Design Goals
Input | Overcurrent Conditions | Output | Supply | Response Time | Total Ionizing Dose | Single Event Immunity | |
---|---|---|---|---|---|---|---|
Iload Min | Iload Max | IOC_TH | Vout_OC | VS | tdelay | TID | SEL |
5A | 10A | 11A | 2.2V | 5V | < 5µs | 50 krad (Si) | 75 MeV×cm2/mg |
Design Description
This is a unidirectional current-sensing solution, generally referred to as overcurrent protection (OCP) that can provide an overcurrent alert signal to power off a system exceeding a threshold current. In this particular setup, the normal operating load is from 5A to 10A, with the overcurrent threshold defined at 11A (IOC_TH). The current shunt monitor and comparator are powered from a single 5-V supply rail. OCP can be applied to both high-side and low-side topologies. The solution presented in this circuit is a high-side implementation, with RL placed as a representation of a purely resistive system load at 4Ω. This circuit is useful in a variety of power applications for telemetry, health monitoring and system diagnostics. 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 comparator function is fulfilled by a LM193QML-SP, but the LM139AQML-SP can be used here as well, if additional comparator device count is needed.
Design Notes
Design Steps
A shunt resistor value of 10mΩ is chosen for the design. Be aware that while a larger shunt provides more utilization of the full scale range, thermal constraints on the shunt will increase proportionally with the resistance. Selection of a higher value will also increase the voltage of the overcurrent trip point, which may cause challenges in satisfying the common-mode requirement of the LM193QML-SP.
The overcurrent condition is set by a voltage divider between R2 and R3 set for the previously-mentioned point. Choose a value for R3 and calculate the needed resistor R2. Here, R3 is chosen at 10kΩ:
Note that the LM193QML-SP requires that at least one input be from 0V to 3V when operating at a supply of 5V to satisfy the input common-mode requirement. This node held at a constant 2.2V satisfies this requirement for all values of comparison.
Often, calculated resistor values will not directly align with available resistor choices. Here, 12.73kΩ is not a standard value, so the closest standard value of 12kΩ is selected. The actual overcurrent point based on the resistor value can be found by combining the previous equations.
This value means that at a measured current of 3.5A for the chosen shunt, offset voltage will contribute a 10% error to the measurement.
Design Simulations
Transient Simulation Results
High-Side OCP Simulation Results
The first simulation examines the ramp response of the circuit, as the current transitions above the overcurrent trigger point.
Next, examining the delay response of the circuit, with a step response delivered at 4.0ms, it is shown that the circuit responds to the event approximately 2.5µs later, so the delay response is satisfied:
Design References
See the TI Precision Labs, Current Sense Amplifiers video series.
INA901-SP | |
---|---|
VS | 2.7V to 16V |
VCM | –15V to 65V |
VOUT | GND+3mV to VS-50mV, typical |
VOS | ±500μV, typical |
Iq | 350μA, typical |
IB | ±8μA, typical |
TID Characterization (ELDRS-Free) | 50krad (Si) |
SEL Immune to LET | 75 MeV-cm2/mg |
https://www.ti.com/product/INA901-SP |
For less harsh radiation environments, TI also offers the INA240-SEP, which offers Single Event Latch-up (SEL) Immunity to 43 MeV-cm2/mg at 125°C. It is ELDRS Free to 30 krad(Si), and Total Ionizing Dose (TID) RLAT for Every Wafer Lot is up to 20 krad(Si):
INA240-SEP | |
---|---|
VS | 2.7V to 5.5V |
VCM | –4V to 80V |
VOUT | GND+1mV to VS-50mV, typical |
VOS | ±5μV, typical |
Iq | 1.8mA, typical |
IB | ±90μA, typical |
TID Characterization (ELDRS-Free) | 30 krad (Si) |
SEL Immune to LET | 43 MeV-cm2/mg |
https://www.ti.com/product/INA240-SEP |
TLV1704-SEP | LM139AQML-SP | |
---|---|---|
VS | 2.2V to 36V | 2V to 36V |
VCM | Rail-to-Rail | 0V to 34V |
VOUT | Open-Collector, Rail-to-Rail | Open-Collector |
VOS | 500μV | 2mV |
Iq | 55μA/channel | 200μA/channel |
tPD(HL) | 460ns | 2.50μs |
TID Characterization (ELDRS-Free) | 30 krad (Si) | 100 krad (Si) |
SEL Immune to LET | 43 MeV-cm2/mg | SEL Immune (Bipolar process) |
https://www.ti.com/product/LM139AQML-SP |
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