The circuit in Fig 1
supplies both 3.3 and 5V to transitional circuits that employ both the
new 3.3V and older 5V devices. Additionally, because the regulator
accepts either 3.3 or 5V inputs, you could plug it into either a new
3.3V system or an old 5V system.The circuit consists of two sections: a
dc/dc converter and a double-pole, double-throw (dpdt) switch. The dpdt
switch comprises a pair of dual n-channel MOSFETs (Q2 and Q3) and their associated high-side drivers.
Upon power-up, the comparator in IC2 determines the state of the
circuit. The comparator’s output, IC2 pin 6, goes to the input of the MOSFET
driver, IC1. The driver internally generates a gatedrive voltage 8.8V
above the device’s supply voltage. This high voltage drives the
appropriate MOSFETs in Q2 and Q3.
IC2 is also the heart of a flying-capacitor, buck/boost dc/dc
converter. Unlike other switching-regulator schemes, this topology needs
no transformers. Transistor Q1 controls this section’s output voltage,
VS. When VIN is at 5V, Q1 is off, forcing the
section to operate as a step-down converter. In this mode, the section
produces 3.3V, which goes to the output through Q3B. Also in this mode,
5V power goes directly through Q2A, and Q2B and Q3A are both off.
When VIN is 3.3V, IC1 turns on Q1,
shorting out the 140-kΩ resistor and forcing the dc/dc-converter section
into step-up mode. In this mode the converter section generates 5V at
VS, powering the 5V output via Q2B. Also in this mode, 3.3V goes
directly from the circuit’s input to the output via Q3A. Q2A and Q3B are
both off.No-load quiescent current consumption is approximately 500 μA.
Lower-frequency converters would reduce power consumption at the
expense of a larger inductor. The efficiency of the dc/dc-converter
section is 73% in either mode. But because this power accounts for only
half of the circuit’s output power, the circuit’s overall efficiency is
approximately 80% with VIN=3.3V and 86% with VIN=5V.