LP78083 datasheet(6/8 Pages) POWER

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Preliminary Datasheet

LP78083

LP78083 – Ver. 1.0 Datasheet

Feb.-2008

Page 6 of 8

Efficiency considerations

The efficiency of a switching regulator is equal to the output

Power divided by the input power times 100%.it is often useful

to analyze individual losses to determine what is limiting the

efficiency and which change would produce the most

improvement efficiency can be expressed as :

Efficiency= 100%- (L1+L2+L3…)

Where L1、L2, etc. are the individual losses as a percentage of

Input power .although all dissipative elements in the for most

of losses: VIN quiescent current and 12R loss dominates the

efficiency loss at medium to high load currents. In a typical

efficiency plot, the efficiency curve at very low load currents

can be misleading since the actual power lost is of no

consequence.

1.The VIN quiescent current is due to two components:

the DC Bias current as given in the electrical

characteristics and the Internal main switch and

synchronous switch gate charge currents. the gate

charge current results from switching the gate

capacitance

the

internal

power

MOSFET

switches .Each time the gate charge current.results

from switching the gate capacitance of the internal

power MOSFET switches. Each time the gate is

switches from high to low to high again, a packet of

charge △Q moves from VIN to ground.

The resulting △Q/△t is the current out of VIN that is typically

larger than the DC bias current. In continuous mode.

LGATCHG=f(QT+QB)

Where QT and QB are the gate charges of the internal top and

bottom switches. Both the DC bias and gate charge losses are

proportional to VIN and thus their effects will be more

pronounced at higher supply voltages.

2. 12Rlosses tae calculated from the resistances of the internal

switches, RSW and external inductor RL. in continuous mode

the average output current flowing through inductor L is

“chopped” between the main switch and the synchronous

switch. Thus, the series resistance looking into the LX pin is a

function of both top and bottom MOSFER RDS(ON) and the duty

cycle (DC) as follows:

RSW=RDS(ON)TOP×DC+RDS(ON)BOT×(1-DC)

The RDS(ON) for both the top and bottom MOSFETS can be

obtained from the typical performance characteristics curves.

thus, to obtain 12R losses, simply add RSW to RL and multiply

the square of the average output current.

Other losses including CIN and COUT ESR dissipative

losses and inductor core losses generally account for

less than 2% of the total loss.

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