LTM8003 datasheet(21/28 Pages) AD

21 / 28 page

LTM8051

Rev. 0

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APPLICATIONS INFORMATION

A few rules to keep in mind are:

1. Place the RFB and RT resistors as close as possible to

their respective pins.

2. Place the CIN capacitor as close as possible to the VIN

and GND connection of the LTM8051.

3. Place the COUT capacitor as close as possible to the

VOUT and GND connection of the LTM8051.

4. Place the CIN and COUT capacitors such that their

ground current flow directly adjacent to or underneath

the LTM8051.

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

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

Avoid breaking the ground connection between the

external components and the LTM8051.

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 con-

nection 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 3. The LTM8051

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 LTM8051. However, these capacitors

can cause problems if the LTM8051 is plugged into a

live 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 LTM8051 can ring to more than twice the nominal

input voltage, possibly exceeding the LTM8051’s rating

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

trolled or the LTM8051 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 rela-

tively high equivalent series resistance damps the circuit

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 LTM8051 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

Characteristics section can be used as a guide. These

curves were generated by the LTM8051 mounted to

Figure 3. Layout Showing Suggested External Components,

GND Plane and Vias

COUT2

COUT3

COUT4

COUT1

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