IN

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(1 DC)

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2fL

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LM2731

SNVS217G – MAY 2004 – REVISED SEPTEMBER 2015

www.ti.com

8.2.2.10 Maximum Switch Current

The maximum FET switch current available before the current limiter cuts in is dependent on duty cycle of the

application. This is illustrated in the graphs below which show typical values of switch current for both the "X" and

"Y" versions as a function of effective (actual) duty cycle:

Figure 28. Switch Current Limit vs Duty Cycle - X Option

Figure 29. Switch Current Limit vs Duty Cycle - Y Option

8.2.2.11 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:

(7)

Where "DC" is the duty cycle of the application. The switch current can be found by:

(8)

Inductor ripple current is dependent on inductance, duty cycle, input voltage and frequency:

(9)

Combining all terms, an expression can be developed which allows the maximum available load current to be

calculated:

(10)

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

various 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:

16

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