LM2743MTCSLASHNOPB datasheet(17/44 Pages) TI2

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IPK-CL = ILIM + (TSW - 200 ns)

VIN - VO

ILIM

Normal Operation

Current Limit

VIN ± 9.5V

RCS

10 mA

LM2743

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SNVS276I – APRIL 2004 – REVISED FEBRUARY 2019

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Feature Description (continued)

For example, a conservative 15-A current limit in a 10-A design with a minimum RDS(ON) of 10 mΩ would require

a 6-k

Ω resistor. To prevent the ISEN pin from sinking too much current when the switch node goes above 9.5 V,

the value of the current limit setting resistor RCS should not be too low. The criterion is as follows,

(10)

where the 10 mA is the maximum current ISEN pin is allowed to sink. For example if VIN = 13.2 V, the minimum

value of RCS 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:

(11)

Where TSW is the inverse of switching frequency fSW. 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.

LM2743MTC/NOPB Datasheet(PDF) 17 Page - Texas Instruments