LTC3549 datasheet(11/16 Pages) LINER

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LTC3549

3549f

LOAD CURRENT (mA)

0.1000

1.0000

0.0100

0.0010

0.1

100

1000

3549 F02

0.0001

2.5V

3.6V

4.2V

VIN BURST

2.5V

3.6V

4.2V

VIN PULSE SKIP VOUT = 1.8V

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 percent-

age of input power.

Although all dissipative elements in the circuit produce

losses, two main sources usually account for most of

the losses in LTC3549 circuits: VIN quiescent current and

I2R losses. The VIN quiescent current loss dominates

the efﬁciency loss at very low load currents whereas the

I2R 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, as illustrated

in Figure 2.

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 of the internal power

MOSFET switches. Each time the gate is switched from

high to low to high again, a packet of charge, dQ, moves

from VIN to ground. The resulting dQ/dt is the current out

of VIN that is typically larger than the DC bias current. In

continuous mode, IGATECHG = 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. I2R losses are 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 SW pin is a function of both

top and bottom MOSFET RDS(ON) and the duty cycle

(DC) as follows:

RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)

(1 – DC)

The RDS(ON)forboththetopandbottomMOSFETscanbe

obtained from the Typical Performance Characteristics.

Thus, to obtain I2R losses, simply add RSW to RL and

multiply the result by the square of the average output

current.

Other losses including CIN and COUT ESR dissipative los-

ses and inductor core losses generally account for less

than 2% total additional loss.

APPLICATIO S I FOR ATIO

Figure 2. Power Loss vs Load Current

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