We describe in this paper an energy-efficient waveform generator which is dedicated to build a low-power intra-cortical implantable microstimulator. It features novel flexible current-mode stimuli such as half-sine and rising exponential pulses. The output stage of the proposed device consists of an electrode-tissue driver that generates high-voltage supplies on-chip in order to increase the voltage swing and maintain the stimulation in high-impedance interfaces which are due to small electrode areas. The current pulse duration may vary between 10 μs and 7 ms, while its amplitude varies from 2 to 200 μA. A current-mode digital to analog converter used to setup the current magnitude presents a DNL of 1.912×10^-4 LSB and an INL of 1.781×10^-4 LSB. The dynamic range of generated exponential current pulses reaches 40.66 dB with a linearity error <; ±0.5 dB. An output voltage swing of 14.5 V is reached allowing electrical stimulation through 150 kΩ microelectrode-tissue interfaces.