Simple Circuit Boosts a 3V Input and Improves Efficiency
In Figure 1, a discrete charge pump composed of D1, D2, C1 and C2 boosts the input voltage. The voltage at the IN pin is 2 times the input voltage, Vin. This is fed to the MAX1967's internal linear regulator input to generate a 5V output, VL. Thus, the 5V gate drive is provided to the external MOSFET, even with a 3V input. D3 is used to start up the IC during the soft start. Once the IC and the charge pump circuit start to operate, VL provides 5V output, which reverses bias diode D3.
Figure 1. Synchronous buck converter with charge pump.
Figure 2 shows conversion efficiency with 3.0V input and 1.8V output. It is shown that the measured efficiency with the charge pump is 3% higher than that of conventional connection (IN and VL tie to input) at a 2.5A load. This results in 0.35W power savings and reduces heat dissipation.
Figure 2. Measured efficiency vs output current.