Current sensor

APPLICATION NOTE—105
Application Note 105
December 2005
Current Sense Circuit Collection
Making Sense of Current
Tim Regan, Editor
INTRODUCTION
Sensing and/or controlling current flow is a fundamental This Application Note Will Change
requirement in many electronics systems, and the tech- This Application Note is a growing and changing docu-
niques to do so are as diverse as the applications them- ment. Many of the chapters listed below are placeholders
selves. This Application Note compiles solutions to cur- for material that will be filled in soon. As the chapters are
rent sensing problems and organizes the solutions by added, their links will be enabled.
general application type. These circuits have been culled
from a variety of Linear Technology documents. Using the Application Note
Click the name of a chapter in the “Circuit Collection In-
Circuits Organized by General Application dex” below to open the PDF version of that chapter.
Each chapter collects together applications that tend to
solve a similar general problem, such as high side cur- Contributors
rent sensing, or negative supply sensing. The chapters Jon Munson, Alexi Sevastopoulos,
are titled accordingly (see “Circuit Collection Index” be- Greg Zimmer, Michael Stokowski
low). In this way, the reader has access to many possible
solutions to a particular problem in one place. , LTC, LTM, LT, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered
trademarks of Linear Technology Corporation. Adaptive Power, C-Load, DirectSense, Easy
Drive, FilterCAD, Hot Swap, LinearView, µModule, Micropower SwitcherCAD, Multimode
It is unlikely that any particular circuit shown will exactly Dimming, No Latency ∆Σ, No Latency Delta-Sigma, No RSENSE, Operational Filter, PanelPro-
tect, PowerPath, PowerSOT, SmartStart, SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT,
meet the requirements for a specific design, but the sug- UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names
gestion of many circuit techniques and devices should may be trademarks of the companies that manufacture the products.

prove useful. Specific circuits may appear in several
chapters if they have broad application.

CIRCUIT COLLECTION INDEX

Current Sense Basics Level Shifting High Speed
High Side High Voltage Fault Sensing
Low Side Low Voltage Digitizing
Negative Voltage High Current (100mA to Amps) Current Control
Unidirectional Low Current (Picoamps to Precision
Bidirectional Milliamps) Wide Range
AC Motors and Inductive Loads
DC Batteries

Introduction-1

APPLICATION NOTE 105: Current Sense Circuit Collection

Current Sense Basics
This chapter introduces the basic techniques used for HIGH SIDE CURRENT SENSING
sensing current. It serves also as a definition of common
Current sensed in the supply path of the power connec-
terms. Each technique has advantages and disadvan-
tion to the monitored load. Current generally flows in just
tages and these are described. The types of amplifiers
one direction (uni-directional). Any switching is per-
used to implement the circuits are provided.
formed on the load-side of monitor.
To see other chapters in this Application Note, return to
DC VSUPPLY
the Introduction.
+
LOW SIDE CURRENT SENSING RSENSE ISENSE OUTPUT ∝ ILOAD

–
Current sensed in the ground return path of the power
connection to the monitored load. Current generally ILOAD LOAD

flows in just one direction (uni-directional). Any switch-
ing is performed on the load-side of monitor.

DC VSUPPLY
High Side Advantages
Load is grounded
ILOAD LOAD VCC Load not activated by accidental short at power con-
+
nection
RSENSE ISENSE OUTPUT ∝ ILOAD
High load current caused by short is detected
–
High Side Disadvantages
. High input common mode voltages (often very high)
Output needs to be level shifted down to system oper-
ating voltage levels
Low Side Advantages
Low input common mode voltage Amplifier Types for High Side Implementation
Ground referenced output voltage Dedicated current sensing amplifiers: LT6100,
Easy single supply design LTC6101, LT1787
Over-the-Top™ op amps: LT1637
Low Side Disadvantages Flying capacitor amplifier: LTC6943
Load lifted from direct ground connection
Load activated by accidental short at ground end load
switch
High load current caused by short is not detected

Amplifier Types for Low Side Implementation
Precision zero-drift op amps: LTC2050, LTC2054
Instrumentation amplifiers: LTC2053, LT1990,
LTC6943
Rail-to-Rail Input op amps: LT1677

Current Sense Basics-1

FULL-RANGE (HIGH AND LOW SIDE) SUMMARY OF CURRENT SENSE SOLUTIONS
CURRENT SENSING
The next few pages contain a table that summarizes cur-
Bi-directional current sensed in a bridge driven load, or rent sense solutions and applicable devices. Look first in
unidirectional high side connection with a supply side the “Type/Circuit” column and the “Gain” column for a
switch. general description of the application. Then scan across
the other columns for applicable devices and their speci-
DC VSUPPLY
fications.

