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
Design Steps
The offset current with this approximation is given by:
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:
So for a successful design, a shunt of at least 1-W rated power is chosen for continuous sensing.
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.
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 |
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