LTM8053 datasheet(18/24 Pages) LINER

18 / 24 page

LTM8063

8063fa

For more information www.linear.com/LTM8063

APPLICATIONS INFORMATION

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 LTM8063.

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 5. The LTM8063 can benefit from the heat-sinking

afforded by vias that connect to internal 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.

Figure 5.

COUT

VOUT

FB SYNC

GND

VIN

TR/SS

GND/THERMAL VIAS

CIN

RUN

8063 F05

Layout Showing Suggested External

Components, GND Plane and Thermal Vias

Hot-Plugging Safely

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of LTM8063. However, these capacitors

can cause problems if the LTM8063 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 LTM8063 can ring to more than

twice the nominal input voltage, possibly exceeding the

LTM8063’sratinganddamagingthepart.Iftheinputsupply

is poorly controlled or the LTM8063 is hot-plugged 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

add an electrolytic bulk cap to the VIN net. This capacitor’s

relativelyhighequivalentseriesresistancedampsthecircuit

and eliminates the voltage overshoot. The extra capacitor

improves low frequency ripple filtering and can slightly

improve the efficiency of the circuit, though it is likely to

be the largest component in the circuit.

Thermal Considerations

The LTM8063 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

derating curves given in the Typical Performance Char-

acteristics section can be used as a guide. These curves

were generated by the LTM8063 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.

For increased accuracy and fidelity to the actual applica-

tion, many designers use FEA (Finite Element Analysis)

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.

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