Datasheet

I
LOAD
(max) = (1 - DC) x (I
SW
(max) - DC (V
IN
- V
SW
))
2fL
LM2733
www.ti.com 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:
I
LOAD
= I
IND
(AVG) x (1 - DC) (7)
Where "DC" is the duty cycle of the application. The switch current can be found by:
I
SW
= I
IND
(AVG) + ½ (I
RIPPLE
) (8)
Inductor ripple current is dependent on inductance, duty cycle, input voltage and frequency:
I
RIPPLE
= DC x (V
IN
-V
SW
) / (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 V
IN
- "X"
Figure 31. Max. Load Current vs V
IN
- "Y"
DESIGN PARAMETERS V
SW
AND I
SW
The value of the FET "ON" voltage (referred to as V
SW
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 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.
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