10

LTC3406

LTC3406-1.5/LTC3406-1.8

3406fa

APPLICATIO S I FOR ATIO

Although all dissipative elements in the circuit produce

losses, two main sources usually account for most of the

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 4.

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

(DC) as follows:

be obtained from the Typical Performance Charateristics

output current.

losses and inductor core losses generally account for less

than 2% total additional loss.

Thermal Considerations

In most applications the LTC3406 does not dissipate

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

where the LTC3406 is running at high ambient tempera-

ture with low supply voltage and high duty cycles, such

as in dropout, the heat dissipated may exceed the maxi-

mum 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.

To avoid the LTC3406 from exceeding the maximum

junction temperature, the user will need to do some

thermal analysis. The goal of the thermal analysis is to

determine whether the power dissipated exceeds the

maximum junction temperature of the part. The tempera-

ture rise is given by:

is the thermal resistance from the junction of the die to the

ambient temperature.

Figure 4. Power Lost vs Load Current

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

internal top and bottom switches. Both the DC bias and

their effects will be more pronounced at higher supply

voltages.

LOAD CURRENT (mA)

0.1

1

0.00001

0.001

1

10

100

1000

3406 F04

0.0001

0.01

0.1