LM2735XMFSLASHNOPB datasheet(43/59 Pages) TI

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RDCR

IND

)

(

DCR

OUT

DSON

x R

x D

)

(

DCR

OUT

DSON

x R

x D

OUT

)

(

DCR

OUT

DSON

x R

x D

OUT

LM2735, LM2735-Q1

www.ti.com

SNVS485G – JUNE 2007 – REVISED AUGUST 2015

Thermal Considerations (continued)

Conversion ratio of the boost converter with conduction loss elements inserted:

(32)

If the loss elements are reduced to zero, the conversion ratio simplifies to:

(33)

And this is known:

(34)

Therefore:

(35)

Calculations for determining the most significant power losses 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:

(36)

The diode, NMOS switch, and inductor DCR losses are included in this calculation. Setting any loss element to

zero will simplify the equation.

VD is the forward voltage drop across the Schottky diode. It can be obtained from the manufacturer’s Electrical

Characteristics section of the data sheet.

The conduction losses in the diode are calculated as follows:

PDIODE = VD × IO

(37)

Depending on the duty cycle, this can be the single most significant 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. Depending 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 specifications, 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:

PIND = IIN

DCR

(38)

(39)

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