VCC

RSENSE
+
LOAD ISENSE OUTPUT ∝ ILOAD

ILOAD –

Full-Range Advantages
Only one current sense resistor needed for bidirec-
tional sensing
Convenient sensing of load current on/off profiles for
inductive loads

Full-Range Disadvantages
Wide input common mode voltage swings
Common mode rejection may limit high frequency
accuracy in PWM applications

Amplifier Types for Bi-directional Implementation
Difference amplifiers-LT1990, LT1991, LT1995,
LT1996
Instrumentation amplifiers: LTC2053
Flying capacitor amplifier: LTC6943



ACCURACY SPEED

OFFSET INPUT DIFFERENTIAL
GAIN DEVICES AND VSUPPLY
TYPE/CIRCUIT VOLTAGE CURRENT BANDWIDTH SLEW RATE VIN RANGE (VCM) VIN RANGE
(V/V) PACKAGES RANGE (VS)
(VOS) (IBIAS) (SURVIVAL)
High Side 10 to 50 LT6100 300µV 5µA 100kHz 0.05V/µs 2.7V to 36V (VS + 1.4V) to 48V ±48V
One Direction
Voltage Out MSOP-8
RSENSE
VIN
DFN
LOAD
(VCC + 1.4V) TO 48V
1 8
VS– VS+
RG1 RG2
5k 5k

– + R
25k
A1

VCC
2
2.7V TO 36V –
VOUT
Q1 RE A2 5
10k +
VO1

RO
R R/3
50k
VEE FIL A2 A4
4 3 6 7
6100 F01

High Side Resistor LTC6101 350µV 250nA 200kHz 2.5V/µs 4V to 70V (VS – 1.5V) to 70V ±70V
One Direction Ratio LTC6101HV 350µV 250nA 200kHz 2.5V/µs 4V to 105V (VS – 1.5V) to 105V ±105V
Current Out
ILOAD VSENSE
SOT23-5
– +
VBATTERY
RSENSE
RIN 5
MSOP-8
10V V+
L
O
A
D
IN – 5k –
3

IN + 5k +
4 IOUT
10V
OUT ROUT
1 VOUT = VSENSE x
RIN
LTC6101/LTC6101HV V–
2 ROUT
6101 BD


ACCURACY SPEED

High Side Fixed 8 LT1787 75µV 20µA 300kHz 0.1V/µs 2.5V to 36V 2.5V to 36V ±10V
Bi-directional or LT1787HV 75µV 20µA 300kHz 0.1V/µs 2.5V to 60V 2.5V to 60V ±10V
Current or Voltage (ROUT = 20k) Scaleable
RSENSE ISENSE SO-8
VS– VS+
MSOP-8
RG1A RG2A
1.25k 1.25k
FIL– FIL+
RG1B RG2B
1.25k 1.25k

– +
A1
IOUT

VBIAS
ROUT
Q1 Q2 20k
VOUT

VEE CURRENT MIRROR
1787 F 01

High Side Resistor LT1494 150µV 250pA 3kHz 0.001V/µs 2.1V to 36V 0 to VS + (36V – VS) 36V
One Direction Ratio LT1636 50µV 5nA 200kHz 0.07V/µs 2.6V to 44V 0 to VS + (44V – VS) 44V
Voltage Out LT1637 100µV 20nA 1MHz 0.35V/µs 1.8V to 44V 0 to VS + (44V – VS) 44V
Over the Top Amplifiers LT1672 150µV 250pA 12kHz 0.005V/µs 2.1V to 36V 0 to VS + (36V – VS) 36V
3V TO 44V LT1782 400µV 8nA 200kHz 0.07V/µs 2.2V to 18V 0 to VS + (18V – VS) 36V
R1
200Ω
LT1783 400µV 45nA 1.25MHz 0.42V/µs 2.2V to 18V 0 to VS + (18V – VS) 36V
3V
RS
+
LT1784 1500µV 250nA 2.5MHz 2.4V/µs 2V to 18V 0 to VS + (18V – VS) 36V
0.2Ω
Q1
LT1637
2N3904
– VOUT
ILOAD
R2
(0V TO 2.7V)
DIP-8
VOUT 2k
LOAD ILOAD =
(RS)(R2/R1) 1637 TA06 MS-8
SO-8
DFN
SOT23-5
SOT23–6


