8.1 Introduction

The ultimate goal of a portable system is to be battery-less, since the battery provides the largest contribution for both volume and weight. Among the choices to achieve this goal, the solar power supply is the most usual but, particularly in bioelectronics, other techniques as radio-frequency have been reported [96].

The biggest issue when considering these power supplies is the voltage level they can generate. The nominal voltage of a single solar cell is about 0.5V [97] (or even lower for the other sources) and it is well behind the nominal voltage of one typical battery cell (1.2 - 1.5V) or even their end-of-discharge values (0.9V) [98].

The design of integrated circuits capable of working at one solar-cell nominal voltages offer a wide range of possibilities in portable analog processing, but whereas the digital circuitry (as shown in Figure 4.5) will naturally tend to those low values (even for other reasons), their analog parts will face severe problems to adopt them.

A possible solution or turnaround method is in-chip voltage multiplication by step-up voltage converters [99]. This technique is, however, noisy and not compatible with the sensitive analog circuits. Furthermore, they give analog designers the additional burden of taking care of the Power Supply Rejection Ratio (PSRR), an issue usually disregarded in battery powered equipment. As no other system level solution exists, one must consider the design of 0.5V analog circuit blocks.