Sometimes, an external device will have an unknown voltage that could be above or below the desired translation voltage, preventing a normal connection of the LSF. Resistors are added on the A side in place of the second supply in this case – this is an example of when LSF single supply operation is utilized, shown in Figure 9-5. In the following figure, a single 3.3 V supply is used to translate between a 3.3 V device and a device that can change between 1.8 V and 5.0 V. R1 and R2 are added in place of the second supply. Note that due to some current coming out of the V_{ref_A} pin, this cannot be treated as a simple voltage divider.

Figure 9-5 Single Supply Translation with 3.3 V Supply

The steps to select the resistor values for R1 and R2 are as follows:

1. Select a value for R1. Typically, 1 MΩ is used to reduce current consumption.
2. Plug in values for your system into the following equation. Note that V_{ref_A} is the lowest voltage in the system. V_CCB is the primary supply and R1 is the selected value from step 1.

The single supply used must be at least 0.8 V larger than the lowest desired translation voltage. The voltage at V_{ref_A} must be selected as the lowest voltage to be used in the system. The LSF evaluation module (LSF-EVM) contains unpopulated pads to place R1 and R2 for single supply operation testing. For an example single supply translation schematic and details, see the Single Supply Translation with the LSF Family video.