ROHM Boasts ±1% Accuracy in New Current Sense Amplifier ICs
Built for industrial applications, the new IC from ROHM is said to offer high-accuracy current sensing while reducing PCB area.
One of the standard circuit blocks implemented in a variety of electronic systems is current sensing. Whether current sensing is used for circuit protection, power management, or control loops, understanding the current flow to important loads is a necessity in most circuit designs.
However, for high-accuracy applications, some of the standard current sensing architectures can fall short if not designed correctly. Recently, ROHM Semiconductor released a new current sensing IC claimed to enable unprecedented accuracy while minimizing PCBA area.
ROHM's accuracy-focused current sense amplifier ICs for industrial applications. Image used courtesy of ROHM Semiconductor
In this article, we’ll take a look at a conventional current sensing architecture, discuss some of its shortcomings, and assess the new product from ROHM.
Standard Current Sense Architecture
While there are many different ways to measure current in an electronic system, one of the most common solutions is through the use of a shunt resistor.
In this architecture, a very low resistance resistor (typically on the order of milliohms) is placed in series with the load. Sharing a series connection, this shunt resistor, also called a sense resistor, experiences the full current to the load and develops a voltage drop across it that is directly proportional to the current (V = I/R). However, since the sense resistor is so small, the voltage drop it experiences is also of extremely low amplitude.
A standard current sensing architecture from a 12 V rail. Image used courtesy of Texas Instruments
To increase the amplitude of this signal, the voltage of each resistor terminal is then fed as inputs to a dedicated operational amplifier (op amp). Here, the signal is amplified, and output as a single-ended signal is fed to an ADC for conversion into the digital domain. With knowledge of the shunt resistor value and the voltage rail for the load, the current can then easily be determined.
This architecture is very popular, owing mostly to its simplicity, low cost, and strong sensing performance.
Self-heating Accuracy Challenges
Despite the popularity of this shunt resistor architecture, in applications where extremely high accuracy is required, it sometimes can fall short. The major reason for this is that the sense resistor in this circuit is subject to variation based on both manufacturing and temperature.
As the sense resistor changes temperature, so too will its resistance value. And, since the current sensing architecture relies on the voltage drop across the resistor, a change in resistance will directly affect the measured current value.
Current measurement error caused by resistor self-heating. Image courtesy of Renesas
The impact here isn’t due to high ambient temperatures only but is also greatly influenced by resistor self-heating. Since the point of this sense resistor is to be exposed to high currents, it will naturally dissipate power as it operates. Here, the temperature coefficient (Tc) of the sense resistor will directly lead to a degradation in the current measurement accuracy. The change in a resistor value due to a temperature rise is calculated using the equation: ∆Rsense = RsenseTc × ∆Temperature
Thus, achieving high accuracy in this architecture requires either a very temperature-stable sense resistor or a means of accounting for resistor self-heating error.
ROHM Releases High-accuracy Current Sensing Amp
This week, ROHM announced a new current sensing IC that aims to tackle challenges with accuracy.
The product, the BD14210G-LA, is a new current sensing amplifier that comes highly integrated to decrease board space. By integrating peripheral discrete components, such as filters and bypass capacitors, ROHM claims that the BD14210G-LA reduces the circuit’s BOM from 11 components to three components. According to ROHM, this reduction in BOM count translates to a 46% reduction in the PCBA area.
Internal block diagram of the BD14210G-LA. Image used courtesy of ROHM Semiconductor
Beyond area savings, the new IC is said to achieve an extremely high accuracy of ±1% across an entire temperature range of -40 °C to +125 °C. ROHM states that this increased accuracy comes from internal circuitry that preserves current detection accuracy even as the temperature fluctuates.
With this new product, ROHM hopes to enable high current sense accuracy in a variety of industrial applications, including wireless base stations, PLCs, and inverters, as well as consumer applications such as home appliances.
