Brian Black at Linear Technology Corp. focuses on how modern current sense amplifiers are meeting the needs of a range of applications
Accurate current sensing is becoming more important in a wide range of applications as traditional ‘open-loop’ systems of the past are being supplanted by smart and efficient ‘closed-loop’ designs. Examples of applications which implement current measurement include motor torque, solenoid force, LED intensity, solar cell exposure and battery power.
To measure current, a sense resistor is placed directly in series with the current, and an amplifier isolates and amplifies the voltage across this resistor (VSENSE). New amplifiers, specifically optimised for this task are becoming widely adopted in many applications from automotive to industrial, and from communications to computing.
Two common methods of current measurement are high-side and low-side sensing. In both cases, a small sense resistor is placed in the current path and the voltage that develops across it can be measured using an amplifier-based circuit. In low-side sensing the sense resistor is placed between the load and ground, while in high-side sensing the sense resistor is placed between the positive supply and the load. These two methods have fundamental system trade-offs, as well as different circuit requirements.
The primary advantage of low-side current sensing is that the amplifier supply voltage and input common mode voltage range can be quite low.
But there are a number of limitations which more than offset this. First, low-side current sensing requires that no ground paths exist that could allow current to be diverted around the sense resistor or that could contribute current from an adjacent circuit.
If a mechanical frame establishes the system ground, it may be impractical to insert this sense resistor. Also, since grounds are not perfect conductors, ground voltage can vary at different points in the system, necessitating the use of a differential amplifier for precision measurements. Perhaps the biggest problem with low-side sensing is that the sense resistor creates an offset voltage between true system ground and the ‘ground’ point seen by the load. This can induce common mode errors in the system and presents a problem for interfacing to other systems requiring the same ground potential.
Since measurement resolution is enhanced by the magnitude of VSENSE, the designer must trade-off ‘ground noise’ for increased resolution. A modest full-scale VSENSE of 100mV translates to 100mV of injected ground noise. The problem of ground variation can be avoided by placing the current sense resistor between the power supply and the load.
This approach is known as high-side current sensing. While it avoids the problems of placing a sense resistor in the ground path listed above, there are other challenges with high-side current sensing. As with low-side current sensing, high-side current sense circuits use a sense resistor to develop a differential voltage which can be directly measured. However, now there is a non-zero common mode voltage across the resistor. This configuration presents the technical challenge that a small differential sense voltage must be discerned from the common mode voltage of the power supply.
For low voltage systems, an instrumentation amplifier or other rail-to-rail differential amplifier may suffice for monitoring a high-side sense resistor. The output of the amplifier must then be translated to ground without adding significant error.
When the supply voltage is very high, circuitry may be required to translate VSENSE down to the input common mode range of the amplifier or to float the amplifier up to the supply voltage. Aside from the added board space and cost, these techniques assume that the common mode voltage will stay within a narrow, specified range.
For most current sense applications, it’s very useful to anticipate large common mode fluctuations. For example, if the current sense circuit can operate when the power supply voltage drops, it can indicate if there is a problem at the supply or at the load; excessive current suggests current limiting and load faults and insufficient current indicates power supply failure.
On the other hand, current sense circuits may face common mode voltages that exceed the supply voltage. Many current-devices, such as motors and solenoids, are inductive by nature; rapid current changes through them will cause inductive fly-back, leading to large voltage swings across the sense resistor. It is precisely in these instances when the amplifier can be most useful.
To address the challenges of current sensing, high-side current sense amplifiers were created. These special amplifiers are designed to extract a small differential voltage, generated by current passing through a small sense resistor, from a high common mode voltage. The sense voltage is then amplified and translated into a ground-referenced signal. Figure 1 illustrates the basic topology of a high-side current sense amplifier. In this case, the amplifier forces a voltage across RIN that is equivalent to VSENSE.
The current through RIN is then forced through ROUT, providing a ground referenced output voltage. For this basic capability, it is clear that high-side current sense amplifiers should have high input impedance, high gain with good gain accuracy, and a wide common mode range with good common mode rejection. What may not be as clear is the importance of the amplifier’s precision.
Ideally, the task of current and voltage sensing should not impact the load to which it’s connected. This means that voltage sense devices should have nearly infinite input impedance; this ensures that no appreciable current can be diverted from the load.
