LM3645 has the ability to turn the DC/DC boost converter typically used to power the four LED outputs into a general purpose fixed output power rail (bit [0] in reg 0x00) for use in other system applications that need voltage conversion. Four user selectable regulation voltage levels can be generated; 4.0 V, 4.5 V, 5.0V and 5.2 V (bits [7:6] in register 0x00). Voltage Mode can be used in conjunction with driving LEDs and will behave based upon the Output Mode Priority Bit (bit [1] in reg 0x00). If the bit is set to '0', maintaining voltage regulation is of highest importance. In Voltage Mode priority mode, care must be taken to manage the on chip thermal dissipation. If the bit is set to '1', the boost is allowed to regulate to a level that is optimized for driving the enabled LEDs. In LED drive priority mode, the output voltage returns to the target voltage mode level at the termination of LED driving.

In voltage mode, there are current limitations for each mode due to the peak switch node voltage. Using a 1-Ω resistor and 4-nF capacitance for the snubber, the maximum allowed currents are as follows:

• 4.0 V mode = 3 A peak current
• 4.5 V mode = 1.5 A peak current
• 5.0 V mode = 750 mA peak current
• 5.2 V mode = Not recommended