7.3.10 Current Limit

Current limit is realized by sensing the voltage across the low-side MOSFET while it is on. The R_DS(ON) of the MOSFET is a known value; hence the current through the MOSFET can be determined as:

The current through the low-side MOSFET while it is on is also the falling portion of the inductor current. The current limit threshold is determined by an external resistor, R_CS, connected between the switching node and the I_SEN pin. A constant current of 40 µA is forced through R_CS, causing a fixed voltage drop. This fixed voltage is compared against V_DS and if the latter is higher, the current limit of the chip has been reached. To obtain a more accurate value for R_CS you must consider the operating values of R_DS(ON) and I_SEN-TH at their operating temperatures in your application and the effect of slight parameter differences from part to part. R_CS can be found by using the following equation using the R_DS(ON) value of the low side MOSFET at it's expected hot temperature and the absolute minimum value expected over the full temperature range for the for the I_SEN-TH which is 25 µA:

For example, a conservative 15-A current limit in a 10-A design with a minimum R_DS(ON) of 10 mΩ would require a 6-kΩ resistor. To prevent the I_SEN pin from sinking too much current when the switch node goes above 9.5 V, the value of the current limit setting resistor R_CS should not be too low. The criterion is as follows,

Equation 10.

where the 10 mA is the maximum current I_SEN pin is allowed to sink. For example if V_IN = 13.2 V, the minimum value of R_CS is 370 Ω. Because current sensing is done across the low-side MOSFET, no minimum high-side on-time is necessary. The LM2743 enters current limit mode if the inductor current exceeds the current limit threshold at the point where the high-side MOSFET turns off and the low-side MOSFET turns on. (The point of peak inductor current, see Figure 28). Note that in normal operation mode the high-side MOSFET always turns on at the beginning of a clock cycle. In current limit mode, by contrast, the high-side MOSFET on-pulse is skipped. This causes inductor current to fall. Unlike a normal operation switching cycle, however, in a current limit mode switching cycle the high-side MOSFET will turn on as soon as inductor current has fallen to the current limit threshold. The LM2743 will continue to skip high-side MOSFET pulses until the inductor current peak is below the current limit threshold, at which point the system resumes normal operation.

Figure 28. Current Limit Threshold

Unlike a high-side MOSFET current sensing scheme, which limits the peaks of inductor current, low-side current sensing is only allowed to limit the current during the converter off-time, when inductor current is falling. Therefore in a typical current limit plot the valleys are normally well defined, but the peaks are variable, according to the duty cycle. The PWM error amplifier and comparator control the off-pulse of the high-side MOSFET, even during current limit mode, meaning that peak inductor current can exceed the current limit threshold. Assuming that the output inductor does not saturate, the maximum peak inductor current during current limit mode can be calculated with the following equation:

Equation 11.

Where T_SW is the inverse of switching frequency f_SW. The 200 ns term represents the minimum off-time of the duty cycle, which ensures enough time for correct operation of the current sensing circuitry.

In order to minimize the time period in which peak inductor current exceeds the current limit threshold, the IC also discharges the soft-start capacitor through a fixed 90-µA sink. The output of the LM2743 internal error amplifier is limited by the voltage on the soft-start capacitor. Hence, discharging the soft-start capacitor reduces the maximum duty cycle D of the controller. During severe current limit this reduction in duty cycle will reduce the output voltage if the current limit conditions last for an extended time. Output inductor current will be reduced in turn to a flat level equal to the current limit threshold. The third benefit of the soft-start capacitor discharge is a smooth, controlled ramp of output voltage when the current limit condition is cleared.