Specifications

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Table 3.3.1-1: LED status code table. Reprinted with permission of Texas
Instruments.
A designed schematic of the portable unit’s power supply is available in Figure
3.3.1-1. Essentially, the external power supply will be connected to the unit.
Inside the unit, the connection point will be to the charging circuit. The charging
circuit will connect to the battery. At this point, two voltage regulators will provide
two voltages for use by the rest of the circuit. The battery is always driving the
load. While a switch was considered to allow the external supply to be able to
directly drive the load, the desire for simplicity meant that an extra switch would
not be ideal. This would have also have had the additional advantage of reducing
the number of cycles on the battery, thus extending its life.
It should be noted that the battery is connected to both the charging circuit and
the primary load of the battery (the remainder of the circuitry). There is a design
assumption that the user will most likely not be using the device while it is
plugged in to charge. Even if it is, however, it would still work as again, the
device is still always being driven from the voltage source provided by the
battery.
In addition, all major elements of the circuit are capable of driving at least the
worst case draw of current, which is mentioned in table 3.3.-1. In practice, it is
unlikely this scenario would occur. Nonetheless, it should be designed as such
as an element of robust design.