8.1.9 Setting the Current Limit Threshold

As mentioned in the Current Sensing section, the LM27402 exploits the filter inductor DCR to detect overcurrent events. If desired, the user can employ inductors with low tolerance DCR to increase the accuracy of the current limit threshold. The most common circuit arrangement for sensing the inductor DCR voltage is shown in Figure 32.

Figure 32. Inductor DCR Current Sensing Circuit

The most accurate sensing of the differential voltage across the inductor DCR is achieved by matching the time constant of the R_SC_S sense filter with the inductor's L/R_DCR time constant. If the time constants are matched, the voltage across the capacitor follows the voltage across the DCR. A typical range of capacitance used in the R_SC_S network is 100 nF to 1µF. The equation to match the time constants is:

Equation 12.

Adjust the current limit threshold to any level with a single resistor from the current limit comparator to the output voltage pin. Use the circuit in Figure 33 to set the current limit.

Figure 33. Adjusting the Current Limit Setpoint

Because the voltage across the inductor DCR follows the current through the inductor, the device trips at the peak of the inductor current. Capacitor C_SBY shown in Figure 33 filters the input to the current sense comparator. A working range for this capacitance is 47 pF to 100 pF. The equation to set the resistor value of R_SET is:

Equation 13.

I_LIMIT is the desired current limit level, R_DCR is the rated DC resistance of the inductor and I_cs- is the 10 µA current source flowing out of the CS– pin. To aid in high frequency common-mode rejection, a series resistor, R_CS, of same resistance as R_SET, is optionally added to the CS+ signal path.

The internal current source I_CS- is powered from the input voltage rail, V_IN. The minimum voltage required to drive that current source is 1 V from V_IN to V_OUT. If a low-dropout condition occurs where V_IN – V_OUT< 1 V, the LM27402 may prematurely initiate hiccup mode. There are multiple options to avoid this situation. The first option is to enable the LM27402 after the input voltage has risen 1 V above the nominal output voltage as seen in Figure 28. The second option is to lower the comparator common-mode voltage shown in Figure 34 such that the I_CS- current source has enough headroom voltage.

Figure 34. Common Mode Voltage Resistor Divider Network

Refer to AN-2060 LM27402 Current Limit Application Circuits (SNVA441) for design guidelines to adjust the common-mode voltage of the current sense comparator.