LM2744EVAL National Semiconductor, LM2744EVAL Datasheet - Page 13

• Manufacturer Part Number: LM2744EVAL
• Description: BOARD EVALUATION LM2744
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM2744EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1.2V
• Current - Output: 3.5A
• Voltage - Input: 1.8 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 1MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM2744
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

circuits that the output voltage has fallen out of regulation,
however the switching of the LM2744 continues regardless of
the state of the Power Good signal. The Power Good flag will
return to logic high whenever the feedback pin voltage is be-
tween 72% and 118% of V
UVLO
The 2.76V turn-on threshold on V
of about 300mV. If V
UVLO mode. UVLO consists of turning off the top and bottom
MOSFETS and remaining in that condition until V
above 2.76V. As with shutdown, the soft-start capacitor is
discharged through an internal MOSFET, ensuring that the
next start-up will be controlled by the soft-start circuitry.
CURRENT LIMIT
Current limit is realized by sensing the voltage across the low-
side MOSFET while it is on. The R
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
ed between the switching node and the I
current of 40 µA is forced through R
drop. This fixed voltage is compared against V
latter is higher, the current limit of the chip has been reached.
R
For example, a conservative 15A current limit in a 10A design
with a minimum R
sistor. Because current sensing is done across the low-side
MOSFET, no minimum high-side on-time is necessary. The
LM2744 enters current limit mode if the inductor current ex-
ceeds 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 10). 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
LM2744 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.
CS
can be found by using the following equation:
R
DSON
CS
V
CC
= R
DS
of 10mΩ would require a 3.74kΩ re-
drops below 2.42V, the chip enters
= I
DSON
REF
OUT
.
x I
* R
LIM
CC
CS
DSON
DSON
/ 40 µA
has a built in hysteresis
, causing a fixed voltage
of the MOSFET is a
SEN
pin. A constant
DS
CS
, connect-
and if the
CC
rises
13
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
200ns 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 LM2744 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
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 LM2744
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 LM2744 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
SD
sees a logic low (see the Electrical Characteristics ta-
OSC
FIGURE 10. Current Limit Threshold
is the inverse of switching frequency F
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