Extend current-sense amplifier's input range down to 0V
Modern electric/hybrid cars often include, in addition to the traditional lead-acid battery, a large capacitor of 40 Farads or more as a back-up power source. Located under the rear seat, the large capacitor gives an extra 10–15 minutes of driving time that allows you to reach the next charging station or gas station. Lead-acid batteries discharge only down to 8V or so, but a capacitor can discharge all the way down to 0V. In turn, this capability requires a current-sense amplifier that can measure inputs all the way down to 0V.
Most high-side current-sense amplifiers operate over a limited range of common-mode input and supply voltages. Consider, for example, the MAX4081, whose common-mode input and supply voltages range from 76V down to 4.5V. To set the zero load-current point (the voltage output corresponding to zero load current, where VSENSE = 0V) for a bidirectional (charge/discharge) current-sensing application, one normally connects an external reference (+2.5V, for instance) to the reference input (REF). For the MAX4081, the lower common-mode limit of 4.5V prohibits its use in applications that require current sensing close to ground.
You can solve this problem by connecting a charge pump (MAX1852) to the circuit (Figure 1). The tiny charge pump (IC2) is powered from +5V, as is the current-sense amplifier. The output of the charge pump (-5V) acts as a negative supply voltage at the GND pin of IC1. REFA and REFB are connected to GND.
Figure 1. Adding a -5V rail to this current-sense amplifier extends its lower common-mode limit from 4.5V to 0V.
The internal op amp (A2) of IC1 now operates from ±5V rails (a 10V span), with its non-inverting input (REF pins) sitting at the 0V midrail level. For VSENSE = 0V, the output voltage is 0V. VSENSE then increases with load current, producing an output of 5x, 20x, or 60x, according to the part number's gain suffix F, T, or S. The effective common-mode range now extends from 0V to +70V, with the original specifications unchanged (VOS < 0.6mV, and Gain Error < 0.6%).
You can also configure this circuit for bidirectional current-sense applications operating from a single supply voltage (Figure 2). The ADC reference output drives the REFA pin to set the zero load-current point for IC1.
Figure 2. Connecting the reference voltage from an ADC to the current-sense amplifier as shown, lets the circuit monitor bidirectional currents.
Test results for the Figure 1 circuit (Figures 3 and 4) show that the current-sense amplifier's common-mode voltage (with GND connected to -5V) can go down to -2.8V. In contrast, common-mode voltage for the standard application (with GND connected to 0V) can go down only to +2.3V. With REFA and REFB shorted to GND, however, the output can swing 5V above and below GND.
Figure 3. As shown in this plot of the Figure 1 output voltage, the addition of a charge pump extends IC1's common-mode range to 2.8V below ground.
Figure 4. For IC1 in Figures 1-2, operating with a gain of 60 and a common-mode voltage of 0V, VOUT vs. VSENSE is linear over the entire VSENSE range (-80mV to +80mV).
IC1's typical supply current (103µA) presents a small load current to IC2, which prevents overloading and voltage drooping at its output. Take care when the output moves below GND. Load current then flows out of IC1's GND terminal and into the charge pump, whose negative output can droop as a result (rise toward 0V). As a countermeasure, you can use bigger capacitors in the charge pump or restrict the sense amplifier's output voltage.