NSC LMP8603QMMX 60v common mode, fixed gain, bidirectional precision current sensing amplifier Datasheet

March 18, 2010
60V Common Mode, Fixed Gain, Bidirectional Precision
Current Sensing Amplifier
General Description
Features
The LMP8602 and LMP8603 are fixed gain precision amplifiers. The parts will amplify and filter small differential signals
in the presence of high common mode voltages. The input
common mode voltage range is –22V to +60V when operating
from a single 5V supply. With a 3.3V supply, the input common mode voltage range is from –4V to +27V. The LMP8602
and LMP8603 are members of the Linear Monolithic Precision
(LMP®) family and are ideal parts for unidirectional and bidirectional current sensing applications. All parameter values
of the parts that are shown in the tables are 100% tested and
all bold values are also 100% tested over temperature.
The parts have a precise gain of 50x for the LMP8602 and
100x for the LMP8603, which are adequate in most targeted
applications to drive an ADC to its full scale value. The fixed
gain is achieved in two separate stages, a preamplifier with a
gain of 10x and an output stage buffer amplifier with a gain of
5x for the LMP8602 and 10x for the LMP8603. The connection
between the two stages of the signal path is brought out on
two pins to enable the possibility to create an additional filter
network around the output buffer amplifier. These pins can
also be used for alternative configurations with different gain
as described in the applications section.
The mid-rail offset adjustment pin enables the user to use
these devices for bidirectional single supply voltage current
sensing. The output signal is bidirectional and mid-rail referenced when this pin is connected to the positive supply rail.
With the offset pin connected to ground, the output signal is
unidirectional and ground-referenced.
The LMP8602 and LMP8603 are available in a 8–Pin SOIC
package and in a 8–Pin MSOP package.
The LMP8602Q and LMP8603Q incorporate enhanced manufacturing and support processes for the automotive market,
including defect detection methodologies. Reliability qualification is compliant with the requirements and temperature
grades defined in the AEC Q100 standard.
Unless otherwise noted, typical values at TA = 25°C,
VS = 5.0V, Gain = 50x (LMP8602), Gain = 100x (LMP8603)
10μV/°C max
■ TCVos
90 dB min
■ CMRR
Input
offset
voltage
1 mV max
■
−4V to 27V
■ CMVR at VS = 3.3V
−22V to 60V
■ CMVR at VS = 5.0V
■ Operating ambient temperature range −40°C to 125°C
■ Single supply bidirectional operation
■ All Min / Max limits 100% tested
■ LMP8602Q and LMP8603Q available in Automotive AECQ100 Grade 1 qualified version
Applications
■
■
■
■
■
■
■
High side and low side driver configuration current sensing
Bidirectional current measurement
Current loop to voltage conversion
Automotive fuel injection control
Transmission control
Power steering
Battery management systems
Typical Applications
30083401
LMP™ is a trademark of National Semiconductor Corporation.
© 2010 National Semiconductor Corporation
300834
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LMP8602/LMP8602Q/LMP8603/LMP8603Q 60V Common Mode, Fixed Gain, Bidirectional
Precision Current Sensing Amplifier
LMP8602/LMP8602Q/
LMP8603/LMP8603Q
LMP8602/LMP8602Q/LMP8603/LMP8603Q
Storage Temperature Range
Junction Temperature (Note 3)
Mounting Temperature
Infrared or Convection (20 sec)
Wave Soldering Lead (10 sec)
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
ESD Tolerance (Note 4)
Human Body
For input pins only
For all other pins
Machine Model
Charge Device Model
Supply Voltage (VS - GND)
Continuous Input Voltage (−IN
and +IN) (Note 6)
Transient (400 ms)
Maximum Voltage at A1, A2,
OFFSET and OUT Pins
Operating Ratings
±4000V
±2000V
200V
1000V
6.0V
235°C
260°C
(Note 1)
Supply Voltage (VS – GND)
3.0V to 5.5V
Offset Voltage (Pin 7 )
0 to VS
Temperature Range (Note 3)
Packaged devices
−40°C to +125°C
Package Thermal Resistance (Note 3)
−22V to 60V
−25V to 65V
VS +0.3V and
GND -0.3V
3.3V Electrical Characteristics
−65°C to 150°C
150°C
8-Pin SOIC (θJA)
190°C/W
8-Pin MSOP (θJA)
203°C/W
(Note 2)
Unless otherwise specified, all limits guaranteed at TA = 25°C, VS = 3.3V, GND = 0V, −4V ≤ VCM ≤ 27V, and RL = ∞, Offset (Pin
7) is grounded, 10nF between VS and GND. Boldface limits apply at the temperature extremes.
