Extending the Supply Voltage Range of a 4-Decade, < 300uA to 3A Resistor-less Current-Sensing Solution

Introduction

Measuring current in a system is fundamental, yet powerful tool in monitoring the system's state. With advanced technology, electronic or electrical systems are extremely reducing in physical size, power dissipation and cost with not much tradeoff in terms of performance. Every electronic device monitors its own health and state, where these diagnostics provide vital information needed to manage the system and even command its future design upgrades.

The need to measure a wide range of current in a system is increased from miniscule current levels to several amperes of current. For example, ascertaining the high dynamic range of current flowing or consumption in a system can be seen in the following cases:

1. Sleep/Inactive currents to determine the overall loading performance and battery/supply power estimation in addition to normal operation.
2. Automatic test equipment (ATE)/Testing environments needed to handle miniscule/low micro-amp current levels to ampere levels of current to necessitate R&D or production-level testing.
3. Production floor environments to catch the production problems (flux trapped under ICs, unwanted solder shorts or open circuits) along with normal operating function tests.
4. Industrial equipment monitoring: The power dissipation during ON and OFF times provides the health of the equipment. For example, normal and leakage currents monitored in equipment to determine its wear and tear over time.

Current Solution

Figure 1 shows a CSA with a sense resistor. In the presence of higher voltage level (common-mode levels) applications up to 80V, a simple CSA on the outside (but a complex integrated circuit design with architecture catered towards precision and accuracy) and a sense resistor are the solution to most problems when measuring current. Current-sense amplifiers come with best-in-class accuracy and precision to tackle the demand of realizing microampere current levels and still maintain better signal-to-noise ratio (SNR) performance to provide the resolution of measurement sought by the system design.

However, it is not an easy task to select an optimized CSA for designers. There are tradeoffs to consider as shown in Figure 2:

1. Available supply.
2. Minimum detectable current (translates to how low the input offset voltage (V_OS) of the device is).
3. Maximum detectable current (translates to the maximum input sense voltage (V_SENSE)).
4. Power dissipation allowable on the R_SENSE.

Since the differential voltage range is set by the choice of the current sense amplifier, increasing the R_SENSE value improves the accuracy of the measurements for lower values of the current. But the power dissipation is higher at higher current, and that may not be acceptable. Also, the range of the sensed current is reduced (I_MIN : I_MAX).

Reducing the R_SENSE value is more beneficial as it reduces the power dissipation of the resistor and increases the sensed current range. Reducing the R_SENSE value reduces the SNR (which can be improved with averaging to average the noise at the input).

Note: During this scenario, the offset of the device affects the accuracy of the measurement.

Often, calibration at room temperature is done to improve system accuracy, cancelling out the offset voltage with the addition of test cost for certain systems.

Also, the input differential voltage range (V_SENSE) is dependent on the supply voltage or the internal/external reference voltage and the gain:

In any application realizing high current ranges, the goal is to maximize the dynamic range for a targeted accuracy budget, which is typically estimated by the following equation:

V_SENSE-RANGE is typically 100mV for most CSAs with an input offset voltage of approximately 10µV.

Note: If V_{SENSE_MIN} is chosen to be a 10xV_OS factor, this provides 3 decades at best for ±10% errors in an uncalibrated system.

Similarly, if a 100xV_OS is selected, a ±1% error range can be achieved, but then the dynamic range decreases to 2 decades. As a result, there is a tradeoff between the dynamic range and accuracy: tightening the accuracy budget reduces the dynamic range dictated by the V_{SENSE_MIN} and vice versa.

Note: In a CSA + RSENSE system, RSENSE (tolerance and temperature coefficient) is usually the restriction in the total accuracy of the system.

This is still the industry's efficient practice to monitor/measure currents in a system due to its simplicity, reliability, and reasonable costs compared to other alternatives such as fuel gauges, CSAs with integrated chip resistors, and discrete implementation of difference amplifiers using operational amplifiers (Op-amps). Higher grade tolerance and temperature coefficient sense resistors are available, but only at steeper prices. The total error budget of the application over temperature needs to be equivalent to the error emerging from the R_SENSE.

Resistor-less Sensing Solution

For the applications that require higher dynamic range of currents to be measured from a couple of hundreds of microamperes to several amperes, an integrated current-sensing device (U1) shown in Figure 3 is a highly useful and effective solution. The solution meets the bill in the following criteria:

1. Integrated sensing element (resistor-less).
2. Greater than 4-decade current-sensing dynamic range.
3. Current output feature (along with 160Ω LOAD provides 0-1V V_OUT, compatible with all analog-to-digital converter (ADC)/microcontroller inputs for current realization).

Instead of an external sense resistor, an integrated sensing device is present across the V_DD input and load (LD) output, capable of measuring the system load current (I_LOAD) from 100µA to 3.3A. An internal gain block with the gain of 1/500 provides the output current at ISH, which is . A 160O resistor connecting from the ISH current output to GND, converts to VISH voltage output from 0 to 1V.

The drop across V_DD and LD on the sensing element device is approximately 60mV at 3A of load current (Plot 1), equivalent to just 180mW power dissipation while at lower current values, the total error observed to sense the 100µA range is in the region of 10% (Plot 2). Together with less power dissipation at higher current loads and still maintaining improved error budget at lower current levels, this scheme prevails the traditional sense circuit of Figure 1. Hence, applications requiring wider current sensing ranges up to 3A of sensing can benefit from this scheme.

Resistor-less Sensing Solution with Extended Line/Input Voltage

Figure 4 is an input voltage range extension of Figure 3, where the supply voltage for U1 can now accept the line voltage higher, up to 6V to 36V. The Zener diode (D1) maintains the voltage across V_DD and gate of the PFET (M1) to 5.6 V. The bulk of the high voltage line is absorbed by M1 with the M1's source clamped to approximately 4V to 4.5V away from the V_DD input voltage, thus maintaining the U1 operating voltage (V_DD-V_SS) within its normal operating range (Plot 3). This M1's source voltage is then biasing the gate voltage for M2 PFET. The M2 PFET source is at V_SS (U1) + V_TH (M2) ensuring the U1 ISH output is within acceptable voltage levels. The ISH current output and R1 generate 0V to 1V output with respect to GND.

Experimental Results

The following are the experimental results from the circuit of Figure 4.

Conclusion

The resistor-less sensing solution using MAX40016 can realize a 4-decade current-sensing solution with extended operating range up to 36V.

著者

Bich Pham

Bich Pham joined Analog Devices as a customer applications engineer in 2000 and is now a senior member of Technical Staff where he remains focused on helping customers solve real-world design challenges. Bich has a BSEE from San Jose State University in California.

Ashwin Badri Narayanan

Ashwin Badrinarayanan was an Application Engineer at Analog Devices, experience in Analog/ Mixed Signal system design, Analog signal conditioning, Linear & Non-linear circuits using Operational Amplifiers, Voltage Reference, Current Sense Amplifiers, Sigma-Delta/ SAR Data ConvertersHe hols an Master of electronics and communication engineering of the Universitiy of Dalles, Texas.