LM2731XMF datasheet(14/16 Pages) NSC

14 / 16 page

Application Hints (Continued)

20059150

Switch Current Limit vs Duty Cycle - "X"

20059151

Switch Current Limit vs Duty Cycle - "Y"

CALCULATING LOAD CURRENT

As shown in the figure which depicts inductor current, the

load current is related to the average inductor current by the

relation:

LOAD =IIND(AVG) x (1 - DC)

Where "DC" is the duty cycle of the application. The switch

current can be found by:

SW =IIND(AVG) +

⁄2 (I

RIPPLE)

Inductor ripple current is dependent on inductance, duty

cycle, input voltage and frequency:

RIPPLE =DCx(VIN-VSW)/(fxL)

combining all terms, we can develop an expression which

allows the maximum available load current to be calculated:

The equation shown to calculate maximum load current

takes into account the losses in the inductor or turn-OFF

switching losses of the FET and diode. For actual load

current in typical applications, we took bench data for vari-

ous input and output voltages for both the "X" and "Y"

versions of the LM2731 and displayed the maximum load

current available for a typical device in graph form:

20059148

Max. Load Current (typ) vs V

IN - "X"

20059149

Max. Load Current (typ) vs V

IN - "Y"

DESIGN PARAMETERS V

SW AND ISW

The value of the FET "ON" voltage (referred to as V

SW in the

equations) is dependent on load current. A good approxima-

tion can be obtained by multiplying the "ON Resistance" of

the FET times the average inductor current.

FET on resistance increases at V

IN values below 5V, since

the internal N-FET has less gate voltage in this input voltage

range (see Typical performance Characteristics curves).

Above V

IN = 5V, the FET gate voltage is internally clamped

to 5V.

The maximum peak switch current the device can deliver is

dependent on duty cycle. For higher duty cycles, see Typical

performance Characteristics curves.

THERMAL CONSIDERATIONS

At higher duty cycles, the increased ON time of the FET

means the maximum output current will be determined by

power dissipation within the LM2731 FET switch. The switch

power dissipation from ON-state conduction is calculated by:

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