Symbol
Parameter
Conditions
Min
Typ
Max
(Note 7) (Note 5) (Note 7)
Units
Overall Performance (From -IN (pin 1) and +IN (pin 8) to OUT (pin 5) with pins A1 (pin 3) and A2 (pin 4) connected)
IS
AV
Supply Current
Total Gain
1
1.3
LMP8602
49.75
50
50.25
LMP8603
99.5
100
100.5
−2.7
±20
Gain Drift (Note 15)
−40°C ≤ TA ≤ 125°C
SR
Slew Rate (Note 8)
VIN = ±0.165V
BW
Bandwidth
VOS
Input Offset Voltage
VCM = VS / 2
TCVOS
Input Offset Voltage Drift (Note 9)
−40°C ≤ TA ≤ 125°C
en
Input Referred Voltage Noise
PSRR
Power Supply Rejection Ratio
V/V
ppm/°C
0.4
0.7
V/μs
50
60
kHz
0.15
±1
mV
2
±10
μV/°C
0.1 Hz − 10 Hz, 6 Sigma
16.4
μVP-P
Spectral Density, 1 kHz
830
nV/√Hz
DC, 3.0V ≤ VS ≤ 3.6V, VCM = VS/2
70
LMP8602
Mid−scale Offset Scaling Accuracy
mA
86
±0.25
Input Referred
LMP8603
±0.45
Input Referred
dB
±1
%
±0.33
mV
±1.5
%
±0.248
mV
Preamplifier (From input pins -IN (pin 1) and +IN (pin 8) to A1 (pin 3))
RCM
Input Impedance Common Mode
−4V ≤ VCM ≤ 27V
250
295
350
kΩ
RDM
Input Impedance Differential Mode
−4V ≤ VCM ≤ 27V
500
590
700
kΩ
VOS
Input Offset Voltage
VCM = VS / 2
±0.15
±1
mV
DC CMRR DC Common Mode Rejection Ratio
−2V ≤ VCM ≤ 24V
86
96
AC Common Mode Rejection Ratio
AC CMRR
(Note 10)
f = 1 kHz
80
94
f = 10 kHz
CMVR
Input Common Mode Voltage Range
for 80 dB CMRR
K1
Gain (Note 15)
RF-INT
Output Impedance Filter Resistor
TCRF-INT
Output Impedance Filter Resistor Drift
A1 VOUT
A1 Output Voltage Swing
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dB
dB
85
27
V
9.95
10.0
10.05
V/V
99
100
101
±5
±50
2
10
−4
RL = ∞
VOL
VOH
3.2
2
3.25
kΩ
ppm/°C
mV
V
Parameter
Conditions
Min
Typ
Max
(Note 7) (Note 5) (Note 7)
Units
Output Buffer (From A2 (pin 4) to OUT ( pin 5 )
VOS
Input Offset Voltage
K2
Gain (Note 15)
IB
Input Bias Current of A2 (Note 11)
A2 VOUT
A2 Output Voltage Swing
(Note 12, Note 13)
0V ≤ VCM ≤ VS
−2
−2.5
LMP8602
LMP8603
Output Short-Circuit Current (Note 14)
5V Electrical Characteristics
2
2.5
4.975
5
5.025
9.95
10
10.05
−40
VOL,
LMP8602
10
40
RL = 100 kΩ
LMP8603
10
80
VOH,
mV
V/V
fA
±20
3.28
3.29
Sourcing, VIN = VS, VOUT = GND
-25
-38
-60
Sinking, VIN = GND, VOUT = VS
30
46
65
RL = 100 kΩ
ISC
±0.5
nA
mV
V
mA
(Note 2)
Unless otherwise specified, all limits guaranteed for at TA = 25°C, VS = 5V, GND = 0V, −22V ≤ VCM ≤ 60V, and RL = ∞, Offset (Pin
7) is grounded, 10nF between VS and GND. Boldface limits apply at the temperature extremes.
