Taking only a cursory look at the input-voltage ratings of your power-supply IC can limit the usable input-voltage range. With careful examination of an IC's operating specifications and circuit topology, you may be able to work around that input-voltage limitation. For instance, The data sheet for TI's TPS61042 shows that it has all the functions necessary for providing a constant-current drive to a white-LED circuit; however, the input voltage of the IC does not meet the input-voltage requirements of this Design Idea. The dual lithium-ion input voltage varies from 6 to 8.4 V, but the TPS61042 input-voltage range is 1.8 to 6 V. Closer examination of the circuit shows that the power stage need not connect to the same voltage rail as the control IC. Figure 1 shows that by separating the input voltage to the TPS61042 from the power stage, you can power the LED driver from an input voltage greater than 6 V.
|Figure 1.||This circuit provides a way to get around a power-supply IC’s input-voltage limitations.|
The IC can receive power from any available system voltage of 1.8 to 6 V by connecting this voltage to the VIN pin. The input to the power stage can now connect directly to the battery. In general, the power stage can connect to any voltage that is lower than the required output voltage. With a boost topology, the input voltage to the power stage must be less than the output voltage, or the inductor and diode pass the input voltage directly to the output. The maximum allowable voltage on the SW pin, 28 V, also limits the maximum input voltage to the power stage.
This technique also improves system efficiency. Efficiency data for this circuit shows that higher input voltages provide higher efficiency. If you have to run the LED driver from a voltage that is lower than 6 V, the power to drive the LED is “double-converted.” It first converts from the raw lithium-ion input into an intermediate system voltage and then converts from the intermediate-system voltage into the LED-drive current. By carefully examining the operating specs for the IC, you can get around around its input-voltage limitation, save cost and board space, and increase system efficiency.