LTM8003 datasheet(19/26 Pages) LINER

19 / 26 page

LTM8073

8073fa

For more information www.linear.com/LTM8073

applicaTions inForMaTion

Hot-Plugging Safely

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of LTM8073. However, these capaci-

tors can cause problems if the LTM8073 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 voltage at the VIN pin of the LTM8073 can ring

to more than twice the nominal input voltage, possibly

exceeding the LTM8073’s rating and damaging the part.

If the input supply is poorly controlled or the LTM8073 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 relatively 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 LTM8073 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 LTM8073 mounted to a

58cm2 4-layer FR4 printed circuit board. Boards of other

sizes and layer count can exhibit different thermal behav-

ior, so it is incumbent upon the user to verify proper oper-

ation over the intended system’s line, load and environ-

mental 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 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 cir-

cuit 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 ther-

mal resistance measured in a one cubic foot sealed enclo-

sure. This environment is sometimes referred to as still

Figure 3. Layout Showing Suggested External Components, GND

Plane and Thermal Vias

SYNC

GND

GND/THERMAL VIAS

RUN

VIN

CIN

BIAS AUX

RFB

COUT

VOUT

RT TR/SS

8073 F03

LTM8003 Datasheet(PDF) 19 Page - Linear Technology

LTM8003 Datasheet(HTML) 19 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Datasheet Download

Link URL