Symbol
Parameter
Conditions
Min
Typ
Max
(Note 7) (Note 5) (Note 7)
Units
Overall Performance (From -IN (pin 1) and +IN (pin 8) to OUT (pin 5) with pins A1 (pin 3) and A2 (pin 4) connected)
IS
AV
Supply Current
Total Gain (Note 15)
1.1
1.5
LMP8602
49.75
50
50.25
LMP8603
99.5
100
100.5
−2.8
±20
Gain Drift
−40°C ≤ TA ≤ 125°C
SR
Slew Rate (Note 8)
VIN = ±0.25V
BW
Bandwidth
VOS
Input Offset Voltage
TCVOS
Input Offset Voltage Drift (Note 9)
eN
Input Referred Voltage Noise
PSRR
Power Supply Rejection Ratio
V/V
ppm/°C
0.6
0.83
V/μs
50
60
kHz
−40°C ≤ TA ≤ 125°C
0.15
±1
mV
2
±10
μV/°C
0.1 Hz − 10 Hz, 6 Sigma
17.5
μVP-P
Spectral Density, 1 kHz
890
nV/√Hz
90
dB
DC 4.5V ≤ VS ≤ 5.5V
70
LMP8602
Mid−scale Offset Scaling Accuracy
mA
±0.25
Input Referred
LMP8603
±0.45
Input Referred
±1
%
±0.50
mV
±1.5
%
±0.375
mV
Preamplifier (From input pins -IN (pin 1) and +IN (pin 8) to A1 (pin 3))
RCM
Input Impedance Common Mode
RDM
Input Impedance Differential Mode
VOS
Input Offset Voltage
DC CMRR DC Common Mode Rejection Ratio
0V ≤ VCM ≤ 60V
250
295
350
kΩ
−20V ≤ VCM< 0V
165
193
250
kΩ
0V ≤ VCM ≤ 60V
500
590
700
kΩ
−20V ≤ VCM < 0V
300
386
500
kΩ
±0.15
±1
mV
VCM = VS / 2
−20V ≤ VCM ≤ 60V
90
105
80
96
AC CMRR
AC Common Mode Rejection Ratio
(Note 10)
f = 1 kHz
CMVR
Input Common Mode Voltage Range
for 80 dB CMRR
K1
Gain (Note 15)
RF-INT
Output Impedance Filter Resistor
f = 10 kHz
dB
dB
83
3
60
V
9.95
10
10.05
V/V
99
100
101
kΩ
−22
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LMP8602/LMP8602Q/LMP8603/LMP8603Q
Symbol
LMP8602/LMP8602Q/LMP8603/LMP8603Q
Symbol
TCRF-INT
A1 VOUT
Parameter
Conditions
Min
Typ
Max
(Note 7) (Note 5) (Note 7)
Units
±5
±50
ppm/°C
2
10
mV
Output Impedance Filter Resistor Drift
A1 Ouput Voltage Swing
RL = ∞
VOL
VOH
4.95
4.985
0V ≤ VCM ≤ VS
−2
−2.5
±0.5
2
2.5
LMP8602
4.975
5
5.025
LMP8603
9.95
10
10.05
V
Output Buffer (From A2 (pin 4) to OUT ( pin 5 )
VOS
Input Offset Voltage
K2
Gain (Note 15)
IB
Input Bias Current of A2 (Note 11)
A2 VOUT
A2 Ouput Voltage Swing
(Note 12, Note 13)
−40
VOL,
LMP8602
10
40
RL = 100 kΩ
LMP8603
10
80
4.98
4.99
Sourcing, VIN = VS, VOUT = GND
–25
–42
–60
Sinking, VIN = GND, VOUT = VS
30
48
65
RL = 100 kΩ
ISC
Output Short-Circuit Current (Note 14)
V/V
fA
±20
VOH,
mV
nA
mV
V
mA
Note 1: “Absolute Maximum Ratings” indicate limits beyond which damage to the device may occur, including inoperability and degradation of the device reliability
and/or performance. Functional operation of the device and/or non-degradation at the Absolute Maximum Ratings or other conditions beyond those indicated in
the Recommended Operating Conditions is not implied. The Recommended Operating Conditions indicate conditions at which the device is functional and the
device should not be beyond such conditions. All voltages are measured with respect to the ground pin, unless otherwise specified.
Note 2: The electrical Characteristics tables list guaranteed specifications under the listed Recommended Operating Conditions except as otherwise modified or
specified by the Electrical Characteristics Conditions and/or Notes. Typical specifications are estimations only and are not guaranteed.
Note 3: The maximum power dissipation must be derated at elevated temperatures and is dictated by TJ(MAX), θJA, and the ambient temperature, TA. The maximum
allowable power dissipation PDMAX = (TJ(MAX) - TA) / θJA or the number given in Absolute Maximum Ratings, whichever is lower.
Note 4: Human Body Model per MIL-STD-883, Method 3015.7. Machine Model, per JESD22-A115-A. Field-Induced Charge-Device Model, per JESD22-C101C.
Note 5: Typical values represent the most likely parameter norms at TA = +25°C, and at the Recommended Operation Conditions at the time of product
characterization and are not guaranteed.
