LTC3603 datasheet(14/22 Pages) LINER

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LTC3603

3603fa

APPLICATIONS INFORMATION

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: VIN operating current and I2R losses.

The VINoperatingcurrentlossdominatestheefﬁciencyloss

at very low load currents whereas the I2R loss dominates

the efﬁciency loss at medium to high load currents.

1. The VIN operating current comprises three components:

The DC supply current as given in the electrical char-

acteristics, the internal MOSFET gate charge currents

and the internal topside MOSFET transition losses. The

MOSFET gate charge current results from switching the

gate capacitance of the internal power MOSFET switches.

The gates of these switches are driven from the INTVCC

supply. Each time the gate is switched from high to

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

INTVCC to ground. The resulting dQ/dt is the current

out of INTVCC that is typically larger than the DC bias

current. In continuous mode, the gate charge current

can be approximated by IGATECHG = f(9.5nC). Since the

INTVCC voltage is generated from VIN by a linear regula-

tor, the current that is internally drawn from the INTVCC

supply can be treated as VIN current for the purposes

of efﬁciency considerations.

Transition losses apply only to the internal topside

MOSFET and become more prominent at higher input

voltages. Transition losses can be estimated from:

Transition Loss = (1.7) VIN2 • IO(MAX) • (120pF) • f

2. I2R losses are calculated from the resistances of the

internal switches, RSW and external inductor RL. In

continuous mode, the average output current ﬂow-

ing 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) for both the top and bottom MOSFETs can

be obtained from the Typical Performance Characteristics

curves. Thus, to obtain I2R losses, simply add RSW to

RL and multiply the result by the square of the average

output current:

I2R Loss = IO2(RSW + RL)

Other losses, including CIN and COUT ESR dissipative

losses and inductor core losses, generally account for

less than 2% of the total power loss.

Thermal Considerations

In most applications, the LTC3603 does not dissipate much

heat due to its high efﬁciency. But, in applications where the

LTC3603 is running at high ambient temperature with low

supply voltage and high duty cycles, such as in dropout,

the heat dissipated may exceed the maximum junction

temperature of the part. If the junction temperature reaches

approximately 150°C, both power switches will be turned

off and the SW node will become high impedance.

TIME

Figure 5b. Ratiometric Tracking

VOUT

TIME

3603 F05b,c

Figure 5c. Coincident Tracking

VOUT

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