−plug event and properly measured bycurrent sense amplifiers as seen from the scope shot in
Figure 6, this can be achieved if current sense amplifiers arefast enough and provide 25−40 kHz 3dB bandwidth.

Once the hot plug event is gone the total current measured
by an instrumentation current probe is settled to 20 A. Since
the current is equally shared by eFuses connected in parallel,each eFuse in this case carries around 10 A current. From
Figure 3, the voltage drop across 20 W RLIMIT resistor forthis eFuse at 10 A load is around 94.3 mV, since we are using
a current sense amplifier with a gain of 50, we see a voltage

www.onsemi.com
4

AND9685/DConclusion

This application note demonstrated a cheap and effective
way of adding a load current measurement solution tocurrent limiting electronic fuses. The solution requires only
one active device in a compact SOT−363 or UQFN10

package and minimum supporting passive components forit, such as decoupling capacitor. This solution provides
measured load current not only in a static configuration but
also during dynamic events.

ON Semiconductor andare trademarks of Semiconductor Components Industries, LLC dba ON Semiconductor or its subsidiaries in the United States and/or other countries.
ON Semiconductor owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property. A listing of ON Semiconductor’s product/patent coverage
may be accessed at www.onsemi.com/site/pdf/Patent−Marking.pdf. ON Semiconductor reserves the right to make changes without further notice to any products herein.
ON Semiconductor makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does ON Semiconductor assume any liability
arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. Buyer
is responsible for its products and applications using ON Semiconductor products, including compliance with all laws, regulations and safety requirements or standards, regardless ofany support or applications information provided by ON Semiconductor. “Typical” parameters which may be provided in ON Semiconductor data sheets and/or specifications can anddo vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s
technical experts. ON Semiconductor does not convey any license under its patent rights nor the rights of others. ON Semiconductor products are not designed, intended, or authorized
for use as a critical component in life support systems or any FDA Class 3 medical devices or medical devices with a same or similar classification in a foreign jurisdiction or any devices
intended for implantation in the human body. Should Buyer purchase or use ON Semiconductor products for any such unintended or unauthorized application, Buyer shall indemnify andhold ON Semiconductor and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees
arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that ON Semiconductor wasnegligent regarding the design or manufacture of the part. ON Semiconductor is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyrightlaws and is not for resale in any manner.

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AND9685/D