Conversely, a current sense device should have nearly zero input impedance; this ensures that the voltage to the load is not significantly reduced. High-side current sense circuits (amplifier + resistor) are subject to both of these requirements. The amplifier used to sense the voltage across RSENSE must have high input impedance. The resistor used to sense the current to the load must be very small.
To fully appreciate this, let’s consider the use of a large sense resistor. As the series resistance is increased, the voltage available to the load is reduced. Extra series resistance is a source of wasted energy; large sense resistors can lead to excessive heat dissipation, with potential long-term reliability concerns.
Is there any reason to use a large sense resistor? The primary advantage is to increase the over-all output voltage (EQ.1).
(EQ.1)
There is a limit to the size of the sense resistor. The input range of the amplifier and the maximum expected current will determine the largest practical sense resistance (EQ.2).
(EQ.2)
As an example, if 50mA is the maximum expected current through the sense resistor (ISENSE_MAX) and the high-side current sense amplifier can accept inputs up to 250mV (VSENSE_MAX), the maximum sense resistance is 5Ohms (RSENSE_MAX).
Ideally, the designer should not be forced to add sense resistance to compensate for the amplifier. As long as the amplifier can operate with sufficient gain and gain accuracy, the designer should instead focus on the minimum acceptable sense resistance. This can be calculated from the current sense amplifier’s input offset voltage and the smallest current that must be resolved:
(EQ.3)
As an example, if 1mA resolution is required (IRES) and the offset voltage of the high-side current sense amplifier is 1mV (VOFFSET), the minimum sense resistance is 1Ohm (RSENSE_MIN). Equation 3 highlights a key point: the minimum sense resistance is directly related to the offset of the high side current sense amplifier.
Modern current sense amplifier
With precision high-side current sensing in mind, new high-side current sense amplifiers offer dramatic improvements in performance over previous generations. For example, Linear Technology’s LTC6102 is a high-side current sense amplifier that incorporates zero-drift technology.
This amplifier has an input offset voltage of only 10µV and an offset drift of 50nV/°C Max. Compared to previous generations of current sense amplifiers, this device can use a significantly smaller sense resistor2. If the system can tolerate a large VSENSE, the LTC6102 can accept sense voltages up to 2V. The combined offset plus this maximum sense voltage provides over 106dB of dynamic range, allowing this device to resolve micro-amps from amps of current.
Sensing very small current is possible since any gain can be selected with external resistors. The gain accuracy can be better than 99 percent by using precision resistors.
High impedance inputs limit the input bias current to less than 300pA. In effect, the LTC6102 can operate with an input common mode voltage up to 105V. The common mode rejection of 130dB contributes less than 32µV of offset error across a full 100V of input common mode voltage range3. For fault protection, the LTC6102 has a 1µsec response time, allowing it to quickly initiate a power shutdown in the event of unexpected load or supply changes.
One of the most challenging current sense applications is to monitor fast switching currents, such as in H-bridge motor control, switching power supplies, solenoids, and battery chargers. Linear Technology’s LT1999 is a high-voltage, bidirectional current sense amplifier, designed to monitor fast switching currents with common-mode voltages from -5V to 80V. To maintain outstanding accuracy, even in the presence of large square wave input voltages, this device has more than 80dB of common mode rejection at 100kHz.
Current sensing may also be important for safety. With actuator control, battery management, power load monitoring, and other current controlled systems, large, unexpected current variation can indicate a serious fault condition. The company’s LT6108 and LT6109 combine precision current sense, fast, precision comparators and a precision voltage reference. When a fault condition arises, these two devices provide fast latch-able output signals.
In some applications, there is a desire to measure the current over a very wide common mode range which includes ground. This allows for current to be accurately measured even when the high-side voltage collapses due to a circuit fault. Amplifiers such as the LT6105 can meet these requirements.
High-side current sense amplifiers offer inherent benefits for monitoring and controlling current. Advancing technologies in battery management and motor control, to name a few examples, are creating a significant demand for current sense amplifiers with higher common mode voltages, higher accuracy, and better precision.
High-side current sense amplifiers have now achieved the performance levels of industry leading precision op-amps, giving designers a simple, versatile and highly accurate alternative to the lower precision or more complicated current sense circuits of the past.
For more information about current sensing, Linear Technology has compiled its ISENSE Application Note, an extensive collection of current sense circuits. This is available for download at: www.linear.com/currentsense
Linear Technology Corporation
