LTC3621/LTC3621-2

12

3621f

For more information www.linear.com/LTC3621

applicaTions inForMaTion

2. The switching current is the sum of the MOSFET driver

and control currents. The power MOSFET driver current

resultsfromswitchingthegatecapacitanceofthepower

MOSFETs. Each time a power MOSFET gate is switched

from low to high to low again, a packet of charge dQ

moves from IN to ground. The resulting dQ/dt is a cur-

rent out of IN that is typically much larger than the DC

the internal top and bottom power MOSFETs and f is

the switching frequency. The power loss is thus:

thus their effects will be more pronounced at higher

supply voltages and higher frequencies.

3. Other “hidden” losses such as transition loss and cop-

per trace and internal load resistances can account for

additional efficiency degradations in the overall power

system. It is very important to include these “system”

level losses in the design of a system. Transition loss

arises from the brief amount of time the top power

MOSFET spends in the saturated region during switch

node transitions. The LTC3621 internal power devices

switch quickly enough that these losses are not sig-

nificant compared to other sources. These losses plus

other losses, including diode conduction losses during

dead-time and inductor core losses, generally account

for less than 2% total additional loss.

Thermal Conditions

In a majority of applications, the LTC3621 does not dis-

sipate much heat due to its high efficiency and low thermal

resistance of its exposed pad package. However, in ap-

plications where the LTC3621 is running at high ambient

maximum output current load, the heat dissipated may

exceed the maximum junction temperature of the part. If

the junction temperature reaches approximately 160°C,

bothpowerswitcheswillbeturnedoffuntilthetemperature

drops about 15°C cooler.

To avoid the LTC3621 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:

As an example, consider the case when the LTC3621









=370mΩ•

1.8V

12V

+150mΩ • 1–

1.8V

12V









=183m

Ω

tion with no load is about 5mA, which includes switching

and internal biasing current loss, transition loss, inductor

core loss and other losses in the application. Therefore,

the total power dissipated by the part is:

= 243mW

TheDFN2mm

×3mmpackagejunction-to-ambientthermal

resistance,

temperature of the regulator operating in a 25°C ambient

temperature is approximately:

Remembering that the above junction temperature is

it increases with temperature. Redoing the calculation