ACCURACY SPEED

High Side Resistor LTC2053 5µV 4nA 200kHz 0.2V/µs 2.7V to 11V 2.7V to 11V 5.5V
One Direction Ratio LTC6800 5µV 4nA 200kHz 0.2V/µs 2.7V to 5.5V 2.7V to 5.5V 5.5V
Voltage Out
Instrumentation Amplifier DFN
5V
0.1µF
MS-8
3
+ 8
7
LTC2053 VOUT
2 6
VIN – 5
4
1

0.1µF VOUT = –VIN

2053 TA07
–5V

High Side or Low Side Unity LTC6943 6pA 90kHz 5V to 18V 5V to 18V 18V
One Direction
Voltage on a TSSOP – 16
capacitor output
Flying Capacitor
E
I
POSITIVE OR
NEGATIVE RAIL RSHUNT

1/2 LTC6943
11 12

10

1µF 1µF E I= E
RSHUNT
9

6 7

14 15
0.01µF

6943 • TA01b


ACCURACY SPEED

High Side or Low Side 1 and 10 LT1990 900µV 2.5nA 105kHz 0.55V/µs 2.4V to 36V –250V to 250V ±250V
Bi-Directional 1 to 13 LT1991 15µV 110kHz 0.12V/µs 2.7V to 36V –60V to 60V ±60V
Voltage Out 1 to 7 LT1995 1000µV 2.5nA 32MHz 1000V/µs 5V to 36V 0V to 36V VS + 0.3V
Difference Amplifiers 9 to 117 LT1996 15µV 38kHz 0.12V/µs 2.7V to 36V –60V to 60V ±60V
V S+
8
9
10
M9
M3
7 Pin Strap SO-8
VIN – M1

VIN + 1
LT1991
6

R1
Configurable DFN
P1
R2* 2 5 10k
10k 3
P3
P9 4
MS–10
V + – VIN –
ILOAD = IN
V S– 10kΩ
*SHORT R2 FOR LOWEST OUTPUT
OFFSET CURRENT. INCLUDE R2 FOR
HIGHEST OUTPUT IMPEDANCE.

Low Side Resistor LTC2050 0.5µV 75pA 3MHz 2V/µs 2.7V to 7V 0V to (VS – 1.3V) VS + 0.3V
One Direction Ratio LTC2054 0.5µV 0.6pA 500kHz 0.5V/µs 2.7V to 7V 0V to (VS – 0.7V) VS + 0.3V
Voltage Out LTC2054HV 0.5µV 0.6pA 500kHz 0.5V/µs 2.7V to 12V 0V to (VS – 0.7V) VS + 0.3V
Zero-Drift Amplifiers
5V SO-8
3
+
5 OUT
3V/AMP
SOT23-5
1
4
LTC2050HV
LOAD CURRENT
IN MEASURED SOT23 – 6
– 2
CIRCUIT, REFERRED
TO –5V
10Ω 10k

TO 3mΩ
MEASURED
CIRCUIT
LOAD CURRENT 0.1µF
– 5V 2050 TA08

Low Side Resistor LT1218 25µV 30nA 300kHz 0.1V/µs 2V to 36V 0V to VS VS + 0.3V
One Direction Ratio LT1677 20µV 2nA 7.2MHz 2.5V/µs 2.5V to 44V 0V to VS VS + 0.3V
Voltage Out LT1800 75µV 25nA 80MHz 25V/µs 2V to 12.6V 0V to VS VS + 0.3V
Rail to Rail I/O Amplifiers LT1806 100µV 1µA 325MHz 125V/µs 1.8V to 12.6V 0V to VS VS + 0.3V
IL 3V
LT6200 1400µV 10µA 110MHz 50V/µs 2.2V to 12.6V 0V to VS VS + 0.3V
0A TO 1A
52.3Ω + LT6220 70µV 15nA 60MHz 20V/µs 2.2V to 12.6V 0V to VS VS + 0.3V
VOUT
LT1800
0V TO 2V
0.1Ω –

