Datasheet

I
PK-CL
= I
LIM
+ (T
SW
- 200 ns)
V
IN
- V
O
L
I
LIM
I
L
D
Normal Operation Current Limit
LM2743
www.ti.com SNVS276G –APRIL 2004–REVISED MARCH 2013
Figure 32. 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:
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.
FOLDBACK CURRENT LIMIT
In the case where extra protection is used to help an output short condition, a current foldback resistor (R
CLF
)
should be considered, see Figure 33. First select the percentage of current limit foldback (P
LIM
):
P
LIM
= I
LIM
x P
where P is a ratio between 0 and 1.
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