LTM8053 datasheet(19/26 Pages) LINER

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LTM8065

8065fa

For more information www.linear.com/LTM8065

APPLICATIONS INFORMATION

4. Place the CIN and COUT capacitors such that their

ground current flow directly adjacent to or underneath

the LTM8065.

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 LTM8065.

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 LTM8065 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.

Hot-Plugging Safely

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of LTM8065. However, these capacitors

can cause problems if the LTM8065 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 LTM8065 can ring to more than

twice the nominal input voltage, possibly exceeding the

LTM8065’sratinganddamagingthepart.Iftheinputsupply

is poorly controlled or the LTM8065 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 LTM8065 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 LTM8065 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.

Figure 5. Layout Showing Suggested External

Components, GND Plane and Thermal Vias

RFB

COUT

VOUT

GND

BIAS AUX

GND

VIN

TR/SS

SYNC

GND/THERMAL VIAS

CIN

RUN

8065 F05

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