lm2735xsdx National Semiconductor Corporation, lm2735xsdx Datasheet - Page 15

• Manufacturer Part Number: lm2735xsdx
• Description: 520khz/1.6mhz - Space-efficient Boost And Sepic Dc-dc Regulator
• Manufacturer: National Semiconductor Corporation

Calculating Efficiency, and Junction
Temperature
The complete LM2735 DC/DC converter efficiency (η) can be
calculated in the following manner.
Power loss (P
converter, switching and conduction. Conduction losses usu-
ally dominate at higher output loads, where as switching
losses remain relatively fixed and dominate at lower output
loads.
Losses in the LM2735 Device: P
Conversion ratio of the Boost Converter with conduction loss
elements inserted:
One can see that if the loss elements are reduced to zero, the
conversion ratio simplifies to:
And we know:
Therefore:
Calculations for determining the most significant power loss-
es are discussed below. Other losses totaling less than 2%
are not discussed.
A simple efficiency calculation that takes into account the
conduction losses is shown below:
LOSS
) is the sum of two types of losses in the
LOSS
= P
COND
+ P
SW
+ P
Q
15
The diode, NMOS switch, and inductor DCR losses are in-
cluded in this calculation. Setting any loss element to zero will
simplify the equation.
V
can be obtained from the manufacturer’s Electrical Charac-
teristics section of the data sheet.
The conduction losses in the diode are calculated as follows:
Depending on the duty cycle, this can be the single most sig-
nificant power loss in the circuit. Care should be taken to
choose a diode that has a low forward voltage drop. Another
concern with diode selection is reverse leakage current. De-
pending on the ambient temperature and the reverse voltage
across the diode, the current being drawn from the output to
the NMOS switch during time D could be significant, this may
increase losses internal to the LM2735 and reduce the overall
efficiency of the application. Refer to Schottky diode
manufacturer’s data sheets for reverse leakage specifica-
tions, and typical applications within this data sheet for diode
selections.
Another significant external power loss is the conduction loss
in the input inductor. The power loss within the inductor can
be simplified to:
The LM2735 conduction loss is mainly associated with the
internal NFET:
D
is the forward voltage drop across the Schottky diode. It
FIGURE 13. LM2735 Switch Current
P
COND-NFET
P
(small ripple approximation)
COND-NFET
P
P
DIODE
IND
= I
= I
= I
2
SW-rms
= V
IN
IN
2
2
R
D
x R
DCR
x I
x R
DSON
O
DSON
x D
x D
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