52.3Ω 1k SO-8
1800 F02 DIP-8
VOUT = 2 • IL
f–3dB = 4MHz
UNCERTAINTY DUE TO VOS, IB < 4mA
SOT23-5
SOT23 – 6



High Side
This chapter discusses solutions for high side current “Classic” Positive Supply Rail Current Sense
sensing. With these circuits the total current supplied to 5V
a load is monitored in the positive power supply line.
200Ω
To see other chapters in this Application Note, return to
the Introduction.
0.2Ω +
LT6100 Load Current Monitor LT1637
Q1
200Ω 2N3904
TO LOAD RSENSE
– 0V TO 4.3V
LOAD ILOAD 2k
C1
+
1 8 0.1µF 5V VOUT = (2Ω)(ILOAD) 1637 TA02

V S– V S+
2
VCC A4
7 This circuit uses generic devices to assemble a function
+
3V
C2 – + similar to an LTC6101. A Rail-to-Rail Input type op amp
0.1µF
3 6
is required since input voltages are right at the upper rail.
FIL A2
The circuit shown here is capable of monitoring up to
44V applications. Besides the complication of extra parts,
4
VEE
OUT 5
OUTPUT the VOS performance of op amps at the supply is gener-
LT6100 ally not factory trimmed, thus less accurate than other
6100 F04
solutions. The finite current gain of the bipolar transistor
This is the basic LT6100 circuit configuration. The inter- is a small source of gain error.
nal circuitry, including an output buffer, typically operates
from a low voltage supply, such as the 3V shown. The Over-The-Top Current Sense
monitored supply can range anywhere from VCC + 1.4V 3V TO 44V
up to 48V. The A2 and A4 pins can be strapped various R1
200Ω
ways to provide a wide range of internally fixed gains.
3V
The input leads become very hi-Z when VCC is powered
RS
down, so as not to drain batteries for example. Access to 0.2Ω +
Q1
an internal signal node (pin 3) provides an option to in- LT1637
2N3904
clude a filtering function with one added capacitor. Small- – VOUT
(0V TO 2.7V)
ILOAD
signal range is limited by VOL in single-supply operation. R2
VOUT 2k
LOAD ILOAD =
(RS)(R2/R1) 1637 TA06

This circuit is a variation on the “classic” high-side cir-
cuit, but takes advantage of Over-the-Top input capability
to separately supply the IC from a low-voltage rail. This
provides a measure of fault protection to downstream
circuitry by virtue of the limited output swing set by the
low-voltage supply. The disadvantage is VOS in the Over-
the-Top mode is generally inferior to other modes, thus
less accurate. The finite current gain of the bipolar tran-
sistor is a source of small gain error.

High Side-1

Self-Powered High Side Current Sense Precision High Side Power Supply Current Sense
1.5mΩ
VREGULATOR

2 – 8
OUT
7 100mV/A
LTC6800
OF LOAD
3 + 6 10k CURRENT
5
4
0.1µF
ILOAD LOAD

150Ω

6800 TA01

This is a low-voltage, ultra-high-precision monitor featur-
This circuit takes advantage of the microampere supply ing a Zero-Drift Instrumentation Amplifier (IA) that pro-
current and Rail-to-Rail input of the LT1494. The circuit vides Rail-to-Rail inputs and outputs. Voltage gain is set
is simple because the supply draw is essentially equal to by the feedback resistors. Accuracy of this circuit is set
the load current developed through RA. This supply cur- by the quality of resistors selected by the user, small-
rent is simply passed through RB to form an output volt- signal range is limited by VOL in single-supply operation.
age that is appropriately amplified. The voltage rating of this part restricts this solution to
applications of <5.5V. This IA is sampled, so the output is
High Side Current Sense and Fuse Monitor discontinuous with input changes, thus only suited to
RSENSE very low frequency measurements.
TO LOAD 2mΩ FUSE
BATTERY
BUS
+
Positive Supply Rail Current Sense
1 8
VS– VS+ VCC
ADC R1
2 7 200Ω
POWER VCC A4
≥2.7V C2 – +
0.1µF
Rs
3 6 0.2Ω – –
FIL A2 Q1
1/2 LT1366 1/2 LT1366
TP0610L
+ +
4
VEE
OUT 5 OUTPUT
2.5V = 25A
ILOAD

R2
VO = ILOAD • RS ( )
R2
R1
LT6100 LOAD 20k = ILOAD • 20Ω
DN374 F02
1366 TA01

The LT6100 can be used as a combination current sensor
and fuse monitor. This part includes on-chip output buff- This is a configuration similar to an LT6100 implemented
ering and was designed to operate with the low supply with generic components. A Rail-to-Rail or Over-the-Top
voltage (≥2.7V), typical of vehicle data acquisition sys- input op amp type is required (for the first section). The
tems, while the sense inputs monitor signals at the first section is a variation on the classic high-side where
higher battery bus potential. The LT6100 inputs are toler- the P-MOSFET provides an accurate output current into
ant of large input differentials, thus allowing the blown- R2 (compared to a BJT). The second section is a buffer
fuse operating condition (this would be detected by an to allow driving ADC ports, etc., and could be configured
output full-scale indication). The LT6100 can also be with gain if needed. As shown, this circuit can handle up
powered down while maintaining high impedance sense to 36V operation. Small-signal range is limited by VOL in
inputs, drawing less than 1µA max from the battery bus. single-supply operation.

High Side-2

Precision Current Sensing in Supply Rails Measuring bias current into an Avalanche Photo
E
Diode (APD) using an instrumentation amplifier.
I
POSITIVE OR
NEGATIVE RAIL 1k
RSHUNT 1%
VIN BIAS OUTPUT
1/2 LTC6943 10V TO 33V TO APD
35V
11 12
– CURRENT
LT1789 MONITOR OUTPUT
10
0mA TO 1mA = 0V TO 1V
1µF 1µF E I= E + A=1
RSHUNT
9 AN92 F02a

1N4684
1k
6 7 3.3V
1%
VIN BIAS OUTPUT
10V TO 35V TO APD

14 15 10M
0.01µF – CURRENT
LT1789 MONITOR OUTPUT
6943 • TA01b
0mA TO 1mA = 0V TO 1V
+ A=1
This is the same sampling architecture as used in the
AN92 F02b
front-end of the LTC2053 and LTC6800, but sans op amp
gain stage. This particular switch can handle up to 18V, The upper circuit uses an instrumentation amplifier (IA)
so the ultra-high precision concept can be utilized at powered by a separate rail (>1V above VIN) to measure
higher voltages than the fully integrated ICs mentioned. across the 1kΩ current shunt. The lower figure is similar
This circuit simply commutates charge from the flying but derives its power supply from the APD bias line. The
sense capacitor to the ground-referenced output capaci- limitation of these circuits is the 35V maximum APD
tor so that under dc input conditions the single-ended voltage, whereas some APDs may require 90V or more.
output voltage is exactly the same as the differential In the single-supply configuration shown, there is also a
across the sense resistor. A high precision buffer ampli- dynamic range limitation due to VOL to consider. The ad-
fier would typically follow this circuit (such as an vantage of this approach is the high accuracy that is
LTC2054). The commutation rate is user-set by the ca- available in an IA.
pacitor connected to pin 14. For negative supply monitor-
ing, pin 15 would be tied to the negative rail rather than
ground.

High Side-3

Simple 500V Current Monitor Bidirectional Battery-Current Monitor
TO RSENSE
CHARGER/
LOAD C1
15V
1 8 1µF
–
FIL FIL+
–
LT1787
2 VS VS+ 7

3 VBIAS 6
DNC
ROUT
4 5
VEE OUTPUT
VOUT
C2 C3*
–5V 1µF 1000pF
1787 F02

*OPTIONAL

This circuit provides the capability of monitoring current
in either direction through the sense resistor. To allow
negative outputs to represent charging current, VEE is
connected to a small negative supply. In single-supply
Adding two external Mosfets to hold off the voltage al- operation (VEE at ground), the output range may be offset
lows the LTC6101 to connect to very high potentials and upwards by applying a positive reference level to VBIAS
monitor the current flow. The output current from the (1.25V for example). C3 may be used to form a filter in
LTC6101, which is proportional to the sensed input volt- conjunction with the output resistance (ROUT) of the part.
age, flows through M1 to create a ground referenced This solution offers excellent precision (very low VOS)
output voltage. and a fixed nominal gain of 8.

