CB2012T2R2M datasheet(10/16 Pages) LINER

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LTC3410

3410fb

APPLICATIO S I FOR ATIO

where L1, L2, etc. are the individual losses as a percentage

of input power.

Although all dissipative elements in the circuit produce

losses, two main sources usually account for most of the

losses in LTC3410 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 3.

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

the DC bias current as given in the electrical character-

istics 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 VINthat 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) for both the top and bottom MOSFETs can

be obtained from the Typical Performance Charateristics

curves. 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

losses and inductor core losses generally account for less

than 2% total additional loss.

Thermal Considerations

In most applications the LTC3410 does not dissipate

much heat due to its high efficiency. But, in applications

where the LTC3410 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

Figure 3. Power Loss vs Load Current

LOAD CURRENT (mA)

0.1

0.00001

0.001

100

1000

3410 F03

0.0001

0.01

0.1

VIN = 3.6V

VOUT = 3.3V

VOUT = 1.8V

VOUT = 1.2V

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