The constant current control design requires a high-precision current sensor, and the selection of the right operational amplifier (op amp) for measuring battery charge or battery discharge current through a shunt resistor is pivotal for accurate and reliable current measurement. The Y14770R00300F9R shunt resistor, with a resistance of 3mΩ made up of a power metal strip, is an excellent choice for current sensing. With a 1% tolerance and a maximum temperature coefficient of ±40ppm/°C, this shunt resistor provides accurate and stable performance.

For amplifying the current signal across the shunt, the INA818, a high-precision instrumentation amplifier, is used. The choice of an instrumentation amplifier with the right common-mode voltage is crucial, because the amplifier directly impacts the input and output voltage limitations. The instrumentation amplifier provides compatibility with the common-mode voltage of the application and is essential for achieving accurate and reliable current measurements in CC control.

Using TI's analog engineer's calculator to determine the input common-mode and limitations of the INA818, Figure 3-1 shows the calculation results. The maximum common-mode voltage is consistent with the maximum voltage on the battery side, which is 15V in this case. The INA818 is powered by 20V and –5V, and the reference voltage generated from the DAC is 2.5V. When the gain is set to 66, the calculation shows the input range from –111.4mV to 112.1mV, fitting within the current signal range (±8A × 3mΩ = 24mV).

The CC control loop also needs a high-precision op amp. The precision drift op amp, OPA192, with maximum input offset drift of ±0.1µV/°C, is used for this function.

Figure 3-2 illustrates the CC control loop schematic. The output voltage, linked to a voltage-controlled voltage source, functions as the tracking input for the DC-DC converter, supplying V_DCDC to the MOSFET drain. An intentional 1V rise is incorporated into the configuration to maintain V_DS at 1V, regardless of load conditions. The reference voltage, I_SET, is generated from a digital-to-analog converter (DAC) to control the output current or voltage depending on the operation mode. Another input, I_SENSE is the current feedback signal acquired through the differential voltage across the current sense resistor. These signals undergo differentiation in the error amplifier, OPA192, to facilitate current loop control. For small signal simulation, a 1TF capacitor and 1TH inductor are connected to the I_SENSE. This purpose is to break the feedback loop as the capacitor is open at DC while the inductor is a short. At high frequencies, the inductor is open and the capacitor is shorted.

Figure 3-3 shows the small signal of the CC simulation. The phase margin for constant current output of 8A is 73.39°, with the crossover frequency of 189.59kHz. This simulation affirms the stability of the control circuit and bandwidth sufficiency.

The voltage to adjust output current can be set from 0V to 5V. Use Equation 1 to calculate the input reference voltage for the desired current settings. In this reference design, to generate 8A output current, I_SET, the DAC output voltage, is set to 4.08V.