LM2733XMF datasheet(13/22 Pages) TI

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ILOAD(max) = (1 - DC) x (ISW(max) - DC (VIN - VSW))

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LM2733

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SNVS209E – NOVEMBER 2002 – REVISED APRIL 2013

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:

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

(7)

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

ISW = IIND(AVG) + ½ (IRIPPLE)

(8)

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

IRIPPLE = DC x (VIN-VSW) / (f x L)

(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 and displayed the maximum

load current available for a typical device in graph form:

Figure 30. Max. Load Current vs VIN - "X"

Figure 31. Max. Load Current vs VIN - "Y"

DESIGN PARAMETERS VSW AND ISW

The value of the FET "ON" voltage (referred to as VSW in the equations) is dependent on load current. A good

approximation can be obtained by multiplying the "ON Resistance" of the FET times the average inductor

current.

FET on resistance increases at VIN values below 5V, since the internal N-FET has less gate voltage in this input

voltage range (see Typical Performance Characteristics curves). Above VIN = 5V, the FET gate voltage is

internally clamped to 5V.

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