Note 6: For the MSOP package, the bare board spacing at the solder pads of the package will be to small for reliable use at higher voltages (VCM >25V) Therefore
it is strongly advised to add a conformal coating on the PCB assembled with the LMP8602 and LMP8603.
Note 7: Datasheet min/max specification limits are guaranteed by test.
Note 8: Slew rate is the average of the rising and falling slew rates.
Note 9: Offset voltage drift determined by dividing the change in VOS at temperature extremes into the total temperature change.
Note 10: AC Common Mode Signal is a 5VPP sine-wave (0V to 5V) at the given frequency.
Note 11: Positive current corresponds to current flowing into the device.
Note 12: For this test input is driven from A1 stage in uni-directional mode (Offset pin connected to GND).
Note 13: For VOL, RL is connected to VS and for VOH, RL is connected to GND.
Note 14: Short-Circuit test is a momentary test. Continuous short circuit operation at elevated ambient temperature can result in exceeding the maximum allowed
junction temperature of 150°C.
Note 15: Both the gain of the preamplifier A1V and the gain of the buffer amplifier A2V are measured individually. The over all gain of both amplifiers AV is also
measured to assure the gain of all parts is always within the AV limits.
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4
LMP8602/LMP8602Q/LMP8603/LMP8603Q
Block Diagram
30083405
K2 = 5 for LMP8602, K2 = 10 for LMP8603
Connection Diagram
8-Pin SOIC / MSOP
30083402
Top View
Pin Descriptions
Pin
Name
Description
2
GND
Power Ground
6
VS
Positive Supply Voltage
1
−IN
Negative Input
8
+IN
Positive Input
3
A1
Preamplifier output
4
A2
Input from the external filter network and / or A1
Offset
7
OFFSET
Output
5
OUT
Power Supply
Inputs
Filter Network
DC Offset for bidirectional signals
Single ended output
5
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LMP8602/LMP8602Q/LMP8603/LMP8603Q
Ordering Information
Package
Part Number
LMP8602MA
8-Pin SOIC
LMP8602MAX
LMP8602QMA
LMP8602QMAX
LMP8602MM
8–Pin MSOP
LMP8602MMX
LMP8602QMM
LMP8602QMMX
Package
Part Number
LMP8603MA
8-Pin SOIC
LMP8603MAX
LMP8603QMA
LMP8603QMAX
LMP8603MM
8–Pin MSOP
LMP8603MMX
LMP8603QMM
LMP8603QMMX
Package Marking
LMP8602MA
LMP8602QMA
Transport Media
NSC Drawing
95 Units/Rail
2.5K Units Tape and Reel
95 Units/Rail
M08A
2.5K Units Tape and Reel
1k Units Tape and Reel
AN3A
3.5K Units Tape and Reel
1k Units Tape and Reel
AF7A
MUA08A
3.5K Units Tape and Reel
Package Marking
LMP8603MA
LMP8603QMA
Transport Media
NSC Drawing
95 Units/Rail
2.5K Units Tape and Reel
95 Units/Rail
M08A
2.5K Units Tape and Reel
1k Units Tape and Reel
AP3A
3.5K Units Tape and Reel
1k Units Tape and Reel
AH7A
MUA08A
3.5K Units Tape and Reel
Automotive Grade (Q) product incorporates enhanced manufacturing and support processes for the automotive market, including
defect detection methodologies. Reliability qualification is compliant with the requirements and temperature grades defined in the
AEC Q100 standard. Automotive Grade products are identified with the letter Q. Fully compliant PPAP documentation is available.
For more information, go to http://www.national.com/automotive.
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6
Unless otherwise specified, all limits guaranteed for at TA = 25°C,
VS = 5V, GND = 0V, −22V ≤ VCM ≤ 60V, and RL = ∞, Offset (Pin 7) connected to VS, 10nF between VS and GND.
