8.2.1.2.1 Select R_SNS and CL setting

The LM5069 monitors the current in the external MOSFET (Q1) by measuring the voltage across the sense resistor (RS), connected from VIN to SENSE. When the voltage difference across the VIN and SENSE pins (VCL) is greater than 55 mV (typical), the LM5069 begins modulating the MOSFET gate. Size R_SNS for maximum or minimum V_CL for applications that require ensured shutoff or ensured conduction. R_SNS is sized to exhibit minimum V_CL across R_SNS at maximum load current in Equation 1.

Equation 1.

Typically sense resistors are only available in discrete value. We choose the next smallest discrete value, 4 mΩ. If a precise current limit is desired, a sense resistor along with a resistor divider can be used as shown in Figure 29.

Figure 29. SENSE Resistor Divider

If using a resistor divider, then the next larger available sense resistor must be chosen (5 mΩ in this example). The ratio of R1 and R2 can then be calculated with Equation 2.

Equation 2.

Note that the SENSE pin pulls 23 µA of current, which creates an offset across R2. TI recommends keeping R2 below 10 Ω to reduce the offset that this introduces. In addition, the 1% resistors add to the current monitoring error. Finally, if the resistor divider approach is used, compute the effective sense resistance (R_{SNS, EFF}) using Equation 3 and use that in all equations instead of R_SNS.

Equation 3.

Note that for many applications, a precise current limit may not be required. In that case, it’s simpler to pick the next smaller available sense resistor.