Texas Instruments | Bidirectional Current Sensing with a Window Comparator Circuit | Application notes | Texas Instruments Bidirectional Current Sensing with a Window Comparator Circuit Application notes

Texas Instruments Bidirectional Current Sensing with a Window Comparator Circuit Application notes
Application Report
SNOAA40 – September 2019
Bidirectional Current Sensing with a Window Comparator
Circuit
Design Goals
SYSTEM CURRENT LEVELS
SUPPLY
Falling OC
Threshold
Falling OC
Recovery
Rising OC
Threshold
Rising OC
Recovery
V+
V-
IG1 < -35A
IG1 > -31A
IG1 > 100A
IG1 < 90A
3.3 V
0V
Design Description
This bidirectional current sensing solution uses a current-sense amplifier and a high speed dual
comparator with a rail-to-rail input common mode range to create over-current (OC) alert signals at the
comparator outputs (OUTA and OUTB) if the input current (IG1) rises above 100A or falls below -35A. In
this implementation, both over-current alert signals are active high, so when the 100A or -35A thresholds
are crossed, the comparator outputs will go high. External hysteresis is implemented on both comparators
so that the comparator outputs will return to logic low states when the current reduces by 10% (90A and
-31A). While the circuit below has shunt resistor R8 connected to ground, the same circuit is applicable for
high side current sensing up to the common mode voltage range of the INA.
VCC
HTV
C9 100n
+ +
TLV3201
OUTB
-
HTV
R7 200k
R5 10k
R2 10k
3.3V
V+
INPB
R4 6.4k
LTV
VCC
R3 806k
R6 4.42k
V+
R1 13k
V+
INNA
C10 1u
V+
V+
OUTA
-
REF2
REF1
INN
TLV3201
OUT
INA240A1
GND
IG1
R8 330u
INP
+ +
LTV
INA_OUT
C2 1u
C1 100n
V+
C7 100n
C8 1u
V+
Design Notes
1. Select a comparator with rail-to-rail input common mode range.
2. Select a current sense amplifier with low offset voltage and a common mode input range that matches
the requirements of the system.
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Design Steps
1. To determine the comparator threshold voltages, first calculate the INA240A1 output voltages that
correspond to the desired current thresholds. The calculations depend on the gain of the INA240 (20,
50, 100, 200 for A1, A2, A3, A4, respectively), the input current (IG1) and sense resistor (R8), and the
reference voltage when the input current is 0 (VREF). Per section 8.3.2 in the INA240 datasheet, R8 is
a function of the differential input voltage and the maximum input current to the INA240. Given that the
input current in this system swings above 100A, by keeping R8 small, the power dissipation across R8
will be lessened.
Using these equations and the desired current thresholds, the following table is generated:
DESCRIPTION
IG1
INA-OUT
VH, CHB
Overcurrent threshold in
forward direction
100 A
1.65 V + 20 x (100 A x 0.33
mΩ) = 2.31 V
VL, CHB
“Recovery threshold” in forward
direction
90 A
1.65 V + 20 x (90 A x 0.33 mΩ)
= 2.244 V
VH, CHA
Overcurrent threshold in
reverse direction
-35 A
1.65 V + 20 x (-35 A x 0.33
mΩ) = 1.419 V
VL, CHA
“Recovery threshold” in reverse
direction
-31.5 A
1.65 V + 20 x (-31.5 A x 0.33
mΩ) = 1.4421 V
First, focus on the top comparator (channel A), which is in an inverting comparator configuration. This
comparator will swing to a logic high when the current in the reverse direction exceeds -35A, and will
return to a logic low when the current in the reverse direction recovers to -31.5A. These current levels
correspond to voltage levels of 1.419 V and 1.4421 V, respectively.
2. Assume a value for R2 (the bottom resistor in the resistor divider). In this circuit, 10 kΩ is chosen.
3. Derive two equations for R1 in terms of V+, VL, VH, R2, R3 by analyzing the circuit when INNA = VL and
when INNA = VH:
4. Set these two equations equal to each other and then solve for R3.
The standard 1% resistor value closest to this is 806 kΩ.
5. Solve for R1 using any of the two equations derived in 3:
The standard 1% resistor value closest to this is 13 kΩ.
The next step is to focus on the bottom comparator (channel B), which is in a non-inverting
configuration. This comparator will swing to a logic high when the current in the forward direction
exceeds 100A, and will return to a logic low when the current in the forward direction recovers to 90A.
These current levels correspond to voltage levels of 2.31 V and 2.244 V, respectively.
2
Bidirectional Current Sensing with a Window Comparator Circuit
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Figure 1.
R4 6.4k
C10 1u
V+
V+
INPB
C9 100n
V+
R6 4.42k
HTV
+ +
INA_OUT
HTV
R5 10k
3.3V
TLV3201
OUTB
-
R7 200k
SBOA306 (High-side current sensing with comparator circuit) derives two equations for VTH (the
voltage on the non-inverting pin) when the comparator output is in a logic low state and a highimpedance state (SBOA306 uses an open-drain comparator). These equations are then set equal to
each other creating a quadratic equation to solve for R6. Since TLV3202 is a push-pull device, the
output will go to a logic high state instead of a high-impedance state. Thus, the pull-up resistor value is
0 and VPU is V+
6. Rewrite the quadratic equation to match this circuit:
7. Choose a value for R7. This resistor dictates the load current of the comparator, and should thus be
large. For this circuit, R7 is assumed to be 200 kΩ.
The standard 1% resistor value closest to this is 4.42kΩ.
8. Calculate VTH using R6.
9. Choose a value for R5. In this case, R5 is chosen to be 10 kΩ.
10. Solve for R4.
The standard 1% resistor value closest to this is 4.64 kΩ.
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Design Simulations
Transient Simulation Results
The below simulation results use a -70A to 130A, 100Hz sine wave for IG1.
Figure 2. Channel A
Figure 3. Channel B
4
Bidirectional Current Sensing with a Window Comparator Circuit
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Design References
See Analog Engineer's Circuit Cookbooks for TI's comprehensive circuit library.
See Circuit SPICE Simulation File SBOMB05.
Design Featured Comparator
TLV320x
VS
2.7 V to 5.5 V
VinCM
200 mV beyond either rail
VOUT
Push-Pull, Rail-to-rail
VOS
1 mV
IQ
40 µA/channel
tPD(HL)
40 ns
#Channels
1, 2
TLV3201-Q1 and TLV3202-Q1
Design Featured Op Amp
INA240
VS
1.6 V to 5.5 V
VinCM
-4 V to 80 V
VOUT
Rail-to-rail
VOS
5 µV
VOS Drift
50 nV/◦C
IQ
260 ns
Gain Options
20 V/V, 50 V/V, 100 V/V, 200 V/V
INA240
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