As described in the Enable, Disable, and Reference Voltage Guidelines section, it is generally recommended that V_{ref_B} > V_{ref_A} + 0.8 V; however, the device can still operate in the condition where V_{ref_B} < V_{ref_A} + 0.8 V as long as additional considerations are made for the design.

Typical Operation (V_{ref_B} > V_{ref_A} + 0.8 V): in this scenario, pullup resistors are not required on the A-side for proper down-translation as is shown for channels 1 and 2 of Figure 9-4. The typical operating mode of the device ensures that when down translating from B to A, the A-side I/O ports will clamp at V_{ref_A} to provide proper voltage translation. For further explanation of device operation, see the Down Translation with the LSF Family video.

Requirements for V_{ref_B} < V_{ref_A} + 0.8 V Operation: in this scenario, there is not a large enough voltage difference between V_{ref_A} and V_{ref_B} to ensure that the A side I/O ports will be clamped at V_{ref_A}, but rather at a voltage approximately equal to V_{ref_B} – 0.8 V. For example, if V_{ref_B} = 1.8 V and V_{ref_A} = 1.2 V, the A-side I/Os will clamp to a voltage around 1.0 V. Therefore, to operate in such a condition, the following additional design considerations must be met:

• V_{ref_B} must be greater than V_{Ref_A} during operation (V_{ref_B} > V_{ref_A})
• Pullup resistors should be populated on A-side I/O ports to ensure the line will be fully pulled up to the desired voltage.

Figure 9-6 shows an example of this setup, where 1.2 V ↔ 1.8 V translation is achieved with the LSF0102-Q1. This type of setup also applies for other voltage nodes such as 1.8 V ↔ 2.5 V, 1.05 V ↔ 1.5 V, and others as long as the Recommended Operating Conditions table is followed.

Figure 9-6 1.2 V to 1.8 V Level Translation with LSF010x