VOS vs. VCM at VS = 3.3V
VOS vs. VCM at VS = 5V
30083424
30083425
Input Bias Current Over Temperature (+IN and −IN pins)
at VS = 3.3V
Input Bias Current Over Temperature (+IN and −IN pins)
at VS = 5V
30083441
30083442
Input Bias Current Over Temperature (A2 pin)
at VS = 5V
Input Bias Current Over Temperature (A2 pin)
at VS = 5V
30083427
30083426
7
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LMP8602/LMP8602Q/LMP8603/LMP8603Q
Typical Performance Characteristics
LMP8602/LMP8602Q/LMP8603/LMP8603Q
Input Referred Voltage Noise vs. Frequency
PSRR vs. Frequency
30083410
30083417
Gain vs. Frequency LMP8602
Gain vs. Frequency LMP8603
30083411
30083412
CMRR vs. Frequency at VS = 3.3V
CMRR vs. Frequency at VS = 5V
30083428
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30083429
8
Step Response at VS = 5V
RL = 10kΩ LMP8602
30083418
30083419
Settling Time (Falling Edge) at VS = 3.3V
LMP8602
Settling Time (Falling Edge) at VS = 5V
LMP8602
30083420
30083421
Settling Time (Rising Edge) at VS = 3.3V
LMP8602
Settling Time (Rising Edge) at VS = 5V
LMP8602
30083422
30083423
9
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LMP8602/LMP8602Q/LMP8603/LMP8603Q
Step Response at VS = 3.3V
RL = 10kΩ LMP8602
LMP8602/LMP8602Q/LMP8603/LMP8603Q
Step Response at VS = 3.3V
RL = 10kΩ LMP8603
Step Response at VS = 5V
RL = 10kΩ LMP8603
30083443
30083444
Settling Time (Falling Edge) at VS = 3.3V
LMP8603
Settling Time (Falling Edge) at VS = 5V
LMP8603
30083445
30083446
Settling Time (Rising Edge) at VS = 3.3V
LMP8603
Settling Time (Rising Edge) at VS = 5V
LMP8603
30083447
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30083448
10
Positive Swing vs. RLOAD VS = 5V
30083413
30083415
Negative Swing vs. RLOAD at VS = 3.3V
Negative Swing vs. RLOAD at VS = 5V
30083414
30083416
Gain Drift Distribution LMP8602
5000 parts
Gain Drift Distribution LMP8603
5000 parts
30083483
30083437
11
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LMP8602/LMP8602Q/LMP8603/LMP8603Q
Positive Swing vs. RLOAD at VS = 3.3V
LMP8602/LMP8602Q/LMP8603/LMP8603Q
Gain error Distribution at VS = 3.3V LMP8602
5000 parts
Gain error Distribution at VS = 3.3V LMP8603
5000 parts
30083484
30083438
Gain error Distribution at VS = 5V LMP8602
5000 parts
Gain error Distribution at VS = 5V LMP8603
5000 parts
30083485
30083439
CMRR Distribution at VS = 3.3V
5000 parts
CMRR Distribution at VS = 5V
5000 parts
30083433
30083432
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LMP8602/LMP8602Q/LMP8603/LMP8603Q
VOS Distribution at VS = 3.3V
5000 parts
VOS Distribution at VS = 5V
5000 parts
30083434
30083435
TCVOS Distribution
5000 parts
30083436
13
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LMP8602/LMP8602Q/LMP8603/LMP8603Q
THEORY OF OPERATION
The schematic shown in Figure 1 gives a schematic representation of the internal operation of the LMP8602/
LMP8603.
The signal on the input pins is typically a small differential
voltage across a current sensing shunt resistor. The input
signal may appear at a high common mode voltage. The input
signals are accessed through two input resistors. The proprietary chopping level-shift current circuit pulls or pushes current through the input resistors to bring the common mode
voltage behind these resistors within the supply rails. Subsequently, the signal is gained up by a factor of 10 (K1) and
brought out on the A1 pin through a trimmed 100 kΩ resistor.
In the application, additional gain adjustment or filtering components can be added between the A1 and A2 pins as will be
explained in subsequent sections. The signal on the A2 pin is
further amplified by a factor (K2) which equals a factor of 5 for
the LMP8602 and a factor of 10 for the LMP8603. The output
signal of the final gain stage is provided on the OUT pin. The
OFFSET pin allows the output signal to be level-shifted to
enable bidirectional current sensing as will be explained below.
Application Information
GENERAL
The LMP8602 and LMP8603 are fixed gain differential voltage precision amplifiers with a gain of 50x for the LMP8602,
and 100x for the LMP8603. The input common mode voltage
range is -22V to +60V when operating from a single 5V supply
or -4V to +27V input common mode voltage range when operating from a single 3.3V supply. The LMP8602 and
LMP8603 are members of the LMP family and are ideal parts
for unidirectional and bidirectional current sensing applications. Because of the proprietary chopping level-shift input
stage the LMP8602 and LMP8603 achieve very low offset,
very low thermal offset drift, and very high CMRR. The
LMP8602 and LMP8603 will amplify and filter small differential signals in the presence of high common mode voltages.
The LMP8602/LMP8602Q/LMP8603/LMP8603Q use level
shift resistors at the inputs. Because of these resistors, the
LMP8602/LMP8602Q/LMP8603/LMP8603Q can easily withstand very large differential input voltages that may exist in
fault conditions where some other less protected high-performance current sense amplifiers might sustain permanent
damage.
