LTC3413 datasheet(10/16 Pages) LINER

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LTC3413

sn3413 3413fs

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 VIN that 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 con-

tinuous 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 Characteristics

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% of the total loss.

Thermal Considerations

In most applications, the LTC3413 does not dissipate

much heat due to its high efficiency.

But, in applications where the LTC3413 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.

To avoid the LTC3413 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:

TR = (PD)(θJA)

where PD is the power dissipated by the regulator and θJA

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

ambient temperature.

The junction temperature, TJ, is given by:

TJ = TA + TR

where TA is the ambient temperature.

As an example, consider the LTC3413 in dropout at an

input voltage of 3.3V, a load current of 3A and an ambient

temperature of 70

°C. From the Typical Performance graph

of switch resistance, the RDS(ON) of the P-channel switch

at 70

°C is approximately 97mΩ. Therefore, power dissi-

pated by the part is:

PD = (ILOAD2)(RDS(ON)) = (3A)2(97mΩ) = 0.87W

For the TSSOP package, the

θJA is 38°C/W. Thus the

junction temperature of the regulator is:

TJ = 70°C + (0.87W)(38°C/W) = 103°C

which is below the maximum junction temperature of

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