High Side-4

LTC6101 Supply Current Simple High Side Current
included as Load in Measurement Sense Using the LTC6101
V+ BATTERY BUS

RIN
RSENSE RIN
RSENSE 100Ω
4 3 0.01Ω
4 3
+ –
LOAD + –
LOAD 2 5
2 5

1 1 VOUT
LTC6101 VOUT LT6101
4.99V = 10A
ROUT
ROUT
4.99k
6101 F06 VOUT = ILOAD(RSENSE • ROUT/RIN) DN374 F01

This is a basic high side current monitor using the
This is the basic LTC6101 high-side sensing supply-
LTC6101. The selection of RIN and ROUT establishes the
monitor configuration, where the supply current drawn
desired gain of this circuit, powered directly from the
by the IC is included in the readout signal. This configu-
battery bus. The current output of the LTC6101 allows it
ration is useful when the IC current may not be negligible
to be located remotely to ROUT. Thus, the amplifier can
in terms of overall current draw, such as in low-power
be placed directly at the shunt, while ROUT is placed near
battery-powered applications. RSENSE should be selected
the monitoring electronics without ground drop errors.
to limit voltage-drop to <500mV for best linearity. If it is
This circuit has a fast 1µs response time that makes it
desirable not to include the IC current in the readout, as
ideal for providing MOSFET load switch protection. The
in load monitoring, pin 5 may be connected directly to V+
switch element may be the high side type connected be-
instead of the load. Gain accuracy of this circuit is limited
tween the sense resistor and the load, a low side type
only by the precision of the resistors selected by the user.
between the load and ground or an H-bridge. The circuit
is programmable to produce up to 1mA of full-scale out-
put current into ROUT, yet draws a mere 250µA supply
current when the load is off.

High Side-5

High-Side Transimpedance Amplifier Intelligent High Side Switch

The LT1910 is a dedicated high side MOSFET driver with
built in protection features. It provides the gate drive for
a power switch from standard logic voltage levels. It pro-
vides shorted load protection by monitoring the current
Current through a photodiode with a large reverse bias
flow to through the switch. Adding an LTC6101 to the
potential is converted to a ground referenced output volt-
same circuit, sharing the same current sense resistor,
age directly through an LTC6101. The supply rail can be
provides a linear voltage signal proportional to the load
as high as 70V. Gain of the I to V conversion, the trans-
current for additional intelligent control.
impedance, is set by the selection of resistor RL.

High Side-6

48V Supply Current Monitor with
Isolated Output and 105V Survivability

The HV version of the LTC6101 can operate with a total
supply voltage of 105V. Current flow in high supply volt-
age rails can be monitored directly or in an isolated fash-
ion as shown in this circuit. The gain of the circuit and
the level of output current from the LTC6101 depends on
the particular opto-isolator used.

High Side-7


Low Side
This chapter discusses solutions for low side current Precision Current Sensing in Supply Rails
sensing. With these circuits the current flowing in the E
I
ground return or negative power supply line is moni- POSITIVE OR
NEGATIVE RAIL
tored. RSHUNT

1/2 LTC6943
To see other chapters in this Application Note, return to 11 12

the Introduction.
10
“Classic” High-Precision Low Side Current Sense 1µF 1µF E I= E
RSHUNT
5V 9

3 5 OUT
+ 3V/AMP 6 7
1 LOAD CURRENT
LTC2050HV
4 IN MEASURED
– 2
CIRCUIT, REFERRED 14 15
TO –5V
0.01µF
10Ω 10k
6943 • TA01b
TO 3mΩ
MEASURED
CIRCUIT
LOAD CURRENT 0.1µF
This is the same sampling architecture as used in the
– 5V 2050 TA08 front-end of the LTC2053 and LTC6800, but sans op amp
gain stage. This particular switch can handle up to 18V,
This configuration is basically a standard non-inverting
so the ultra-high precision concept can be utilized at
amplifier. The op amp used must support common-mode
higher voltages than the fully integrated ICs mentioned.
operation at the lower rail and the use of a Zero-Drift type
This circuit simply commutates charge from the flying
(as shown) provides excellent precision. The output of
sense capacitor to the ground-referenced output capaci-
this circuit is referenced to the lower Kelvin contact,
tor so that under dc input conditions the single-ended
which could be ground in a single-supply application.
output voltage is exactly the same as the differential
Small-signal range is limited by VOL for single-supply
across the sense resistor. A high precision buffer ampli-
designs. Scaling accuracy is set by the quality of the
fier would typically follow this circuit (such as an
user-selected resistors.
LTC2054). The commutation rate is user-set by the ca-
pacitor connected to pin 14. For negative supply monitor-
ing, pin 15 would be tied to the negative rail rather than
ground.

