LTM8048 datasheet(16/22 Pages) LINER

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LTM8045

8045fa

For more information www.linear.com/8045

APPLICATIONS INFORMATION

Hot-Plugging Safely

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of the LTM8045. However, these capaci-

tors can cause problems if the LTM8045 is plugged into a

live input supply (see Application Note 88 for a complete

discussion).Thelowlossceramiccapacitorcombinedwith

stray inductance in series with the power source forms an

underdamped tank circuit, and the voltage at the VIN pin

of the LTM8045 can ring to more than twice the nominal

input voltage, possibly exceeding the LTM8045’s rating

and damaging the part. If the input supply is poorly con-

trolled or the user will be plugging the LTM8045 into an

energized supply, the input network should be designed

to prevent this overshoot. This can be accomplished by

installing a small resistor in series with VIN, but the most

popular method of controlling input voltage overshoot is

to add an electrolytic bulk capacitor to the VIN net. This

capacitor’s relatively high equivalent series resistance

damps the circuit and eliminates the voltage overshoot.

The extra capacitor improves low frequency ripple filter-

ing and can slightly improve the efficiency of the circuit,

though it is physically large.

Thermal Considerations

The LTM8045 output current may need to be derated if

it is required to operate in a high ambient temperature or

deliver a large amount of continuous power. The amount

of current derating is dependent upon the input voltage,

output power and ambient temperature. The temperature

rise curves given in the Typical Performance Character-

istics section can be used as a guide. These curves were

generated by a LTM8045 mounted to a 25.8cm2 4-layer

FR4 printed circuit board with a copper thickness of 2oz

for the top and bottom layer and 1oz for the inner layers.

Boards of other sizes and layer count can exhibit differ-

ent thermal behavior, so it is incumbent upon the user to

verify proper operation over the intended system’s line,

load and environmental operating conditions.

ThethermalresistancenumberslistedinthePinConfigura-

tion section of the data sheet are based on modeling the

µModule package mounted on a test board specified per

JESD 51-9 (“Test Boards for Area Array Surface Mount

PackageThermalMeasurements”).Thethermalcoefficients

provided in this page are based on JESD 51-12 (“Guide-

lines for Reporting and Using Electronic Package Thermal

Information”).

Forincreasedaccuracyandfidelitytotheactualapplication,

many designers use FEA to predict thermal performance.

To that end, the Pin Configuration section of the data sheet

typically gives four thermal coefficients:

•

θJA – Thermal resistance from junction to ambient

•

θJCbottom – Thermal resistance from junction to the

bottom of the product case

•

θJCtop – Thermal resistance from junction to top of the

product case

•

θJB – Thermal resistance from junction to the printed

circuit board.

While the meaning of each of these coefficients may seem

to be intuitive, JEDEC has defined each to avoid confusion

and inconsistency. These definitions are given in JESD

51-12, and are quoted or paraphrased below:

•

θJA is the natural convection junction-to-ambient air

thermal resistance measured in a one cubic foot sealed

enclosure.Thisenvironmentissometimesreferredtoas

“still air” although natural convection causes the air to

move.Thisvalueisdeterminedwiththepartmountedto

a JESD 51-9 defined test board, which does not reflect

an actual application or viable operating condition.

•

θJCbottomisthethermalresistancebetweenthejunction

and bottom of the package with all of the component

power dissipation flowing through the bottom of the

package. In the typical µModule converter, the bulk of

the heat flows out the bottom of the package, but there

is always heat flow out into the ambient environment.

As a result, this thermal resistance value may be useful

for comparing packages but the test conditions don’t

generally match the user’s application.

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