PERFORMANCE GUARANTIES
To guaranty the high performance of the LMP8602/LMP8602Q/LMP8603/LMP8603Q, all minimum and maximum
values shown in the parameter tables of this data sheet are
100% tested where all bold limits are also 100% tested over
temperature.
30083405
K2 = 5 for LMP8602, K2 = 10 for LMP8603
FIGURE 1. Theory of Operation
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30083455
K1 = 10, K2 = 5 for LMP8602, K2 = 10 for LMP8603
FIGURE 2. Second Order Low Pass Filter
When the corner frequency of the additional filter is much
lower than 60 kHz, the transfer function of the described amplifier van be written as:
For any filter gain K > 1x, the design procedure can be very
simple if the two capacitors are chosen to in a certain ratio.
Inserting this in the above equation for Q results in:
Where K1 equals the gain of the preamplifier and K2 that of
the buffer amplifier.
The above equation can be written in the normalized frequency response for a 2nd order low pass filter:
Which results in:
The Cutt-off frequency ωo in rad/sec (divide by 2π to get the
cut-off frequency in Hz) is given by:
In this case, given the predetermined value of R1 = 100 kΩ
( the internal resistor), the quality factor is set solely by the
value of the resistor R2.
and the quality factor of the filter is given by:
15
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LMP8602/LMP8602Q/LMP8603/LMP8603Q
It is also possible to create an additional second order SallenKey low pass filter as shown in Figure 2 by adding external
components R2, C1 and C2. Together with the internal
100 kΩ resistor R1, this circuit creates a second order lowpass filter characteristic.
ADDITIONAL SECOND ORDER LOW PASS FILTER
The LMP8602/LMP8602Q/LMP8603/LMP8603Q has a third
order Butterworth low-pass characteristic with a typical bandwidth of 60 kHz integrated in the preamplifier stage of the part.
The bandwidth of the output buffer can be reduced by adding
a capacitor on the A1 pin to create a first order low pass filter
with a time constant determined by the 100 kΩ internal resistor and the external filter capacitor.
LMP8602/LMP8602Q/LMP8603/LMP8603Q
For C2 the value is calculated with:
R2 can be calculated based on the desired value of Q as the
first step of the design procedure with the following equation:
Or for a gain = 5:
For the gain of 5 for the LMP8602 this results in:
and for a gain = 10:
For the gain of 10 for the LMP8603 this is:
Note that the frequency response achieved using this procedure will only be accurate if the cut-off frequency of the second
order filter is much smaller than the intrinsic 60 kHz low-pass
filter. In other words, to have the frequency response of the
LMP8602/LMP8602Q/LMP8603/LMP8603Q circuit chosen
such that the internal poles do not affect the external second
order filter.
For a desired Q = 0.707 and a cut off frequency = 3 kHz, this
will result for the LMP8602 in rounded values for R2 = 51
kΩ, C1 = 1.5 nF and C2 = 3.9 nF
And for the LMP8603 this will result in rounded values for R2
= 22 kΩ, C1 = 3.3 nF and C2 = 0.39 nF
For instance, the value of Q can be set to 0.5√2 to create a
Butterworth response, to 1/√3 to create a Bessel response,
or a 0.5 to create a critically damped response. Once the
value of R2 has been found, the second and last step of the
design procedure is to calculate the required value of C to give
the desired low-pass cut-off frequency using:
Which for the gain = 5 will give:
GAIN ADJUSTMENT
The gain of the LMP8602 is 50 and the gain of the LMP8603
is 100, however, this gain can be adjusted as the signal path
in between the two internal amplifiers is available on the external pins.
Reduce Gain
Figure 3 shows the configuration that can be used to reduce
the gain of the LMP8602 and the LMP8603.
and for the gain = 10:
30083456
FIGURE 3. Reduce Gain
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16
and for the LMP8603:
For the LMP8603:
From this equation, for a desired value of the gain, the required value of Ri can be calculated for the LMP8602 with:
Given a desired value of the reduced gain Gr, using this equation the required value for Rr can be calculated for the
LMP8602 with:
and for the LMP8603 with:
and for the LMP8603 with:
It should be noted from the equation for the gain Gi that for
large gains Ri approaches 100 kΩ. In this case, the denominator in the equation becomes close to zero. In practice, for
large gains the denominator will be determined by tolerances
in the value of the external resistor Ri and the internal 100
kΩ resistor. In this case, the gain becomes very inaccurate. If
the denominator becomes equal to zero, the system will even
become instable. It is recommended to limit the application of
this technique to gain increases of a factor 2.5 or smaller.
