LM2746MXAX datasheet(14/24 Pages) NSC

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Application Information (Continued)

as inductor current has fallen to the current limit threshold.

The LM2746 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

SW 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 LM2746 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 LM2746

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 LM2746 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

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).

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

IN) to 1.2V

OUT) at a maximum load of 4A with an efficiency of 89%

and a switching frequency of 300kHz. The same procedures

can be followed to create many other designs with varying

input voltages, output voltages, and load currents.

Duty Cycle Calculation

The complete duty cycle for a buck converter is defined with

the following equation:

where V

SWL and VSWH are the respective forward voltage

drops that develop across the low side and high side MOS-

FETs. Assuming the inductor ripple current is 20% to 30% of

the output current, therefore:

SWL =IOUT xRDS(on)LOW

(Low-Side MOSFET)

SWH =IOUT xRDS(on)HIGH

(High-Side MOSFET)

To calculate the maximum duty cycle use the estimated ’hot’

DS(on) value of the MOSFETs, the minimum input voltage,

and maximum load. As shown in Figure 11, the worst case

maximum duty cycles of the LM2746 occurs at 125˚C junc-

tion temperature vs V

CC (IC control section voltage). Ensure

that the operating duty cycle is below the curve in Figure 11,

if this condition is not satisfied, the system will be unable to

develop the required duty cycle to derive the necessary

system power and so the output voltage will fall out of

regulation.

20147788

FIGURE 10. Current Limit Threshold

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