Challenges in Realizing a Chronic High-Resolution Retinal Prosthesis - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 142

Choice of Storage Capacitor
The capacitor(s) C S shown in Figure 7.7 stores the charge that is to be delivered
to the tissue through electrodes. It also acts as a filter to remove the ripple from
the output of the diode. Due to the varying magnitude of the stimulus current,
the capacitor needs to be large enough to store charge to avoid unacceptable
voltage drop when charge is delivered to the electrodes. Figure 7.8 illustrates
this problem for the dual-diode topology case. For the single diode topology, the
analysis is similar but the requirement of the capacitance is even higher since
the capacitor has to supply both anodic and cathodic currents. If N is number of
electrodes stimulated, the total charge delivered to the tissue by the stimulator
Q stim =

Q stim /T stim .
The voltage drop V cap during the stimulation can be calculated as V cap =
1/C S

I stim · N · PW stim . The average current from source is I s_avg =

· I stim · N − I s_avg ∗ PW stim .IfN = 1000I stim = 100A PW stim = 1

millisecond, T stim = 16 milliseconds and the maximum voltage drop the system

can tolerate is 1 V, then the required capacitance is 9375F. This is far too
large a capacitance to be implemented in an IC. Even an off-chip capacitor
of this size will consume a large area for an implant. Fortunately, this is an
extreme condition, when all the 1000 electrodes are stimulated at the same
time, which is unlikely. In addition, if the stimulation of electrodes can be

Figure 7.8. Storage capacitor and related waveforms.