Datasheet

I
PK-CL
= I
LIM
+ (T
SW
- 200 ns)
V
IN
- V
O
L
I
LIM
I
L
D
Normal Operation Current Limit
LM2745, LM2748
www.ti.com SNOSAL2E –APRIL 2005–REVISED APRIL 2013
For example, a conservative 15A current limit in a 10A design with a R
DSON-HOT
of 10 mΩ would require a 6 kΩ
resistor. The minimum value for R
CS
in any application is 1 kΩ. Because current sensing is done across the low-
side MOSFET, no minimum high-side on-time is necessary. The LM2745/8 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 LM2745/8 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:
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 LM2745/8 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.
SHUTDOWN
If the shutdown pin is pulled low, (below 0.8V) the LM2745/8 enters shutdown mode, and discharges the soft-
start capacitor through a MOSFET switch. The high and low-side MOSFETs are turned off. The LM2745/8
remains in this state as long as V
SD
sees a logic low (see the Electrical Characteristics table). To assure proper
IC start-up the shutdown pin should not be left floating. For normal operation this pin should be connected
directly to V
CC
or to another voltage between 1.3V to V
CC
(see the Electrical Characteristics table).
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