A negative charge pump is responsive to a pump enable signal. A voltage controlled current source provides a current. A resistor is coupled between a node from the voltage controlled current source and a negative charge output from the negative charge pump. A capacitor is placed in parallel with the resistor. A comparator generates the pump enable signal to control the negative charge pump. The comparator is coupled to the resistor and the capacitor and measures an IR drop thereacross and compares this measurement against a reference threshold. A level of the pump enable signal can be variable by tuning an amount of resistance of the resistor or capacitor or adjusting the reference threshold. A memory can be driven by a method of the negative charge pump.