Low Side-1

–48V Hot Swap Controller
GND
RIN
3× 1.8k IN SERIES
+ CL
1/4W EACH 100µF

CIN LOAD
GND 1 1µF R3
(SHORT PIN) 5.1k
R1 VIN EN
402k LTC4252-1 VOUT
1% *
8 2
OV PWRGD
9 7 RD 1M
UV DRAIN
R2 10 6 Q1
32.4k TIMER GATE
IRF530S
1% CT 3 4
SS VEE SENSE
0.33µF RC RS
C1 CSS 5 10Ω 0.02Ω
10nF 68nF CC
18nF

–48V
* M0C207

This load protecting circuit employs low-side current event of supply or load faults. An internal shunt regulator
sensing. The N-MOSFET is controlled to soft-start the establishes a local operating voltage.
load (current ramping) or to disconnect the load in the

–48V Low Side Precision Current Sense

The first stage amplifier is basically a complementary and furnishes a positive output voltage for increasing
form of the “classic” high-side current sense, designed load current. . A dual op amp cannot be used for this im-
to operate with telecom negative supply voltage. The plementation due to the different supply voltages for
Zener forms an inexpensive “floating” shunt-regulated each stage. This circuit is exceptionally precise due to the
supply for the first op amp. The N-MOSFET drain delivers use of Zero Drift op amps. The scaling accuracy is estab-
a metered current into the virtual ground of the second lished by the quality of the user-selected resistors. Small-
stage, configured as a trans-impedance amplifier (TIA). signal range is limited by VOL in single-supply operation
The second op amp is powered from a positive supply of the second stage.

Low Side-2

Fast Compact –48V Current Sense
VOUT = 3V – 0.1Ω • ISENSE
ISENSE = 0A TO 30A
ACCURACY ≈ 3%
VOUT
Q1 R1 1k
FMMT493 4.7k 1%
VS = 3V
30.1Ω
1% –
3.3k R1 REDUCES Q1 DISSIPATION
0805
LT1797
×3
+
0.1µF
SETTLES TO 1% IN 2µs,
BZX84C6V8 1V OUTPUT STEP
VZ = 6.8V 0.003Ω
1% 3W
–48V SUPPLY –48V LOAD
(–42V TO –56V) – + 1797 TA01

ISENSE

This amplifier configuration is essentially the comple- tance (1kΩ in this circuit). In this circuit, the output volt-
mentary implementation to the classic high-side configu- age is referenced to a positive potential and moves
ration. The op amp used must support common-mode downward when representing increasing –48V loading.
operation at its lower rail. A “floating” shunt-regulated Scaling accuracy is set by the quality of resistors used
local supply is provided by the Zener diode, and the tran- and the performance of the NPN transistor.
sistor provides metered current to an output load resis-

–48V Current Monitor

Low Side-3

In this circuit an economical ADC is used to acquire the and/or higher efficiency operation, the ADC may be pow-
sense resistor voltage drop directly. The converter is ered from a small transformer circuit as shown below.
powered from a “floating” high-accuracy shunt-regulated
supply and is configured to perform continuous conver-
sions. The ADC digital output drives an opto-isolator,
level-shifting the serial data stream to ground. For wider
supply voltage applications, the 13k biasing resistor may
be replaced with an active 4mA current source such as
shown to the right. For complete dielectric isolation

GND
RIN
3× 1.8k IN SERIES
+ CL
1/4W EACH 100µF

CIN LOAD
GND 1 1µF R3
(SHORT PIN) 5.1k
R1 VIN EN
402k LTC4252-1 VOUT
1% *
8 2
OV PWRGD
9 7 RD 1M
UV DRAIN
R2 10 6 Q1
32.4k TIMER GATE
IRF530S
1% CT 3 4
SS VEE SENSE
0.33µF RC RS
C1 CSS 5 10Ω 0.02Ω
10nF 68nF CC
18nF

