The input voltage limiting capability is necessary to protect the voltage regulator in the charging circuit from input voltage above the maximum allowable value. This value is determined by the input output differential. It may not exceed 36V.
The figure in the next page shows the voltage clamp circuit that was taken out of our prototype at the last minute. The specification of the pass-through transistor added a voltage drop of 4V at the maximum current, which was not acceptable since the charging circuit requires 17V for steady voltage regulation. This circuit compares the zener diode reference to a fraction of the input voltage. Based on this comparison, the op-amp controls transistor Q2, which either turns off or turns on the transistor Q1; therefore disconnecting the charging circuit from the input.