Precision current detection amplifier design based on lithium battery detection

While traveling through cities around the world, it's impossible not to notice the emergence of hybrid electric vehicles (HEVs) and electric vehicles (EVs). With the rapid growth of HEVs and EVs in the automotive market, systems such as battery management have become important.

Take the battery of our mobile phones as an example. We constantly check its status to make sure we can use the phone throughout the day. Now translate that to a car and imagine how important this information becomes.

There are several current sensing technologies that can monitor the status of an HEV or EV battery. The solution varies depending on the voltage and capacity of the battery. As shown in Figure 1, we can measure current at two main locations: the top of the stack (high-side sensing) and the bottom of the stack (low-side sensing).

Figure 1: Top of stack versus bottom of stack in battery management system

Typically, a typical hybrid and electric vehicle battery management system (BMS) can have high voltage and low voltagesubsystem. Generally, the low-voltage subsystem refers to the responsible 12v to 48v systemUsed to manage powertrains, lighting, air conditioning and infotainment systems. Load current can varyMuch depends on the operating conditions of the vehicle. To accurately measure current over a wide dynamic range, a precision current sense amplifier (CSA) that is stable over temperature is desirable. CSA can be configured as any high-side or low-side BMS system. Batteries in current electric vehicles range from 400 V to 800 V. In such systems, isolated current sensing solutions are preferred when performing top-of-stack current measurements.

TI offers several options for isolated current sensing. The DRV425 is an integrated fluxgate sensor IC that, when used in pairs, can be used for high-precision busbar measurements. TI also offers a series of isolated current-sense amplifiers  that monitor shunts on top of high-voltage battery packs.

Isolated current sensing is generally not required for bottom-of-stack current sensing in EV systems and high-side and low-side current sensing in 48-V/12-V HEV systems. 

Current sense amplifiers have been implemented in applications for current and power measurements for many years. These simple and affordable solutions enable designers to implement real-time overcurrent protection, system optimization and current measurement for closed-loop circuits with excellent linearity and accuracy. 

The type of current sense amplifier required will vary based on system requirements and designer preference. TI offers a broad portfolio of current sense amplifiers with common mode range, offset voltage, gain error and drift options. For HEV/EV battery management systems, it may be important to select a current sense amplifier with analog or digital output.

As shown on the left side of Figure 2, a current-sense amplifier with an analog output integrates a gain-setting resistor and sends an amplified signal to a single-ended analog-to-digital converter (ADC) resistor based on the differential voltage measured across the shunt. . For analog output current sense amplifiers, the value of the shunt resistor depends on the full-scale output, maximum input current, and gain. The minimum current is limited by the value of the shunt and the offset voltage of the device. The ADC reference will be an additional error source that needs to be evaluated in the signal path. While current-sense amplifiers with analog outputs remain highly accurate and widely used in battery management systems, current-sense amplifiers with digital outputs may provide additional value.

TI's digital output current sense amplifier (shown on the right side of Figure 2) integrates a specialized delta-sigma ADC to maximize the ADC's full-scale input range across the shunt resistor without amplifying the input signal. Due to their delta-sigma architecture, digital output devices have lower input offset voltages, allowing for higher accuracy measurements at low currents. Therefore, we can use smaller value shunt resistors to increase system efficiency.

Figure 2: Analog and Digital Output Current Sense Amplifiers

Current measurement applications such as battery management systems require strong performance from current sense amplifiers. TI's current sense amplifier portfolio covers needs ranging from wide common-mode range, low offset voltage and small gain error. Current sensing in BMS subsystems of hybrid and electric vehicles faces the challenge of large dynamic range. Digital high-precision current sense amplifier with ultra-low drift can greatly improve overall performance while reducing system complexity.