A voltage monitoring circuit for use with a system that delivers therapeutic pulses to a patient of the type that includes: a voltage regulator couplable to a battery, the voltage regulator is configured to produce a variable supply voltage (VDD,401); a current-generating circuit configured to supply a requested current to one or more electrodes, wherein the current-generating circuit includes one or more transistors (M1,M2) coupled to the variable supply voltage and configured to be in saturation when supplying the requested current, the voltage monitoring circuit comprising: a reference current path that includes a transistor (M5,M6) coupled to the variable supply voltage (VDD,401) that is configured to produce a voltage at a terminal that tracks the variable supply voltage (VDD,401); and a comparator circuit configured to produce an output signal (VNC,404) when the supply voltage (VDD,401) produced by the voltage regulator is insufficient to allow the current-generating circuit to supply the requested current, wherein the comparator circuit has: a first input (A,-) coupled to the terminal of the transistor (M5,M6) in the reference current path to sense the voltage that tracks the variable supply voltage (VDD,401); and a second input (B,+) coupled to a terminal of the one or more transistors (M1,M2) in the current-generating circuit to sense a voltage decrease across the one or more transistors (M1,M2) in the current-generating circuit, and wherein, when the difference between the variable supply voltage (VDD,401) and the voltage at the second input (B,+) becomes less than the difference between the variable supply voltage (VDD,401) and the voltage at the first input (A,-), the comparator circuit is configured to produce the signal (VNC,404) that indicates the variable supply voltage (VDD,401) produced by the voltage regulator is insufficient to allow the current-generating circuit to supply the requested current. a voltage regulator couplable to a battery, the voltage regulator is configured to produce a variable supply voltage (VDD,401); a current-generating circuit configured to supply a requested current to one or more electrodes, wherein the current-generating circuit includes one or more transistors (M1,M2) coupled to the variable supply voltage and configured to be in saturation when supplying the requested current, the voltage monitoring circuit comprising: a reference current path that includes a transistor (M5,M6) coupled to the variable supply voltage (VDD,401) that is configured to produce a voltage at a terminal that tracks the variable supply voltage (VDD,401); and a comparator circuit configured to produce an output signal (VNC,404) when the supply voltage (VDD,401) produced by the voltage regulator is insufficient to allow the current-generating circuit to supply the requested current, wherein the comparator circuit has: a first input (A,-) coupled to the terminal of the transistor (M5,M6) in the reference current path to sense the voltage that tracks the variable supply voltage (VDD,401); and a second input (B,+) coupled to a terminal of the one or more transistors (M1,M2) in the current-generating circuit to sense a voltage decrease across the one or more transistors (M1,M2) in the current-generating circuit, and a first input (A,-) coupled to the terminal of the transistor (M5,M6) in the reference current path to sense the voltage that tracks the variable supply voltage (VDD,401); and a second input (B,+) coupled to a terminal of the one or more transistors (M1,M2) in the current-generating circuit to sense a voltage decrease across the one or more transistors (M1,M2) in the current-generating circuit, and wherein, when the difference between the variable supply voltage (VDD,401) and the voltage at the second input (B,+) becomes less than the difference between the variable supply voltage (VDD,401) and the voltage at the first input (A,-), the comparator circuit is configured to produce the signal (VNC,404) that indicates the variable supply voltage (VDD,401) produced by the voltage regulator is insufficient to allow the current-generating circuit to supply the requested current.