9

LTC3405A-1.375

3405a1375f

LOAD CURRENT (mA)

0.1

10

1000

1

0.1

0.01

0.001

0.0001

3405A1375 F02

1

100

APPLICATIO S I FOR ATIO

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

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 percentage

of input power.

Although all dissipative elements in the circuit produce

losses, two main sources usually account for most of the

the efficiency loss at very low load currents whereas the

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.

Figure 2. Power Lost vs Load Current

internal top and bottom switches. Both the DC bias and

their effects will be more pronounced at higher supply

voltages.

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-1.375 does not

dissipate much heat due to its 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-1.375 from exceeding the maxi-

mum 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: