The TPS61045DRBT is a step-up (boost) switching regulator from Texas Instruments that takes an input from 1.8V to 6V and produces an adjustable output up to 28V. It delivers up to 300mA through the internal switch. No synchronous rectifier is used, so the Schottky diode is external—this is typical for boost converters in this power class and keeps the BOM simple.
The 300mA switch current is the peak current the internal FET can handle—it is not the continuous output current. For a boost from 3.3V to 12V, expect roughly 100–150mA output before the switch hits its limit; the exact number depends on the inductor and duty cycle. If your load needs more, you step up to a part with a higher switch rating. Output voltage is adjustable via an external resistor divider from 1.8V up to 28V. The 28V ceiling is useful for applications like OLED bias (typically 15–20V) or avalanche photodiode bias (up to 25V). The wide input range (1.8V–6V) means it works from a single alkaline cell, two NiMH cells, or a regulated 3.3V/5V rail.