Today’s microprocessors need a regulated 3.3-V supply that can provide several amperes of current. To obtain this, a synchronous switching regulator (LTC1266-3.3) using a 5-V supply is usually used with a p-channel top-side MOSFET and an n-channel bottom-side MOSFET. However, because a p-channel MOSFET has higher RDS(on) and higher gate capacitance than that of a comparable n-channel MOSFET, there would be a higher voltage drop across the MOSFET. This contributes to higher overall power loss. Such a drop in efficiency becomes even more obvious under high current situations. The ideal configuration would use n-channel MOSFETs for both top- and bottomside switches.
To replace the p-channel MOSFET with an n-channel, a secondary input voltage capable of fully enhancing the n-channel MOSFET is needed. Because this secondary input only drives the gate of the charging MOSFET, little current would be drawn. A simple and compact charge pump (LTC1263) that converts 5 V to 12 V does the job. As shown, both the switcher and the charge pump are powered by the 5-V supply while the 12-V output of the charge pump drives the Vin pin of the switcher.
The charge pump has a somewhat lower efficiency than that of an inductor-based switcher. However, it’s preferred over a boost switching regulator because of the charge pump’s simplicity and its need for fewer and smaller external components. These advantages outweigh the drawback of its lower efficiency.
Since both the LTC1266-3.3 switcher and the LTC1263 charge pump have shutdown pins, both of them can be connected directly to an I/O line from a microprocessor. This way, the system can save much current and power during standby mode.
With these two new regulated supplies of 12 V/60 mA and 3.3 V/5 A generated from the original 5-V supply, many previously tough power supply problems are eliminated.
