Sep 1999 Micropower, Precision Current Sense Amplifier Operates from 2.5V to 60V

DESIGN FEATURES
Micropower, Precision Current Sense
Amplifier Operates from 2.5V to 60V
by Richard Markell, Glen Brisebois and Jim Mahoney
Introduction
The LT1787 is a precision, high-side
current sense amplifier designed for
monitoring of the current either into
or out of a battery or other element
capable of sourcing or sinking current. The LT1787 features a miniscule
40µV (typical) input offset voltage with
a 128mV full-scale input voltage. (The
part is generally used at ±128mV fullscale, although it is specified for
500mV minimum full-scale.) This
translates to a 12-bit dynamic range
in resolving currents. A hefty 60V
maximum input voltage specification
allows the part to be used not only in
low voltage battery applications but
also in telecom and industrial applications where higher voltages may be
present.
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 130dB.
The LT1787 allows the use of a
user-selectable sense resistor, the
value of which depends on the current to be monitored. This allows the
input voltage to be optimized to the
128mV value, maximizing dynamic
RSENSE
VS–
VS+
RG1
RG2
FIL–
FIL+
RG1A
RG2A
–
+
A1
VBIAS
Q1
Q2
ROUT
VOUT
VEE
1:1 MIRROR
1787 F 01
Figure 1. LT1787 function diagram
Linear Technology Magazine • September 1999
range. The part has a fixed voltage
gain of eight 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. The
part is available in both 8-lead SO
and MSOP packages.
Operation of the LT1787
Figure 1 shows a function diagram of
the LT1787 integrated circuit. When
current is flowing from VS+ to VS–, a
sense voltage of VSENSE = ISENSE •
RSENSE is generated. Because amplifier A1’s positive and negative inputs
are forced equal by feedback, VSENSE
also appears across the RG side with
the higher VS potential. Hence, for the
situation where VS+ is greater than
VS–, VSENSE appears across RG2A and
RG2B. This current flows through Q2
and becomes the output current, IOUT.
Q1 is kept off by the amplifier and
does not contribute to IOUT.
For split-supply operation, where
VS+ and VEE range from ±2.5V to
±30V; with VBIAS at ground, VOUT
becomes (IOUT • ROUT). In this case,
and for the above described direction
of sense current, VOUT is positive or
“above” VBIAS; thus Q2 is sourcing
current.
When current flows from VS– to
+
VS , input VS– is at a higher potential
than VS+, so VSENSE will appear across
RG1A and RG1B. The current, IOUT =
VSENSE/(RG1A + RG1B), will be conducted
through Q1, while Q2 remains off.
IOUT then duplicates itself through a
one-to-one current mirror at VOUT.
VOUT is negative or “below” VBIAS; Q1
sources current and the mirror sinks
the current at the VOUT node.
The output voltage across ROUT is
related to the input sense voltage by
the following relationships:
VOUT – VBIAS = IOUT • ROUT
VSENSE = ISENSE • RSENSE
IOUT = VSENSE/RG,
RG = RG1A + RG1B = RG2A + RG2B
RG(TYP) = 2.5k
VOUT – VBIAS = (ROUT) (VSENSE)/
RG, ROUT/RG = 8
ROUT(TYP) = 20k
VOUT – VBIAS = 8 (VSENSE),
VSENSE = VS+ – VS–
VOUT = 8 (VSENSE) + VBIAS
Selection of RSENSE
Maximum sense current can vary for
each application. To sense the widest
dynamic range, it is necessary to select
an external sense resistor that fits
each application. 12-bit dynamic
range performance can be achieved
regardless of the maximum current to
be sensed, whether it is 10mA or 10A.
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 (usually 128mV).
For instance, the value of the sense
resistor to sense a maximum current
of 10mA is 128mV/10mA = 12.8Ω.
Since the LT1787 is capable of 12-bit
resolution, the smallest measurable
current is 10mA/4096 counts =
2.44µA/count. In terms of sense voltage, this translates to 128mV/4096
= 31.25µV/count. Other current
ranges can be accommodated by a
simple change in value in the sense
resistor. Care should also be taken to
ensure that the power dissipated in
the sense resistor, IMAX.2 • RSENSE,
does not exceed the maximum power
rating of the resistor.
7
DESIGN FEATURES
Application Circuits
and negative current flow being from
VS– to VS+, where the output varies
from 0V to –1.024V for VSENSE = 0mV
to –128mV. Figure 3 shows the output voltage versus VSENSE for this
configuration.
