10

LTC3405A-1.5/LTC3405A-1.8

sn3405a1518 3405a1518fs

APPLICATIO S I FOR ATIO

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 LTC3405A series parts do not

dissipate much heat due to their high efficiency. But, in

applications where they run at high ambient temperature

with low supply voltage, 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.

To keep the LTC3405A series parts 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.

As an example, consider the LTC3405A-1.8 with an input

voltage of 2.7V, a load current of 300mA and an ambient

temperature of 70

°C. From the typical performance graph

at 70

N-channel synchronous switch is approximately 0.75

Ω.

The series resistance looking into the SW pin is:

Therefore, power dissipated by the part is:

For the SOT-23 package, the

junction temperature of the regulator is:

which is well below the maximum junction temperature of

125

°C.

Note that at higher supply voltages, the junction tempera-

Checking Transient Response

The regulator loop response can be checked by looking at

the load transient response. Switching regulators take

several cycles to respond to a step in load current. When

equal to (

tored for overshoot or ringing that would indicate a stability

problem. For a detailed explanation of switching control

loop theory, see Application Note 76.