Increase Gain
Figure 4 shows the configuration that can be used to increase
the gain of the LMP8602/LMP8602Q/LMP8603/LMP8603Q.
Ri creates positive feedback from the output pin to the input
of the buffer amplifier. The positive feedback increases the
gain. The increased gain Gi for the LMP8602 becomes:
30083457
FIGURE 4. Increase Gain
17
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LMP8602/LMP8602Q/LMP8603/LMP8603Q
Rr creates a resistive divider together with the internal
100 kΩ resistor such that, for the LMP8602, the reduced gain
Gr becomes:
LMP8602/LMP8602Q/LMP8603/LMP8603Q
connected to the signal source. If the LMP8602/LMP8602Q/
LMP8603/LMP8603Q is driving such ADCs the sudden current that should be delivered when the sampling occurs may
disturb the output signal. This effect was simulated with the
circuit shown in Figure 5 where the output is to a capacitor
that is driven by a rail to rail square wave.
BIDIRECTIONAL CURRENT SENSING
The signal on the A1 and OUT pins is ground-referenced
when the OFFSET pin is connected to ground. This means
that the output signal can only represent positive values of the
current through the shunt resistor, so only currents flowing in
one direction can be measured. When the offset pin is tied to
the positive supply rail, the signal on the A1 and OUT pins is
referenced to a mid-rail voltage which allows bidirectional
current sensing. When the offset pin is connected to a voltage
source, the output signal will be level shifted to that voltage
divided by two. In principle, the output signal can be shifted
to any voltage between 0 and VS/2 by applying twice that
voltage from a low impedance source (Note 16) to the OFFSET pin.
With the offset pin connected to the supply pin (VS) the operation of the amplifier will be fully bidirectional and symmetrical
around 0V differential at the input pins. The signal at the output will follow this voltage difference multiplied by the gain and
at an offset voltage at the output of half VS.
Example:
With 5V supply and a gain of 50x for the LMP8602, a differential input signal of +10 mV will result in 3.0V at the output
pin. similarly -10 mV at the input will result in 2.0V at the output
pin.
With 5V supply and a gain of 100x for the LMP8603, a differential input signal of +10 mV will result in 3.5V at the output
pin. similarly -10 mV at the input will result in 1.5V at the output
pin.
30083460
FIGURE 5. Driving Switched Capacitive Load
This circuit simulates the switched connection of a discharged
capacitor to the LMP8602/LMP8602Q/LMP8603/LMP8603Q
output. The resulting VOUT disturbance signals are shown in
Figure 6 and Figure 7.
Note 16: The OFFSET pin has to be driven from a very low-impedance
source (<10Ω). This is because the OFFSET pin internally connects directly
to the resistive feedback networks of the two gain stages. When the OFFSET
pin is driven from a relatively large impedance (e.g. a resistive divider
between the supply rails) accuracy will decrease.
POWER SUPPLY DECOUPLING
In order to decouple the LMP8602/LMP8602Q/LMP8603/LMP8603Q from AC noise on the power supply, it is recommended to use a 0.1 µF bypass capacitor between the VS and
GND pins. This capacitor should be placed as close as possible to the supply pins. In some cases an additional 10 µF
bypass capacitor may further reduce the supply noise.
30083430
FIGURE 6. Capacitive Load Response at 3.3V
LAYOUT CONSIDERATIONS
The two input signals of the LMP8602/LMP8602Q/LMP8603/
LMP8603Q are differential signals and should be handled as
a differential pair. For optimum performance these signals
should be closely together and of equal length. Keep all
impedances in both traces equal and do not allow any other
signal or ground in between the traces of this signals.
The connection between the preamplifier and the output
buffer amplifier is a high impedance signal due to the 100
kΩ series resistor at the output of the preamplifier. Keep the
traces at this point as short as possible and away from interfering signals.
The LMP8602/LMP8602Q/LMP8603/LMP8603Q is available
in a 8–Pin SOIC package and in a 8–Pin MSOP package. For
the MSOP package, the bare board spacing at the solder
pads of the package will be too small for reliable use at higher
voltages (VCM > 25V) In this situation it is strongly advised to
add a conformal coating on the PCB assembled with the
LMP8602/LMP8602Q/LMP8603/LMP8603Q in MSOP package.
