Specifications

38
Fig. 4.1: Diagram of the power subsystem.
with inputs ranging from 6.0-15.0V. The switchers have the ability to be deactivated us ing
control lines. T he battery supervisor circuit sends a digital signal when the power after
the RVP circuit drops below a minimum threshold. In sleep mode, the entire power supply
consumes 13.2µA.
Reverse Voltage Protection Circuit
This circuit traditionally is a d iode placed in series between the positive input and the
voltage regulation devices. An inherent characteristic of diodes that can be problematic in
low power applications is that diodes have a forward voltage drop of 0.7-1.5V. This reduces
the total available voltage from the battery pack, thus reducing operational lifetime. This
is not so crucial for the auxiliary power port, but it is a concern for when operating from
the battery pack.
In an effort to extend the life of this product, the designers searched for ways to get
around this dilemma. A resolution was fou nd by using two HexFET power transistors [36]
in series with their gates tied together. This resulted in a circuit with a switch-able diode-
like behavior with a 50mV drop. Figure 4.2 illustrates the back to back transistors with
supplementary BJT transistors that serves as an auto-select feature when the “power select”
signal is eithier not driven or is tied low. If the control line is driven high, the circuit switches
off the path from VBAT to VIN. Comparing this drop to a standard diode, the b attery life