In the classical current-sensing circuit, designing for the sense resistor can bring unwelcome challenges. This saves board space and provides a four-decade sensing range.
Current sense amplifiers (CSAs) are specialized amplifiers that monitor current flow by measuring a voltage drop across a sensor element. You will find CSAs across multiple applications, such as base-stations, server backplanes, automotive, mobile, and RF power equipment, where it is important to keep track of instantaneous system current, albeit in many instances over time. Although these devices operate at differing signal frequencies, the CSA bandwidth’s operating range is only up to one megahertz. CSA’s primary job is to monitor the power supply’s wellness, where typical variations are slower than the circuit’s signal frequency activity.
The CSA’s sensing element is either an ultra-low value external resistor (RSENSE) or an integrated current-sensing transistor. Generally, these integrated CSA devices are easy to use, more precise than their discrete versions, and less prone to noise. CSA’s typical current ranges vary from tens of micro-amperes to hundreds of amperes. These devices provide common-mode voltage ranges that can extend across +100V.
CSA using an external current sensing resistor
The CSA that uses an external sense resistor has a classic, difference amplifier at its input to sense the small voltage drop across RSENSE. Figure 1 shows a generalized CSA circuit using an ultra-low sensing resistor.
In Figure 1, a careful selection of RSENSE establishes minimal IBAT* RSENSE or VRSENSE voltage drop to preserve the load’s power supply voltage. This system effectively creates a two-decade current sensor. Additionally, the absolute resistance difference between RSENSE and R1 and R2 is high enough to ensure that the battery current to the load nearly exclusively flows through RSENSE.
The Figure 1 configuration provides ample common-mode range, low offset voltage, gain, and temperature stability to safeguard good power supply current measurements.
The common-mode-range defines the DC voltage range at IN+ and IN-, with respect to ground. The design of Figure 1’s CSA typically supports common-mode voltages beyond the power supply. This characteristic allows high-side (near the positive power supply) and low-side (near the negative supply) measurements. Figure 1 shows a high-side measurement, where IN+ and IN- are effectively biased to VBAT1, with the load’s power supply biased to the load voltage, VBAT2. With a small voltage drop across RSENSE, this circuit places the common-mode voltage slightly above the positive supply.
The offset-voltage is a DC error between IN+ and IN-. The CSA offset voltage and the IBAT * RSENSE voltage (VRSENSE) combine to create the voltage difference between IN+ and IN-. Determining the difference between the amplifier’s with high offset voltage necessitates the use of a high RSENSE resistive value. A high RSENSE resistor does overcome high amplifier offset voltages, however, the gain errors increase, and the voltage magnitude of VBAT2 lessens. These errors can be critical in battery powered applications.
The resistors R1, R2, R3, and R4 establish the CSA gain. Nominally R1 equals R2 and R3 equals R4. The value of R3 / R4, in Figure 1, establishes the CSA fixed gain. The controlled process variations of these on-chip resistors ensure robust over temperature performance and lower gain errors. For this device, gain can range from 0.2 V/V to 1000 V/V with gain errors as low as 0.01%.
Selecting the external sensing resistor, RSENSE, requires some finessing. The first design step for this CSA circuit determines the maximum battery current, CSA’s Gain, and the maximum CSA output voltage. With the determination of these values RSENSE equals:
VOUT is the maximum allowable CSA output swing, Gain is the CSA signal gain, and IL(MAX) is the maximum load current.
The magnitude of RSENSE and the maximum power dissipation (RSENSE × IL(MAX)2) define the physical dimensions of the RSENSE.
The amplifier’s errors define the low-range accuracy of this system. The most significant amplifier error is the input offset voltage. The system offset-error percentage equals:
The magnitude of this error impacts the accuracy of the lower value battery currents and consequently impacts the dynamic range of the CSA system.
An alternative to the classical CSA that uses an external sensing resistor is to move the sensing element into the CSA circuit. You can do this by creating an ultra-low value resistor in the integrated chip (IC). However, this technique creates a device that is limited to a small number of applications. An alternative is to utilize available IC transistors to implement current mirrors to measure the battery current.
CSA using internal current sensing transistor
The proposal for this new device is to throw away the two-decade, RSENSE resistor solution and replace it with a four-decade, integrated current-sensing element (Figure 2).
Figure 2 Four-decade current-sensing device
A four-decade current-sense device accepts the power-supply current, through an active on-chip transistor. The device shown in Figure 3 maintains accuracy from 300μA to 3A with a voltage drop of 35mV to 60mV across the transistor sensing element. The current mirror topology eliminates the offset limitations and reduces the noise thereby extending the dynamic range to at least four decades.
Having an integrated sense element allows factory trimming, which saves the user from having to calibrate independent CSA sense resistors. The device shown contains a four-decade current-sense element and uses external resistors (RH, RM, and RL) to select the full-scale current range.
The integrated current-sense element saves significant board space compared to the expensive external RSENSE (Figure 3).
In Figure 3, the combination of the CSA in a SOT-23 package with a 1Ω current-sense resistor consumes ~30mm2 of PCB real estate. The four-decade CSA consumes 10 to 20 times less PCB real estate than the typical CSA plus RSENSE.
The drift of the big 1Ω current-sense resistor is typically very high, from 20ppm/°C to 400ppm/°C, with the least expensive resistors drifting the most. The resistor price increases for good initial accuracy and lower temperature drift.
In the classical current-sensing circuit, designing for the sense resistor can bring unwelcome challenges. The upfront design effort is often tedious, and the sensing resistor used is not only costly in multiple applications, but consumes significant PCB area. Replacing the sensing resistor and CSA with an integrated current mirror is a liberating and straightforward option. This small, compact solution saves ~20 times the board space and provides a four-decade sensing range.
Bonnie Baker has been working with analog and digital designs and systems for more than 30 years and is writing this blog on behalf of Maxim Integrated.