Dual-Supply,
Bidirectional Current Sense
Figure 2 shows the schematic diagram of the LT1787 operated with
dual supplies. This circuit can sense
current in either direction, positive
current flow being from VS+ to VS–,
where the output ranges from 0V to
1.024V for VSENSE = 0mV to 128mV,
Operation with Bias
If a negative supply is not available, a
voltage reference may be connected
to the VBIAS pin of the LT1787. The
value of the reference is not critical, it
simply biases the output of the part to
a new “zero” point (for VSENSE = 0V);
zero is now at VBIAS = VREF, which, for
the case of Figure 4, is equal to 1.25V.
This configuration can be used for
both unipolar and bipolar current
sensing, with VOUT ranging either
“above” or “below” VBIAS, depending
on the direction of the current flow.
This can be seen from the graph shown
in Figure 5. (Note that the reference
must be able to both source and sink
1.5
1.0
C1
1µF
1
2
3
4
FIL+
FIL–
15V
8
+ 7
VS–
VS
LT1787
6
VBIAS
DNC
5
VOUT
VEE
OUTPUT
C2
1µF
–5V
OUTPUT VOLTAGE (V)
RSENSE
TO
CHARGER/
LOAD
VS = ±1.65V TO ±30V
TA = – 40°C TO 85°C
0.5
0
–0.5
–1.0
1000pF
–1.5
–128 –96 –64 –32 0
32 64 96
SENSE VOLTAGE (VS+ – VS–) (mV)
Figure 2. Split-supply, bidirectional operation
128
Figure 3. VOUT vs VSENSE in bipolar mode
2.75
C1
1µF
1
FIL–
FIL+
8
3.3V
TO
60V
OUTPUT VOLTAGE (V)
RSENSE
TO
CHARGER/
LOAD
VS = 3.3V TO 60V
TA = – 40°C TO 85°C
2.25
3.3V
RBIAS
20k
5%
2
7
VS+
VS–
LT1787HV
3
6
VBIAS
DNC
4
5
VOUT
VEE
C2
1µF
LT1634-1.25
1.75
1.25
0.75
0.25
1000pF
–0.25
–128 –96 –64 –32 0
32 64 96
SENSE VOLTAGE (VS+ – VS–) (mV)
OUTPUT
Figure 4. Bidirectional operation with a reference
128
Figure 5. VOUT vs VSENSE in bipolar mode
4500
1Ω
IS
4000
7
3
4
FIL–
VS+
DNC
VEE
LT1787
FIL+
8
3500
5V
3000
VS– 2
VBIAS 6
100pF
5
VCC
COUNTS
1
SINGLE 5V SUPPLY
VREF = 1.25V
I–V CONVERTER
USING INTERNAL ROUT
–
VOUT
36k
VCC
+
LT1495
LTC1404
2500
2000
1500
1000
500
LT1634-1.25
Figure 6. Operation with a buffer
8
0
0
–250 –200 –150 –100 –50
INPUT CURRENT (mA)
50
100
Figure 7. Output counts vs input current
for Figure 6’s circuit
Linear Technology Magazine • September 1999
DESIGN FEATURES
2.5V TO
60V
C
0.1µF
FIL+
FIL–
8
2
7
VS+
VS–
LT1787HV
3
6
VBIAS
DNC
4
VEE
VOUT
5
VOUT
Figure 8. Output voltage referred to ground—
unidirectional sensing mode
current from the VBIAS pin—refer to
the block diagram in Figure 1.)
Operation with a Buffer
Figure 6 uses a rail-to-rail op amp,
the LT1495, as an I/V converter to
buffer the LT1787’s output. The
LT1634-1.25 reference is used to bias
the LT1787’s output so that zero current is now represented by a 1.25V
output. This allows the device to monitor current in either direction while
the circuit operates on a single supply. This also allows lower voltage
operation, since VOUT of the LT1787 is
held constant by the op amp. Figure
7 shows input current versus output
counts (from the LTC1404 A/D converter), showing excellent linearity.
