Datasheet

LM2738
www.ti.com SNVS556B –APRIL 2008–REVISED APRIL 2013
Calculating Efficiency, and Junction Temperature
The complete LM2738 DC/DC converter efficiency can be calculated in the following manner.
(26)
Or
(27)
Calculations for determining the most significant power losses are shown below. Other losses totaling less than
2% are not discussed.
Power loss (P
LOSS
) is the sum of two basic types of losses in the converter: switching and conduction.
Conduction losses usually dominate at higher output loads, whereas switching losses remain relatively fixed and
dominate at lower output loads. The first step in determining the losses is to calculate the duty cycle (D):
(28)
V
SW
is the voltage drop across the internal NFET when it is on, and is equal to:
V
SW
= I
OUT
x R
DSON
(29)
V
D
is the forward voltage drop across the Schottky catch diode. It can be obtained from the diode manufactures
Electrical Characteristics section. If the voltage drop across the inductor (V
DCR
) is accounted for, the equation
becomes:
(30)
The conduction losses in the free-wheeling Schottky diode are calculated as follows:
P
DIODE
= V
D
x I
OUT
x (1-D) (31)
Often this is the single most significant power loss in the circuit. Care should be taken to choose a Schottky
diode that has a low forward voltage drop.
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