4 How is Power Consumption Calculated for LSF Devices?

The LSF Family translators use passive components to perform level translation therefore these devices do not consume much power on their own. Instead there are other areas where power is dissipated. One of which, is the small current that flows through the internal biasing FET of the device. During normal operation (LSF010x, LSF0204), there is a leakage current from V_REB to V_REFA as depicted in the Figure 4-1. This current can be calculated as I_CC = (V_REFB-V_REFA-V_TH)/ 200kΩ, where R_BIAS is recommended to be a value of 200kΩ to minimize the leakage current from flowing back into VCCA.

Figure 4-1 Leakage Current from Higher to Lower Power Supplies

The second leakage current sinks into the driver side whenever an input low signal is being propagated through the device. During a logic LOW, the FET is turned on due to voltage difference between the source (driver) and the gate of the FET due to V_GS > V_TH. The FET operates similar to a small resistor, directly connecting the input to the output. During this time, there is current flow from R_B1 on B side, through the FET, combining with the current flowing through pullup R_A1 and sunk into the driver (A_N) side.

Figure 4-2 Leakage Current Into Driver Side During an Input Logic Low

The worst case is when all channels of the LSF device internal FET is on, where power dissipation, P_D = N x I_CH² x R_ON where I_CH is I_B, the current flowing through the FET or V_B/R_B, and N is the number of channels actively switching. R_ON values can be used through data sheet values.

When a logic HIGH is being propagated, the FET transitions into cutoff mode, where the input and output are separated from one another, meaning negligible amount of current sinks back into the driver side (A_N). The calculation of this leakage current is insignificant and can be disregarded during power consumption calculation.

Figure 4-3 Leakage Current During an Input Logic High