LM2831XMF datasheet(14/27 Pages) NSC

14 / 27 page

Calculating Efficiency, and

Junction Temperature

The complete LM2831 DC/DC converter efficiency can be

calculated in the following manner.

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):

SW is the voltage drop across the internal PFET when it is

on, and is equal to:

SW =IOUT xRDSON

D is the forward voltage drop across the Schottky catch

diode. It can be obtained from the diode manufactures Elec-

trical Characteristics section. If the voltage drop across the

inductor (V

DCR) is accounted for, the equation becomes:

The conduction losses in the free-wheeling Schottky diode

are calculated as follows:

DIODE =VD xIOUT x (1-D)

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.

Another significant external power loss is the conduction

loss in the output inductor. The equation can be simplified to:

IND =IOUT

2 xR

DCR

The LM2831 conduction loss is mainly associated with the

internal PFET:

If the inductor ripple current is fairly small, the conduction

losses can be simplified to:

COND =IOUT

2 xR

DSON xD

Switching losses are also associated with the internal PFET.

They occur during the switch on and off transition periods,

where voltages and currents overlap resulting in power loss.

The simplest means to determine this loss is to empirically

measuring the rise and fall times (10% to 90%) of the switch

at the switch node.

Switching Power Loss is calculated as follows:

SWR = 1/2(VIN xIOUT xFSW xTRISE)

SWF = 1/2(VIN xIOUT xFSW xTFALL)

SW =PSWR +PSWF

Another loss is the power required for operation of the inter-

nal circuitry:

Q =IQ xVIN

Q is the quiescent operating current, and is typically around

2.5mA for the 0.55MHz frequency option.

Typical Application power losses are:

Power Loss Tabulation

5.0V

OUT

3.3V

OUT

4.125W

OUT

1.25A

0.45V

DIODE

188mW

550kHz

2.5mA

12.5mW

RISE

4nS

SWR

7mW

FALL

4nS

SWF

7mW

DS(ON)

150m

Ω

COND

156mW

IND

DCR

70m

Ω

IND

110mW

0.667

LOSS

481mW

88%

INTERNAL

183mW

ΣP

COND +PSW +PDIODE +PIND +PQ =PLOSS

ΣP

COND +PSWF +PSWR +PQ =PINTERNAL

INTERNAL = 183mW

Thermal Definitions

J = Chip junction temperature

A = Ambient temperature

RθJC = Thermal resistance from chip junction to device case

RθJA = Thermal resistance from chip junction to ambient air

Heat in the LM2831 due to internal power dissipation is

removed through conduction and/or convection.

Conduction: Heat transfer occurs through cross sectional

areas of material. Depending on the material, the transfer of

heat can be considered to have poor to good thermal con-

ductivity properties (insulator vs. conductor).

Heat Transfer goes as:

Silicon

→ package → lead frame → PCB

Convection: Heat transfer is by means of airflow. This could

be from a fan or natural convection. Natural convection

occurs when air currents rise from the hot device to cooler

air.

Thermal impedance is defined as:

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