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LM2733

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SNVS209F – NOVEMBER 2002 – REVISED DECEMBER 2014

8.2.2.10 Maximum Switch Current

The maximum FET swtch 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 both the typical and specified values of switch

current for both the "X" and "Y" versions as a function of effective (actual) duty cycle:

Figure 26. Switch Current Limit vs Duty Cycle - "X"

Figure 27. Switch Current Limit vs Duty Cycle - "Y"

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, we can develop an expression 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 LM2733 device and displayed the

maximum load current available for a typical device in graph form:

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