Single-Supply Current Sense
The circuit in Figure 8 provides good
accuracy near full-scale, but has a
limited dynamic range. In this circuit,
the LT1787 is operated from a single
supply of 2.5V minimum to 60V maximum. Current is allowed to flow
through RSENSE in both directions but
7
VS+
3
VEE
4
DNC
VEE
LT1787
0.20
0.15
0.10
0.05
IDEAL
0
0
Figure 10 shows the details of an
LT1787 connected to a LTC1404 12bit serial A/D converter. Details of the
circuit are similar to those shown
previously in Figures 2 and 8 and in
the text detailing these circuits. The
main difference in the applications is
that the circuits in Figures 2 and 8
provide an analog output voltage proportional to the current, whereas the
circuit shown in Figure 10 digitizes
that analog voltage to provide a digital output. Figures 11 and 12 show
the output of the LTC1404 A/D converter. The data in Figure 11 was
collected with VEE operated from –5V;
in other words, both the LT1787 and
the LTC1404 used a negative supply
of –5V. Similarly, the data in Figure
12 was taken with VEE connected to
ground.
Connecting the optional section of
the schematic (still operating the circuit from a single supply) allows the
A/D’s reference to “bias up” the
LT1787 exactly as shown in Figure 5.
Of course, the graph of the output
would then be recast as similar to
Figure 11 (counts versus VSENSE).
5
10
15
20
25
30
VSENSE (mV)
Figure 9. Expanded scale of VOUT vs VSENSE,
unidirectional current sensing mode
Auto Shutdown
Linear Regulator
Figure 13 shows the details of a linear
regulator with high-side current
sensing and latched shutdown capability. The circuit shuts down power
to the load when the current reaches
its overcurrent trip point. Power can
then be restored only by cycling the
main power off and on again. This
circuit features the LT1787 and the
LTC1440 precision comparator with
on-chip reference. The LT1528 is a 3A
low dropout linear regulator with
shutdown.
The circuit uses the LT1787, U1,
as a precision current sensor; the
gain of the LT1787 allows the use of a
0.05Ω sense resistor, which dissipates a mere 0.312W of power. The
LTC1440 ultralow power comparator, U2, with its internal reference, is
used as a precision trigger, followed
by a 74HC00 connected as an RS flipflop (U3C and U3D) to latch the error
condition until power is removed and
reapplied. The other two NAND gates
VCC = 5V
1Ω 1%
FIL–
0.25
Operation with
an A/D Converter
IS
1
0.30
FIL+
VS– 2
VOUT 5
~20k
1
8
1000
10µF
16V
800
600
VOUT (±1V)
6
VBIAS
CONV
2
AIN
3
VREF
4
10k
10µF
16V
CLK
DOUT
GND
8
10k
OPTIONAL:
CONNECT TO VBIAS;
DISCONNECT VBIAS
FROM GROUND
LTC1404
7
6
5
10µF
16V
DOUT
200
0
–200
–400
–600
VEE
Figure 10. Connection to an A/D converter (current-to-counts converter)
Linear Technology Magazine • September 1999
VCC = 5V
VEE = –5V
400
CLOCKING
CIRCUITRY
COUNTS
1
is measured in a single direction only,
with current flow from VS+ to VS–. In
this connection, VBIAS and VEE are
grounded. The output voltage (VOUT )
of the LT1787 for this circuit is equal
to 8 • VSENSE, for VSENSE = VS+ – VS– =
0mV to 128mV. The dynamic range
limits of this circuit can be seen in the
graph shown in Figure 9.
OUTPUT VOLTAGE (V)
RSENSE
TO
LOAD
–800
–1000
–128 –96 –64 –32 0
32
VSENSE (mV)
64
96
128
Figure 11. LT1787 input to LTC1404 ADC
9
DESIGN FEATURES
1000
Battery Fuel Gauge
VCC = 5V
VEE = –5V
COUNTS
800
600
400
200
0
0
20
40
60
80
VSENSE (mV)
100
120
Figure 12. LT1787 input to LTC1404 ADC—
single supply
are used to provide a power-on reset
of the RS flip-flop. The 1M resistor,
R7, and the 0.33µF capacitor, C3, at
pin 2 of U3A provide a long enough
time constant to properly initialize
the flip-flop.
As shown, the circuit’s trip point
for shutdown is just under 2.5A. This
may be changed by altering the value
of current sense resistor R1. Consult
the LT1787 data sheet for details on
how to alter this resistor to sense
different current ranges.
The industry standard method for
gauging battery charge is to keep
track of the endpoints, the full charge
and discharged states, and, in
between, to measure how much discharge has occurred since the last
full charge (and vice versa). In an
automobile, this would be analogous
to having only a “full” reading and an
“empty” reading on the gas tank, and,
in between, to keep track of mileage.
Applying this strategy to batteries is
called “Coulomb-counting”; it is
achieved by measuring (and
numerically accumulating in a
microcontroller) the current flow from
the battery over time. Keeping track
of the history of battery currents and
voltages allows the present state of
the battery charge to be determined.
