LM2744 datasheet(13/22 Pages) NSC

13 / 22 page

Application Information (Continued)

POWER GOOD SIGNAL

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 or falls 28% below that value 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 regu-

lation, however the switching of the LM2744 continues re-

gardless 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 V

REF.

UVLO

The 2.76V turn-on threshold on V

CC has a built in hysteresis

of about 300mV. If V

CC drops below 2.42V, the chip enters

UVLO mode. UVLO consists of turning off the top and bot-

tom MOSFETS and remaining in that condition until V

rises 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

DSON of the MOSFET

is a known value; hence the current through the MOSFET

can be determined as:

DS =IOUT *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, R

CS,

connected between the switching node and the I

SEN pin. A

constant current of 40 µA is forced through R

CS, causing a

fixed voltage drop. This fixed voltage is compared against

DS and if the latter is higher, the current limit of the chip has

been reached. R

CS can be found by using the following

equation:

CS =RDSON xILIM /40µA

For example, a conservative 15A current limit in a 10A

design with a minimum R

DSON of 10m

Ω would require a

3.74k

Ω resistor. Because current sensing is done across the

low-side MOSFET, no minimum high-side on-time is neces-

sary. The LM2744 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 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 opera-

tion.

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:

where T

OSC is the inverse of switching frequency FSW. The

200ns 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 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

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 dis-

charge 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 capaci-

tor through a MOSFET switch. The high and low-side MOS-

FETs are turned off. The LM2744 remains in this state as

long as V

SD sees a logic low (see the Electrical Character-

istics table). To assure proper IC start-up the shutdown pin

20106088

FIGURE 10. Current Limit Threshold

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