LTM8046 datasheet(13/20 Pages) LINER

13 / 20 page

LTM8046

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applicaTions inForMaTion

GND planes at these locations, due to their proximity

to internal power handling components. The optimum

number of thermal vias depends upon the printed

circuit board design. For example, a board might use

very small via holes. It should employ more thermal

vias than a board that uses larger holes.

The printed circuit board construction has an impact on

theisolationperformanceoftheendproduct.Forexample,

increased trace and layer spacing, as well as the choice

of core and prepreg materials (such as using polyimide

versus FR4) can significantly affect the isolation withstand

of the end product.

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 LTM8046. However, these capaci-

tors can cause problems if the LTM8046 is plugged into a

live supply (see Linear Technology Application Note 88 for

a complete discussion). The low loss ceramic capacitor

combined with stray inductance in series with the power

source forms an underdamped tank circuit, and the volt-

age at the VIN pin of the LTM8046 can ring to more than

twice the nominal input voltage, possibly exceeding the

LTM8046’s rating and damaging the part. If the input

supply is poorly controlled or the user will be plugging

the LTM8046 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

to VIN, but the most popular method of controlling input

voltage overshoot is adding an electrolytic bulk capacitor

to VIN. This capacitor’s relatively high equivalent series

resistance damps the circuit and eliminates the voltage

overshoot. The extra capacitor improves low frequency

ripplefilteringandcanslightlyimprovetheefficiencyofthe

circuit, though it can be a large component in the circuit.

Thermal Considerations

The LTM8046 output current may need to be derated if it

is required to operate in a high ambient temperature. 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

Characteristicssectioncanbeusedasaguide.Thesecurves

were generated by the LTM8046 mounted to a 58cm2

4-layer FR4 printed circuit board. Boards of other sizes

and layer count can exhibit different thermal behavior, so

it is incumbent upon the user to verify proper operation

over the intended system’s line, load and environmental

operating conditions.

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 bot-

tom 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 as follows:

θJA is the natural convection junction-to-ambient air

thermal resistance measured in a one cubic foot sealed

enclosure. This environment is sometimes referred to

as still air although natural convection causes the air to

move. This value is determined with the part mounted to a

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

actual application or viable operating condition.

θJCbottom is the junction-to-board thermal resistance with

allofthecomponentpowerdissipationflowingthroughthe

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 envi-

ronment. 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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