Design Goals
Input |
Output |
Supply |
Common-Mode
Voltage |
Error |
Total Ionizing
Dose |
Single Event
Immunity |
ILOAD,
MIN |
ILOAD,
MAX |
VOUT,
min |
VOUT,
max |
VS |
VCM |
Output Error |
TID |
SEL |
–7.5 A |
7.5 A |
500 mV |
3.5 V |
5 V |
0 V |
< 2% |
50 krad (Si) |
75 MeV × cm2
/mg |
Design
Description
This circuit
utilizes passive components (R1 and R2 in the following schematic) to produce a
referred to input (RTI) offset that biases the sense input, and achieves
bidirectional sensing from a unidirectional current sense amplifier, the INA901-SP.
In this design, the normal operating load is from –7.5 A to 7.5 A, with a supply
voltage of 5 V. This topology may be used for other supply voltages, but the
equations corresponding to offset generation will change accordingly. In addition to
this functionality, this circuit implements the INA901-SP, which is a
Radiation-Hardness-Assured (RHA), 50-krad (Si) capable device at Low Dose Rate, that
is also Single Event Latch-up (SEL) Immune to 75 MeV-cm2/mg at 125°C. The
solution presented in this circuit is a low-side implementation, with a common-mode
voltage of approximately 0 V.
Design Notes
- This topology is meant for low
side implementation, where the IN– pin of the INA901-SP is referenced to GND.
While other implementations may be successful, this design is unable to be
implemented where VCM > Vs, as the current must flow
toward the IN+ pin to achieve the necessary offset. Therefore, the common-mode
voltage of the IN+ pin must be less than that of the supply voltage to ensure
valid operation.
- The offset current created by the
design process shows that, as a consequence, the offset voltage point deviates
slightly with the sense voltage, producing a linear error along with the sense
voltage.
- Because this is a low-side
implementation, the trade-offs that come with measuring on the low side are in
effect here. This includes the inability of the load to detect ground
faults.
- When designing with the
INA901-SP, be careful to ensure the full-scale input range of the device remains
above 20 mV for best performance. If VSENSE is allowed below 20 mV,
the device may produce additional errors inside of this operating condition. See
the INA901-SP Radiation Hardened, –15-V to 65-V Common Mode,
Unidirectional Current-Shunt Monitor data sheet for more
information.
- While this circuit provides the
ability to measure in multiple directions, it does not change the fact that the
INA901-SP is inherently a unidirectional device. This infers that one direction
of current sensing will be more accurate than the other, as one direction will
measure towards the offset voltage of the device, leading to increased error in
this direction. The effects of this may be mitigated in part by the use of a
one-point calibration, and is discussed later in this document.
Design Steps
- Choose RSHUNT to
Optimize Input Range: With the RTI offset designed, a shunt may now be
chosen for the desired sensing range. For the INA901-SP, maintain the lower
sensing bound to > 20 mV for optimal performance. This shows that the amount
of sensing headroom from VREF is:
From the
desired –7.5 A design target, it is calculated that the maximum allowable shunt to
achieve this goal is:
As the
ILOAD design target in the remaining direction is symmetric about
VREF, 10 mΩ is selected to complete the design. The final input
VSENSE swing is calculated with the following:
The expected corresponding output is as
follows:
- Verify Shunt Derating is
Sufficient: A necessary aspect of proper shunt design is ensuring that
the design choice has sufficient margin for power derating. As the device heats
in the environment, the amount of power the shunt is capable of dissipating is
derated by a certain factor. For continuous sensing, this factor can be as high
as 0.6. Taking this into account, for the design, a shunt must be chosen that is
rated for at least the following:
So for a
successful design, a shunt of at least 1-W rated power is chosen for continuous
sensing.
- Examine Error. Calibrate if
Necessary: As discussed in previous sections, it is expected that the
current-sense amplifier will be more accurate in the direction of measurement as
the output is driven toward supply. Less accurate measurements are expected as
the sense voltage decreases, and errors from the offset voltage begin to
dominate the measurement. A potential solution for this is to perform at least a
one-point calibration in logic to reduce the effects of the offset voltage.
A one-point calibration is performed by applying
the condition,
to the system, capturing the actual value output by
the INA901-SP, and maintaining the difference between this value and the calculated
ideal in memory. The output of the device is then consistently shifted by this
amount. An example of the effects of this are shown in the following simulated
results.
Design Simulations
DC Sweep Results, –7.5 A <
ILOAD < 7.5 A
Simulated results show that, as
expected, there is a slight change to the offset current as the voltage of IN+
shifts with VSHUNT. Calibration of the raw data results in a full-scale
error of < 1%, thus meeting the design goal. It should be noted that parameters
such as device offset and input bias currents in TINA-TI models reflect typical data
sheet parameters, and additional error may be exhibited pre- and post-calibration
due to variation in these parameters. For additional information on one-point
calibration and examples involving real bench tested data, see the Bidirectional Topologies for the INA901-SP.
Design
References
See the TI
Precision Labs, Current Sense Amplifiers video
series.
Design Featured Current Sense Amplifier
INA901-SP |
VS |
2.7 V to 16 V |
VCM |
–15 V to 65 V |
VOUT |
GND+3 mV to VS –
50 mV, typical |
VOS |
±500 μV, typical |
Iq |
350 μA, typical |
IB |
±8 μA, typical |
TID
Characterization (ELDRS-Free) |
50 krad
(Si) |
SEL Immune to
LET |
75
MeV-cm2/mg |
INA901-SP |