In addition to the traditional lead-acid battery, modern electric/hybrid cars often include a large capacitor (40 F or more) as a backup power source. Located under the rear seat, this capacitor ensures an extra 10 to 15 minutes of driving time that allows you to reach the next charging station or gas station.
Lead-acid batteries discharge down to only 8 V or so, but a capacitor can discharge all the way down to 0 V. This capability requires a current-sense amplifier that can measure inputs to 0 V.
Most high-side current-sense amplifiers, however, 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 76 V down
to 4.5 V. To set a zero load-current point
(that is, the voltage output corresponding
to zero load current, where VSENSE = 0 V)
for a bidirectional (charge/discharge) current-sensing application, one normally
connects an external reference (+2.5 V,
for instance) to the reference input (REF).
For the MAX4081, the lower common-mode limit of 4.5 V normally prohibits its
use in applications that require current
sensing close to ground.
Designers can solve this problem by
connecting a charge pump to the circuit
(). The tiny charge pump (IC2) is
powered from +5 V, as is the current-sense amplifier. The output of the
charge pump ( 5 V) acts as a negative
supply voltage at the "GND" pin of IC1. REFA and REFB of IC1 are connected to
the circuit GND.
The internal op amp (A2) of IC1 now
operates from 5-V rails (a 10-V span),
with its non-inverting input (REF pins) sitting at the 0-V mid-rail level. For VSENSE =
0 V, the output voltage is 0 V. 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 0 V to +70 V, with the
original specifications unchanged (VOS
Tests on the circuit in showed that the current-sense amplifier's common-mode voltage (with "GND" connected to 5 V) can go down to 2.8 V (). In contrast, common-mode voltage for
the standard application (with GND connected to 0 V) can go down only to +2.3 V. With REFA and REFB shorted to GND,
however, the output can swing 5 V above
and below GND. shows the output of IC1 at 0-V common-mode voltage
over the entire range of sense voltage.
IC1's typical supply current (103 µA)
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 0 V). As a countermeasure, you can use bigger capacitors in the
charge pump or restrict the sense amplifier's output voltage.