A coulomb counter can measure charge that flows into or out of a battery. Small, dedicated devices such as LTC2941 or LTC2943 can directly interface with low to medium battery voltages, up to about 20V. You can extend the input operating range of the measurement circuit by using a high-voltage amplifier as a level shift. The LT6375 has some handy features that make this work accurately up to surprisingly wide voltage ranges.
Figure 1 shows the typical connection of a low voltage coulomb counter, the LTC2941. Coulomb counters operate by measuring the voltage across a sense resistor as an indication of the current that needs to be integrated. The key point is that the coulomb counter really measures voltage, and only interprets it as current and then later reports it as charge. If you remove the sense resistor, and somehow drive another voltage across the coulomb counter’s sense pins, it will still interpret that voltage as a current and report an accumulated charge.
In Figure 2, the LT6375 output is connected to the sense pins of the coulomb counter. The LT6375 is a difference amp, which means an op amp plus precision resistors connected in a way to level shift a differential input voltage. The operation of the difference amp is such that it drives its output to a value of:
OUT = REF + GAIN* (IN+ – IN–)
The LT6375 drives its output pin, but the REF pin must be connected to a low impedance source. Similarly, the LTC2941 expects a low impedance source at its SENSE+ pin, which is also the supply pin for that part. You can connect both REF and SENSE+ to the same logic rail as used for the I2C interface, for example 3.3V. By connecting the LT6375’s OUT pin to the SENSE– pin, the LT6375 will impose the difference between its inputs across the inputs of the LTC2941. In effect, the LT6375 acts as a fake sense resistor.
The accuracy of a difference amp depends greatly on the resistor matching. It is obvious that resistor mismatch directly affects gain accuracy. But perhaps less obvious, and in fact more serious, is that resistor mismatch causes offset errors. A 1% resistor mismatch would cause an output offset equal to 1% of the voltage over which you level shift. For example a 48V input level-shifted to 3V would result in a 450mV offset error, which is much too large for this kind of measurement. The LT6375A specifies a minimum CMRR of 97dB, which means that a 45V level shift causes an offset of less than 640µV.
Input Voltage Range
When designing a high-voltage level shift circuit, precautions are needed to ensure that the op amp inputs stay within their valid operating range. The LT6375 offers the flexibility to make that happen. First, the supply pin itself is rated up to 60V, so in some cases you can power the LT6375 from the voltage that is being measured. This is the arrangement in Figure 2, where the LT6375 measures current from a 48V supply. Furthermore, the op amp inside the LT6375 has the unique feature that the inputs can operate at voltages higher than the supply pin itself (Over-The-Top®). Finally, the LT6375 includes additional precision resistors that can be pin-configured to divide the input common mode range, while keeping the differential gain equal to unity. In Figure 3, the auxiliary reference pins are all connected to the 5V supply, which divides the –42V input voltage to bring it within the op amp’s supply range. All in all, the LT6375 can monitor supplies across a +/–270V input range.
The combination of very accurate resistors with flexible features to handle high voltages allows the LT6375 to precisely level shift a sense resistor’s voltage down to a level where it can be processed by low-voltage coulomb counters. The result is a compact circuit to measure accumulated charge at any voltage between +/–270V.