–48V
* M0C207


Low Side-4

Simple Telecom Power Supply Fuse Monitor
47k
–48V 5V
RETURN FUSE
STATUS
R1 R2
100k 100k 3 MOC207
SUPPLY A SUPPLY B
RTN 47k VA VB STATUS STATUS
1 4 5V
VA OUT F OK OK 0 0
SUPPLY A OK UV OR OV 0 1
8 STATUS UV OR OV OK 1 0
VB UV OR OV UV OR OV 1 1
LTC1921 OK: WITHIN SPECIFICATION
2 MOC207
OV: OVERVOLTAGE
FUSE A
47k UV: UNDERVOLTAGE
5V
7 5 VFUSE A VFUSE B FUSE STATUS
FUSE B OUT A SUPPLY B
STATUS = VA = VB 0
= VA ≠ VB 1
≠ VA = VB 1
MOC207 ≠ VA ≠ VB 1*
6
OUT B 0: LED/PHOTODIODE ON
R3
47k 1: LED/PHOTODIODE OFF
F1 D1 *IF BOTH FUSES (F1 AND F2) ARE OPEN,
SUPPLY A 1/4W
–48V OUT ALL STATUS OUTPUTS WILL BE HIGH
–48V SINCE R3 WILL NOT BE POWERED
F2 D2
SUPPLY B = LOGIC COMMON
–48V

The LTC1921 provides an all-in-one telecom fuse and status flags are generated that indicate the condition of
supply-voltage monitoring function. Three opto-isolated the supplies and the fuses.

Low Side-5


Negative Voltage
This chapter discusses solutions for negative voltage To see other chapters in this Application Note, return to
current sensing. the Introduction.

Telecom Supply Current Monitor
+ 5V

LOAD IL 48V

– 3
+
7
G2
5 6
RS
LT1990 VOUT
2 8
– 4
G1
1
–77V ≤ VCM ≤ 8V REF
VOUT = VREF – (10 • IL • RS) VREF = 4V
4 5
IN OUT 174k 1nF
LT6650 1
GND FB
2
20k
1990 AI01

1µF

The LT1990 is a wide common-mode range difference mately 4V by the LT6650. The output signal moves
amplifier used here to amplify the sense resistor drop by downward from the reference potential in this connection
10. To provide the desired input range when using a sin- so that a large output swing can be accommodated.
gle 5V supply, the reference potential is set to approxi-

GND
RIN
3× 1.8k IN SERIES
+ CL
1/4W EACH 100µF

CIN LOAD
GND 1 1µF R3
(SHORT PIN) 5.1k
R1 VIN EN
402k LTC4252-1 VOUT
1% *
8 2
OV PWRGD
9 7 RD 1M
UV DRAIN
R2 10 6 Q1
32.4k TIMER GATE
IRF530S
1% CT 3 4
SS VEE SENSE
0.33µF RC RS
C1 CSS 5 10Ω 0.02Ω
10nF 68nF CC
18nF

–48V
* M0C207


Negative Voltage-1

–48V Low Side Precision Current Sense

The first stage amplifier is basically a complementary and furnishes a positive output voltage for increasing
form of the “classic” high-side current sense, designed load current. . A dual op amp cannot be used for this im-
to operate with telecom negative supply voltage. The plementation due to the different supply voltages for
Zener forms an inexpensive “floating” shunt-regulated each stage. This circuit is exceptionally precise due to the
supply for the first op amp. The N-MOSFET drain delivers use of Zero Drift op amps. The scaling accuracy is estab-
a metered current into the virtual ground of the second lished by the quality of the user-selected resistors. Small-
stage, configured as a trans-impedance amplifier (TIA). signal range is limited by VOL in single-supply operation
The second op amp is powered from a positive supply of the second stage.

Fast Compact –48V Current Sense
VOUT = 3V – 0.1Ω • ISENSE
ISENSE = 0A TO 30A
ACCURACY ≈ 3%
VOUT
Q1 R1 1k
FMMT493 4.7k 1%
VS = 3V
30.1Ω
1% –
3.3k R1 REDUCES Q1 DISSIPATION
0805
LT1797
×3
+
0.1µF
SETTLES TO 1% IN 2µs,
BZX84C6V8 1V OUTPUT STEP
VZ = 6.8V 0.003Ω
1% 3W
–48V SUPPLY –48V LOAD
(–42V TO –56V) – + 1797 TA01

ISENSE

This amplifier configuration is essentially the comple- tance (1kΩ in this circuit). In this circuit, the output volt-
mentary implementation to the classic high-side configu- age is referenced to a positive potential and moves
ration. The op amp used must support common-mode downward when representing increasing –48V loading.
operation at its lower rail. A “floating” shunt-regulated Scaling accuracy is set by the quality of resistors used
local supply is provided by the Zener diode, and the tran- and the performance of the NPN transistor.
sistor provides metered current to an output load resis-

Negative Voltage-2

Current sensor

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