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

Abstract

This application note introduces a resistor-less, greater than 4-decade dynamic range current-sensing solution and describes a simple method to extend its supply voltage range up to 6V to 36V using only a Zener diode and two MOSFETs. The MAX40016 is featured as an example with a schematic and test results.

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

Current-Sense Amplifier (CSA) + sense resistor

Figure 1. Current-Sense Amplifier (CSA) + sense resistor.

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 (VOS) of the device is).
  3. Maximum detectable current (translates to the maximum input sense voltage (VSENSE)).
  4. Power dissipation allowable on the RSENSE.

Design constraints to consider when using CSA and RSENSE

Figure 2. Design constraints to consider when using CSA and RSENSE.

Since the differential voltage range is set by the choice of the current sense amplifier, increasing the RSENSE 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 (IMIN : IMAX).

Reducing the RSENSE value is more beneficial as it reduces the power dissipation of the resistor and increases the sensed current range. Reducing the RSENSE 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 (VSENSE) 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:

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

Note: If VSENSE_MIN is chosen to be a 10xVOS factor, this provides 3 decades at best for ±10% errors in an uncalibrated system.

Similarly, if a 100xVOS 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 VSENSE_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 RSENSE.

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 VOUT, compatible with all analog-to-digital converter (ADC)/microcontroller inputs for current realization).

2.5V to 5.5V current-sensing system with integrated current-sensing element

Figure 3. 2.5V to 5.5V current-sensing system with integrated current-sensing element..

Instead of an external sense resistor, an integrated sensing device is present across the VDD input and load (LD) output, capable of measuring the system load current (ILOAD) 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 VDD 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 VDD 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 VDD input voltage, thus maintaining the U1 operating voltage (VDD-VSS) 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 VSS (U1) + VTH (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.

A 6V to 36V current-sensing system with integrated current-sensing element

Figure 4. A 6V to 36V current-sensing system with integrated current-sensing element.

Table 1. Suggested Components Used in Figure 4
Reference Device Description
D1 CMFZ4690 5.6V Zener
M1 BSP322PH6327XTSA1 MOSFET P-CH 100V 1A SOT-223
M2 BSP322PH6327XTSA1 MOSFET P-CH 100V 1A SOT-223
U1 MAX40016ANL+ 4-Decade Resistor-less CSA in WLP Package

Experimental Results

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

Voltage drop across the internal sense element vs. load current

Plot 1. Voltage drop across the internal sense element vs. load current.

Gain error at ISH output vs. load current at different temperatures

Plot 2. Gain error at ISH output vs. load current at different temperatures.

Function of the MAX40016 supply voltage (VDD-VSS) vs. VLINE

Plot 3. Function of the MAX40016 supply voltage (VDD-VSS) vs. VLINE.

Load transient response with ILOAD step change from 0 to 3A

Plot 4. Load transient response with ILOAD step change from 0 to 3A.

Power-up transient response with 3A ILOAD

Plot 5. Power-up transient response with 3A ILOAD.

Conclusion

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