Download DN451 - Current Sense Amp Inputs Work from –0.3V to 44V Independent of Supply

Current Sense Amp Inputs Work from –0.3V to 44V Independent
of Supply
Design Note 451
Glen Brisebois
Introduction
Monitoring current flow in electrical and electromechanical systems is commonly used to provide feedback to
improve system operation, accelerate fault detection
and diagnosis, and raise efficiency. A current monitoring
circuit usually involves placing a sense resistor in series
with the monitored conductor and determining the voltage
across the sense resistor. To minimize power loss in the
sense resistor it is kept as small as possible, resulting
in a small differential voltage that must be monitored on
top of what may be a fairly large varying common mode
voltage. The LT®6105 is an ideal current sense amplifier
for this application. Just give it any reasonable supply
voltage, say 3V, and its inputs can monitor small sense
voltages at common modes of –0.3V to 44V and anything
in between. The accuracy of the LT6105 over this range
is displayed in Figure 1.
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Solenoid Monitoring
The large input common mode range of the LT6105
makes it suitable for monitoring currents in quarter,
half and full bridge inductive load driving applications.
Figure 2 shows an example of a quarter bridge. The
MOSFET pulls down on the bottom of the solenoid to
increase solenoid current. It lets go to decrease current,
and the solenoid current freewheels through the Schottky
diode. Current measurement waveforms are shown in
Figure 3. The small glitches occur due to the action of
the solenoid plunger, and this provides an opportunity for
mechanical system monitoring without an independent
sensor or limit switch.
Figure 4 shows another solenoid driver circuit, this time
with one end of the solenoid grounded and a P-channel
MOSFET pulling up on the other end. In this case, the
inductor current freewheels around ground, imposing a
negative input common mode voltage of one Schottky
24VDC
1N5818
–
GAIN ERROR (%)
3 VSENSE = 50mV
RIN = 100Ω
A V = 50V
2
200Ω
1%
TA = – 40°C
1
–IN
TA = 25°C
0
0V/OFF
5V/ON
LT6105
200Ω
1%
+IN
2N7000
–1
TA = 125°C
–2
TA = 85°C
5VDC
–3
V+
V–
VOUT
–4
0
5
10
15
20
25
30
35
VS+ INPUT VOLTAGE (V)
40
45
DN451 F02
DN451 F01
Figure 1. Gain Error vs Input Common Mode
10/08/451
24V, 3W
SOLENOID
1Ω
1%
V+ = 12V
+
4
VOUT = 25mV/mA
4.99k
1%
Figure 2. Solenoid is Pulled Low, Freewheels High.
Input Travels from 0V to 24.3V
www.BDTIC.com/Linear
monitor, and another LT6105 as a simple negative supply
monitor. Note that the schematics are practically identical
and both have outputs conveniently referred to ground.
The only requirement for negative supply monitoring, in
addition to the usual constraints of the absolute maximum
ratings, is that the negative supply to the LT6105 must be
at least as negative as the supply it is monitoring.
5V/DIV
10V/DIV
2V/DIV
DN451 F03
50ms/DIV
Figure 3. Solenoid Waveforms: MOSFET Gate,
Solenoid Bottom and Current Sense Amp Output.
Bumps in the Current Result from Plunger Travel
diode drop. This voltage may exceed the input range of
the LT6105. This does not endanger the device, but it
severely degrades its accuracy. In order to avoid violating
the input range, pull-up resistors can be used as shown
in Figure 4.
Conclusion
Current measurement is popular because it offers
improved real time insight into matters of efficiency,
operation and fault diagnosis. The wide input range of
the LT6105 and its accuracy over that range make it easy
to measure currents in a variety of applications.
VOUT = 1V/A
VOUT
LT6105
4.99k
1%
Supply Monitoring
The input common mode range of the LT6105 allows it
to monitor either positive or negative supplies. Figure 5
shows one LT6105 applied as a simple positive supply
+IN
24VDC
–IN
2k
1%
LT6105
1N5818
–15V
NEGATIVE
SUPPLY
100Ω
1%
+IN
V–
VOUT
–IN
VOUT = 25mV/mA
DN451 F04
20mΩ
+ 1%
100Ω
1%
–
TO +15V
LOAD
CURRENT FLOW
V+
5VDC
5VDC
–IN
–
–
2k
1%
V+
CURRENT FLOW
200Ω
1%
1N914
24V, 3W
SOLENOID
+15V
POSITIVE
SUPPLY
TP0610L
+
200Ω
1%
24V/OFF
–15V
4.99k
1%
Figure 4. Solenoid is Pulled High to 24V, Freewheels
Low to a Schottky Below Ground. LT6105 Inputs are
Kept Within Range by 2k Pull Ups
5VDC
V+
–15V
V–
20mΩ
1%
LT6105
+
19V/ON
1Ω
1%
100Ω
1%
V–
100Ω
1%
TO –15V
LOAD
+IN
VOUT
VOUT = 1V/A
DN451 F05
4.99k
1%
Figure 5. The LT6105 Can Monitor the Current of
Either Positive or Negative Supplies, Without a
Schematic Change. Just Ensure that the Current
Flow is in the Correct Direction
Data Sheet Download
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call (408) 432-1900, Ext. 3755
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