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LTC3415

3415fa

3) I2R losses are calculated from the DC resistances of

the internal switches, RSW, and external inductor, RL.

In continuous mode, the average output current ﬂows

through inductor L but is “chopped” between the internal

top and bottom switches. 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)forboththetopandbottomMOSFETscanbe

obtained from the Typical Performance Characteristics

curves. Thus, to obtain I2R losses:

I2R losses = IOUT2(RSW + RL)

4) Other “hidden” losses such as copper trace and in-

ternal battery resistances can account for additional

efﬁciency degradations in portable systems. It is very

important to include these “system” level losses in

the design of a system. The internal battery and fuse

resistance losses can be minimized by ensuring that

CIN has adequate charge storage and very low ESR at

the switching frequency. Other losses including diode

conduction losses during dead-time and inductor

core losses generally account for less than 2% total

additional loss.

Thermal Considerations

In the majority of applications, the LTC3415 does not

dissipate much heat due to its high efﬁciency. However,

in applications where the LTC3415 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 LTC3415 from exceeding the maximum junc-

tion 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 temperature rise is

given by:

TRISE = 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 = TRISE + TAMBIENT

As an example, consider the case when the LTC3415 is in

dropout at an input voltage of 3.3V with a load current of

5A. From the Typical Performance Characteristics graph

of Switch Resistance, the RDS(ON) resistance of the P-

channel switch is 0.03. Therefore, power dissipated by

the part is:

PD = I2 • RDS(ON) = 750mW

The QFN 5mm ×7mmpackagejunction-to-ambientthermal

resistance, θJA, is around 34°C/W. Therefore, the junction

temperature of the regulator operating in a 50°C ambient

temperature is approximately:

TJ = 0.75 • 34 + 50 = 75.5°C

Remembering that the above junction temperature is

obtained from an RDS(ON) at 25°C, we might recalculate

the junction temperature based on a higher RDS(ON) since

it increases with temperature. However, we can safely as-

sume that the actual junction temperature will not exceed

the absolute maximum junction temperature of 125°C.

Solder the LTC3415’s bottom exposed pad to ground for

optimal thermal performance.

APPLICATIONS INFORMATION

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