Hence the need for an accurate current sense amplifier like the LT1787.
Figure 14 shows a schematic for
measuring battery current using the
LT1787 and measuring battery voltage using the micropower LT1635
with 200mV internal reference. The
LT1787 is configured for single-supply, bidirectional operation, with the
2.0V reference coming from U2B, the
LT1635 (created by amplifying its internal 200mV reference by a gain of
ten.) Note that the reference voltage is
fed to the ADC, so its absolute value
is not critical, except in that it will
form the center point for the battery
voltage measurement and will thus
determine the valid battery input voltage range. Resistors R1 and R2 form
a divide-by-five, bringing the battery
voltage down from ~10.8V to ~2.1V to
put it within the input range of a
downstream ADC. Op amp U2A with
resistors R3 and R4 level shift this by
the reference voltage and apply a gain
of five. If a 12-bit ADC with a 5V
reference is used, the following equations apply:
VBATT = 4 (VREF ) + VB
= (5/4095) (4(Ch1) + Ch2)
IBATT = VS / RS
= (1/8) (VI – VREF) / 0.05
= 2.5 (VI – VREF)
= 2.5 (5/4095) (Ch0 – Ch1)
where Ch0, Ch1 and Ch2 are in counts
from 0 to 4095.
5V
5V
R7
1M
R1 0.05Ω
VIN
5V
POWER-ON
R8
RESET
10k
1
3
2 U3A
C7
0.1µF
R2
20k
C1 0.1µF
U2 LTC1440
U1 LT1787
8
FIL–
FIL+
7
2
+
–
VS
VS
3
5
DNC
VOUT
4
6
VEE
VBIAS
3 IN+
1
4 IN–
+
C4
0.01µF 5V
–
6 REF
2
R10
180k
10
9
R4 3.24k
R5 20k
R3
2.4M
C2
1µF
R9
10k
5
U3B 4
6
OUT 8
5 HYST
R6
430Ω
5V
C3
0.33µF
V+ 7
8
D1
1N5712
2
V–
U3C
GND
13
12
U3D
11
U3 = 74HC00
U4 LT1528
5
4
C5
1µF
VIN
OUTPUT
SHDN
1
2
SENSE
GND
R11
3
330Ω
VOUT
3.3V/2.5A
+
C6
100µF
LATCHED SHUTDOWN
Figure 13. LT1787 auto shutdown with latch
10
Linear Technology Magazine • September 1999
DESIGN FEATURES
The LT1787 high-side current sense
amplifier provides an easy-to-use
method of sensing current with 12bit resolution for a multiplicity of
application areas. The part can oper-
14
VOLTAGE (V)
Conclusion
ate to 60V, making it ideal for higher
voltage topologies such as might be
used in telecom or industrial applications. Additionally, the part can find
homes 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.
12
VBATT
10
8
6
4
CURRENT (A)
Figure 15 shows a typical discharge
and charge cycle for a 10.8V, 4A-hour
Li-Ion battery.
2
IBATT
0
–2
–4
0
30
60
90
120
TIME (MINUTES)
150
180
Figure 15. Discharge and charge cycle
of a 10.8V Li-Ion battery
SENSE VBATT AT
THE POSITIVE BATTERY – V +
S
TERMINAL
RS 0.05Ω
VBATT
IBATT
SONY LIP9020
10.8V, 4AH
Li-Ion BATTERY
STAR GROUND
AT NEGATIVE
BATTERY
TERMINAL
LOAD
CHARGE
(12.5V MAX)
C1 0.22µF
U1 LT1787
8
FIL–
FIL+
7
–
+
VS
VS
3
5
DNC
VOUT
4
6
VEE
VBIAS
C2 100pF
1
R5 9.09k
2
ADC CH0
R9 1k
VI
C6
0.22µF
7
R8 1k
1
VREF
VBATT
C5
0.22µF
U2B
1/2 LT1635
R6
1k
1%
+
R2
249k
1%
8
–
ADC CH1
R1
1M
1%
5V
200mV
3
2
+
U2A
1/2 LT1635
–
6
R3
1M
1%
R7 1k
ADC CH2
4
VB
C3
100pF
C4
0.22µF
R4
249k 1%
Figure 14. Battery “fuel gauging” system
For more information on parts featured in this issue, see
http://www.linear-tech.com/go/ltmag
http://www.linear-tech.com/ezone/zone.html
Articles, Design Ideas, Tips from the Lab…
Linear Technology Magazine • September 1999
11