The LT3759 is a wide input range, current mode, DC/DC controller which is capable of regulating either positive or negative output voltages from a single feedback pin. It can be configured as a boost, SEPIC or inverting converter. An internal LDO regulator draws power from VIN or DRIVE to provide up to a 4.75V supply for the gate driver enabling the LT3759’s to work down to 1.6VIN. The fixed frequency, current-mode architecture, accurate SENSE threshold and external DRIVE pin make it well suited for stable operation over a wide range of supply and output voltages. When configured as a boost with a charge pump stage, the LT3759 can easily step up 1.6V input to 48V output at currents greater than 100mA. A window comparator on the FBX pin reports via the PGOOD pin, providing output voltage status indication. The device is available in a 12-Lead exposed pad MSOP package.
Charge Pump Stage
The charge pump is needed to reduce the maximum voltage seen by the MOSFET. It also creates an intermediate voltage to power the DRIVE pin, which is rated for 42V (absolute maximum).
1 Ohm Resistor
A small resistor is needed to limit the current into the sense resistor when the MOSFET turns on. Without it, there would be erratic switching as the current comparator is falsely tripped.
2.5V Startup Voltage
In order to start, there must be sufficient voltage to turn on the MOSFET. This voltage comes from looking at the Transfer Characteristics (drain current vs. gate-to-source voltage) graph in the datasheet. The typical value is around 1.5V, but some margin is needed so 2.5V was selected. Once the DRIVE voltage is present, it is used to power the internal circuitry.
The LT3759 configured as a flyback converter would also be a good option but would require a transformer with an auxilary winding to power the DRIVE pin. It would likely result in a a larger and more complex solution.
Voltage Sources that Work Down To 1.6V And Provides 3A+
Certain NiMH and NiCD batteries operate down to 1.6V and provide 3A+.