LTM8049 datasheet(13/18 Pages) LINEAR

13 / 18 page

LTM8049

8049f

For more information www.linear.com/LTM8049

APPLICATIONS INFORMATION

4. Place the CIN and COUT capacitors such that their

ground current flow directly adjacent or underneath

the LTM8049.

5. Connect all of the GND connections to as large a copper

pour or plane area as possible on the top layer. Avoid

breaking the ground connection between the external

components and the LTM8049.

6. Use vias to connect the GND copper area to the board’s

internal ground planes. Liberally distribute these GND

vias to provide both a good ground connection and

thermal path to the internal planes of the printed circuit

board. Pay attention to the location and density of the

thermal vias in Figure 2. The LTM8049 can benefit from

theheat-sinkingaffordedbyviasthatconnecttointernal

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.

Hot-Plugging Safely

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of LTM8049. However, these capacitors

can cause problems if the LTM8049 is plugged into a live

input supply (see Application Note 88 for a complete dis-

cussion). The low loss ceramic capacitor combined with

stray inductance in series with the power source forms an

underdamped tank circuit, and the voltage at the VIN pin

of the LTM8049 can ring to more than twice the nominal

inputvoltage,possiblyexceedingtheLTM8049’sratingand

damaging the part. If the input supply is poorly controlled

or the user will be plugging the LTM8049 into an energized

supply,theinputnetworkshouldbedesignedtopreventthis

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 to add an electrolytic

bulk capacitor to the VINnet.Thiscapacitor’srelativelyhigh

equivalentseriesresistancedampsthecircuitandeliminates

the voltage overshoot. The extra capacitor improves low

frequency ripple filtering and can slightly improve the ef-

ficiency of the circuit, though it is physically large.

Thermal Considerations

The LTM8049 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 LTM8049 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’sline,loadandenvironmentaloperatingconditions.

The thermal resistance numbers listed in Page 2 of the

data sheet are based on modeling the µModule package

mounted on a test board specified per JESD51-9 (Test

Boards for Area Array Surface Mount Package Thermal

Measurements). The thermal coefficients provided in this

page are based on JESD 51-12 (Guidelines for Reporting

and Using Electronic Package Thermal Information).

Forincreasedaccuracyandfidelitytotheactualapplication,

many designers use FEA to predict thermal performance.

To that end, Page 2 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. This environment is sometimes referred to as

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