MAXIMUM SWITCH CURRENT

The maximum FET swtch current available before the cur-

rent limiter cuts in is dependent on duty cycle of the appli-

cation. This is illustrated in the graphs below which show

both the typical and guaranteed values of switch current for

both the "X" and "Y" versions as a function of effective

(actual) duty cycle:

20055425

Switch Current Limit vs Duty Cycle - "X"

20055426

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:

I

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

current can be found by:

I

1

Inductor ripple current is dependent on inductance, duty

cycle, input voltage and frequency:

I

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 LM2733 and displayed the maximum load

current available for a typical device in graph form:

20055434

Max. Load Current vs V

20055433

Max. Load Current vs V

DESIGN PARAMETERS V

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

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

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

range (see Typical performance Characteristics curves).

Above V

to 5V.

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