30083431
FIGURE 7. Capacitive Load Response at 5.0V
DRIVING SWITCHED CAPACITIVE LOADS
Some ADCs load their signal source with a sample and hold
capacitor. The capacitor may be discharged prior to being
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18
lated to control the average current flowing through the inductive load which is connected to a relatively high battery
voltage. The current through the load is measured across a
shunt resistor RSENSE in series with the load. When the power
transistor is on, current flows from the battery through the inductive load, the shunt resistor and the power transistor to
ground. In this case, the common mode voltage on the shunt
is close to ground. When the power transistor is off, current
flows through the inductive load, through the shunt resistor
and through the freewheeling diode. In this case the common
mode voltage on the shunt is at least one diode voltage drop
above the battery voltage. Therefore, in this application the
common mode voltage on the shunt is varying between a
large positive voltage and a relatively low voltage. Because
the large common mode voltage range of the LMP8602/
LMP8603 and because of the high AC common mode rejection ratio, the LMP8602/LMP8603 is very well suited for this
application.
For this application the following example can be used for the
calculation of the output signal:
When using a sense resistor, RSENSE, of 0.01 Ω and a current
of 1A, then the output voltage at the input pins of the LMP8602
is: RSENSE * ILOAD = 0.01 Ω * 1A = 0.01V
With the gain of 50 for the LMP8602 this will give an output of
0.5V. Or in other words, VOUT = 0.5V/A.
For the LMP8603 the calculation is similar, but with a gain of
100, giving an output of 1 V/A.
30083461
FIGURE 8. Reduce Error When Driving ADCs
The external capacitor absorbs the charge that flows when
the ADC sampling capacitor is connected. The external capacitor should be much larger than the sample and hold
capacitor at the input of the ADC and the RC time constant of
the external filter should be such that the speed of the system
is not affected.
LOW SIDE CURRENT SENSING APPLICATION WITH
LARGE COMMON MODE TRANSIENTS
Figure 9 illustrates a low side current sensing application with
a low side driver. The power transistor is pulse width modu-
30083452
FIGURE 9. Low Side Current Sensing Application with Large Common Mode Transients
19
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LMP8602/LMP8602Q/LMP8603/LMP8603Q
These figures can be used to estimate the disturbance that
will be caused when driving a switched capacitive load. To
minimize the error signal introduced by the sampling that occurs on the ADC input, an additional RC filter can be placed
in between the LMP8602/LMP8602Q/LMP8603/LMP8603Q
and the ADC as illustrated in Figure 8.
LMP8602/LMP8602Q/LMP8603/LMP8603Q
this application the common mode voltage on the shunt drops
below ground when the driver is switched off. Because the
common mode voltage range of the LMP8602/LMP8603 extends below the negative rail, the LMP8602/LMP8603 is also
very well suited for this application.
LOW SIDE CURRENT SENSING APPLICATION WITH
NEGATIVE COMMON MODE TRANSIENTS
Figure 10 illustrates the application of the LMP8602/
LMP8603 in a high side sensing application. This application
is similar to the low side sensing discussed above, except in
30083453
FIGURE 10. High Side Current Sensing Application with Negative Common Mode Transients
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20
30083454
FIGURE 11. Battery Current Monitor Application
21
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LMP8602/LMP8602Q/LMP8603/LMP8603Q
for such applications. If the load current of the battery is higher
then the charging current, the output voltage of the
LMP8602/LMP8603 will be above the “half offset voltage” for
a net current flowing out of the battery. When the charging
current is higher then the load current the output will be below
this “half offset voltage”.
BATTERY CURRENT MONITOR APPLICATION
This application example shows how the LMP8602/
LMP8603 can be used to monitor the current flowing in and
out a battery pack. The fact that the LMP8602/LMP8603 can
measure small voltages at a high offset voltage outside the
parts own supply range makes this part a very good choice
LMP8602/LMP8602Q/LMP8603/LMP8603Q
P8603Q is digitized with the A/D converter and used as an
input for the charge controller. The charge controller can be
used to regulate the charger circuit to deliver exactly the current that is required by the load, avoiding overcharging a fully
loaded battery.
ADVANCED BATTERY CHARGER APPLICATION
The above circuit can be used to realize an advanced battery
charger that has the capability to monitor the exact net current
that flows in and out the battery as show in Figure 12. The
output signal of the LMP8602/LMP8602Q/LMP8603/LM-
30083403
K2 = 5 for LMP8602
K2 = 10 for LMP8603
FIGURE 12. Advanced Battery Charger Application
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22
LMP8602/LMP8602Q/LMP8603/LMP8603Q
Physical Dimensions inches (millimeters) unless otherwise noted
8Pin SOIC
NS Package Number M08A
8Pin MSOP
NS Package Number MUA08A
23
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LMP8602/LMP8602Q/LMP8603/LMP8603Q 60V Common Mode, Fixed Gain, Bidirectional
Precision Current Sensing Amplifier
Notes
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