Switch-Mode Battery Charger Delivers 5A

The fast-charge controller IC3 (Figure 1) normally directs current to the battery via an external pnp transistor. In this circuit, the transistor is replaced with a 5A switching regulator (IC1) that delivers equivalent power with higher efficiency.

IC1 is a 5A buck switching regulator whose output is configured as a current source. Its internal power switch (an npn transistor) is relatively efficient because VCE(SAT) is small in comparison with the 15V-to-40V inputs. (For applications that require 2A or less, the low-saturation, non-Darlington power switch of a MAX726 offers better efficiency.)