DN227 - Sense Milliamps to Kiloamps and Digitize to 12 Bits

Sense Milliamps to Kiloamps and Digitize to 12 Bits – Design Note 227
Richard Markell and Glen Brisebois
Introduction
The LT®1787 high side current sense amplifier provides
a precision measurement of current into or out of a
power source. The part generates an output voltage that
is directly proportional to the current flowing through
an external current sense resistor. With a miniscule
40μV (typical) input offset voltage, the LT1787 has a
better than 12-bit, zero-cross accurate dynamic range
at 250mV full-scale input. A hefty 60V maximum supply
voltage specification allows the part to be used not only
in low voltage battery applications but also in higher
voltage telecom and industrial applications. Independent
VEE and VBIAS pins make the application of the LT1787
extremely versatile.
Operation with an A/D Converter
Figure 1 shows a detailed schematic of a high resolution
(12-bit), bidirectional current-to-bits converter using
an LT1787, the LT1783 SOT-23 1.2MHz micropower,
rail-to-rail op amp and the LTC1404 SO-8 packaged,
12-bit analog-to-digital converter with shutdown. The
circuit, as shown, allows digitization of input current
from approximately –3A to approximately 2A with
12-bit resolution.
The LT1787’s output voltage is buffered by the LT1783,
filtered and applied to the LTC1404’s AIN pin. The precision bias voltage applied to the VBIAS pin of the LT1787
is obtained from the LTC1404’s reference output and is
typically 2.43V. This bias voltage sets the zero-current
output of the LT1787 to 2.43V, or approximately midrange for the A-to-D converter, allowing measurement
of current in both the positive and negative directions.
Note that the LTC1404’s internal reference voltage is
4.096V; thus the output of the converter is 1 count
per millivolt of input voltage from the LT1787’s output,
or 8 counts/mV of input voltage across RSENSE. Zero
sense-current translates to 2.43V or 2430 counts from
the converter. Full-scale positive current is 4096 counts
and full-scale negative current is 0 counts.
The device is self-powered from the supply that it is
monitoring and requires only 60μA of supply current.
The power supply rejection ratio of the LT1787 is in
excess of 120dB. The part has a fixed voltage gain of
8 from input to output.
Additional LT1787 features include provisions for input
noise filtering (both differential and common mode) and
the ability to operate over a very wide supply range of
2.5V to 60V. A functional diagram of the part and its
theory of operation is detailed in the side bar. The part
is available in both 8-lead SO and MSOP packages.
RSENSE = 0.1Ω
1%
TO LOAD
ISENSE
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of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
LT1787
1
FIL–
FIL+
8
2
VS–
VS+
7
3
4
DNC
VEE
VBIAS
VOUT
60V
MAX
VCC = 5V
10μF
16V
VCC = 5V
6
5
1
0.1μF
3
+
3
6
1
LT1783
4
–
5
2 SHDN
100Ω
0.1μF
2
AIN
4
CONV
VREF
LTC1404
GND
VSS
CLK
DOUT
7
6
5
8
CLOCKING
CIRCUITRY
DN227 F01
DOUT
Figure 1. LT1787 Connected to LTC1404 ADC: Bidirectional Current-to-Bits Converter
04/00/227_conv
Figure 2 shows current versus output code for the
complete system. The LTC1404 has a digital interface
consisting of the CLK and CONV input and the DOUT
serial digital output. The signals provide wide flexibility
in allowing the part to be interfaced to most microprocessors and DSPs.
The output voltage, VOUT, of the LT1787 is related to
the input sense voltage by the following relationships:
VSENSE = ISENSE • RSENSE
VOUT = 8(VSENSE) + VBIAS
Although a –3A to 2A range was selected for this illustration, other current ranges can be accommodated
by a simple change in value of the sense resistor. The
correct RSENSE value is derived so that the product of
the maximum sense current and the sense resistor
value is equal to the desired maximum sense voltage
(250mV for 12-bit resolution). For instance, the value
of the sense resistor to sense a maximum current of
10A is 250mV/10A = 0.025Ω. The smallest measurable
current is then 10A/4096 counts = 2.44mA/count. If only
10-bit resolution is desired, then the full-scale voltage
can be reduced to 60mV and RSENSE reduced to 0.006Ω.
Ensure that the power dissipated in the sense resistor,
IMAX 2 • RSENSE, does not exceed the maximum power
rating of the resistor.
4500
OUTPUT CODE (COUNTS)
4000
3500
RS = 0.1Ω
VS = 5V
VBIAS = 2.43V
REF FROM LTC1404
can operate to 60V, making it ideal for higher voltage
systems in telecom or industrial applications. Additionally, the part can find application in battery-powered,
handheld equipment and computers, where the need
for gauging the amount of current consumed and/or the
amount of charge remaining in the battery is critical.
Theory of Operation (See Figure 3)
Inputs VS+ and VS– apply the sense voltage to matched
resistors RG1 and RG2. The opposite ends of resistors
RG1 and RG2 are forced to be at equal potentials by
the voltage gain of amplifier A1. The currents through
RG1 and RG2 are forced to flow through transistors
Q1 and Q2 and are summed at node VOUT by the 1:1
current mirror. The net current from RG1 and RG2
flowing through resistor ROUT gives a voltage gain
of eight. Positive sense voltages result in VOUT being
positive with respect to pin VBIAS.
Pins VEE, VBIAS and VOUT may be connected in a
variety of ways to interface with subsequent circuitry.
Split supply and single supply output configurations
are easily supported.
Supply current for amplifier A1 is drawn from the
VS – pin. The user may choose to include this current
in the monitored current (through RSENSE) by careful
choice of connection polarity.
RSENSE
VS–
3000
2500
ISENSE
RG1A
1.25k
RG2A
1.25k
RG1B
1.25k
RG2B
1.25k
VS+
FIL–
2000
1500
FIL+
1000
–
500
+
IOUT
A1
0
–3
–2
0
–1
SENSE CURRENT (A)
2
1
Q1
Q2
DN227 F02
VBIAS
ROUT
20k
VOUT
Figure 2. Current vs Output Code
VEE
Conclusion
The LT1787 high side current sense amplifier provides
an easy-to-use method of sensing current with 12-bit
resolution for a multiplicity of application areas. The part
Data Sheet Download
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Linear Technology Corporation
CURRENT MIRROR
DN227 F03
Figure 3. LT1787 Functional Diagram
For applications help,
call (408) 432-1900
dn227f_conv LT/TP 0400 370K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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© LINEAR TECHNOLOGY CORPORATION 2000
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