on. (The point of peak inductor current, see Figure 12). Note
that in normal operation mode the high-side MOSFET al-
ways 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 LM2747 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 opera-
tion.
Application Information (Continued)
POWER GOOD SIGNAL
The open drain output on the Power Good pin needs a
pull-up resistor to a low voltage source. The pull-up resistor
should be chosen so that the current going into the Power
Good pin is less than 1 mA. A 100 kΩ resistor is recom-
mended for most applications.
The Power Good signal is an OR-gated flag which takes into
account both output overvoltage and undervoltage condi-
tions. If the feedback pin (FB) voltage is 18% above its
nominal value (118% x VFB = 0.708V) or falls 28% below that
value (72% x VFB = 0.42V) the Power Good flag goes low.
The Power Good flag can be used to signal other circuits that
the output voltage has fallen out of regulation, however the
switching of the LM2747 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 between
72% and 118% of 0.6V.
UVLO
The 2.79V turn-on threshold on VCC has a built in hysteresis
of about 300 mV. If VCC drops below 2.42V, the chip defi-
nitely enters UVLO mode. UVLO consists of turning off the
top and bottom MOSFETS and remaining in that condition
until VCC rises above 2.79V. As with normal shutdown initi-
ated by the SD pin, 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 RDSON of the MOSFET
is a known value; hence the current through the MOSFET
can be determined as:
20150988
FIGURE 12. 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 typi-
cal 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:
VDS = IOUT x RDSON
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, RCS
,
connected between the switching node and the ISEN pin. A
constant current (ISEN-TH) of 40 µA typical is forced through
RCS, causing a fixed voltage drop. This fixed voltage is
compared against VDS and if the latter is higher, the current
limit of the chip has been reached. To obtain a more accurate
value for RCS you must consider the operating values of
RDSON and ISEN-TH at their operating temperatures in your
application and the effect of slight parameter differences
from part to part. RCS can be found by using the following
equation using the RDSON value of the low side MOSFET at
it’s expected hot temperature and the absolute minimum
value expected over the full temperature range for the for the
ISEN-TH which is 25 µA:
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.
RCS = RDSON-HOT x ILIM / ISEN-TH
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 LM2747 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
For example, a conservative 15A current limit in a 10A
design with a RDSON-HOT of 10 mΩ would require a 6 kΩ
resistor. The minimum value for RCS 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
LM2747 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
13
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