LM2743MTCX/NOPB National Semiconductor, LM2743MTCX/NOPB Datasheet - Page 14

• Manufacturer Part Number: LM2743MTCX/NOPB
• Description: IC REG CTLR BUCK N-CH 14-TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: Step-Down (Buck)r

Specifications of LM2743MTCX/NOPB

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 13.5 V
• Current - Output: 20A
• Frequency - Switching: 50kHz ~ 1MHz
• Voltage - Input: 1 ~ 16 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 14-TSSOP
• For Use With: LM2743EVAL - BOARD EVALUATION LM2743LM2743-19AEVAL - BOARD EVALUATION LM2743-19A
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: LM2743MTCX

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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.
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 thresh-
old. 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
ns term represents the minimum off-time of the duty cycle,
which ensures enough time for correct operation of the cur-
rent sensing circuitry.
In order to minimize the time period in which peak inductor
current exceeds the current limit threshold, the IC also dis-
charges 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 condi-
tions last for an extended time. Output inductor current will be
reduced in turn to a flat level equal to the current limit thresh-
old. The third benefit of the soft-start capacitor discharge is a
SW
FIGURE 10. Current Limit Threshold
is the inverse of switching frequency f
SW
. The 200
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14
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
considered, see
rent limit foldback (P
where P is a ratio between 0 and 1.
Obtain the R
where I
following criterion must be satisfied:
The equation for calculating the foldback resistance value is:
SHUTDOWN
If the shutdown pin is pulled low, (below 0.8V) the LM2743
enters shutdown mode, and discharges the soft-start capac-
itor through a MOSFET switch. The high and low-side MOS-
FETs are turned off. The LM2743 remains in this state as long
as V
ble). 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
to V
DESIGN CONSIDERATIONS
The following is a design procedure for all the components
needed to create the Typical Application Circuit shown on the
front page. This design converts 3.3V (V
a maximum load of 4A with an efficiency of 89% and a switch-
ing frequency of 300 kHz. The same procedures can be
CC
SD
(see the Electrical Characteristics table).
FIGURE 11. Foldback Current Limit Circuit
sees a logic low (see the Electrical Characteristics ta-
SEN
= 40 μA. If the switch node goes above 9.5V the
CS
with the following equation:
Figure
LIM
CC
P
):
11. First select the percentage of cur-
LIM
or to another voltage between 1.3V
= I
LIM
x P
IN
) to 1.2V (V
CLF
) should be
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OUT
) at