LTM8031 datasheet(14/22 Pages) LINER

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LTM8031

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applicaTions inForMaTion

rings as high as 35V and the input current peaks at 20A.

One method of damping the tank circuit is to add another

capacitor with a series resistor to the circuit, as shown

in Figure 4b. A 0.7Ω resistor is added in series with the

input to eliminate the voltage overshoot (it also reduces

the peak input current). A 0.1µF capacitor improves high

frequency filtering. For high input voltages its impact on

efficiency is minor, reducing efficiency less than one-half

percent for a 5V output at full load operating from 24V.

By far the most popular method of controlling overshoot

is shown in Figure 4c, where an aluminum electrolytic

capacitor has been connected to FIN. This capacitor’s high

equivalent series resistance damps the circuit and elimi-

nates 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. Placing the electrolytic capacitor

at the FIN terminals can also improve the LTM8031’s EMI

filtering as well as guard against overshoots caused by

the Q of the integrated filter.

Thermal Considerations

The LTM8031 output current may need to be derated if it is

requiredtooperateinahighambienttemperatureordeliver

alargeamountofcontinuouspower.Theamountofcurrent

deratingisdependentupontheinputvoltage,outputpower

and ambient temperature. The temperature rise curves

given in the Typical Performance Characteristics section

can be used as a guide. These curves were generated by a

LTM8031 mounted to a 35cm2 4-layer FR4 printed circuit

board. Boards of other sizes and layer count can exhibit

differentthermalbehavior,soitisincumbentupontheuser

to verify proper operation over the intended system’s line,

load and environmental operating conditions.

The junction-to-air and junction-to-board thermal resis-

tances given in the Pin Configuration diagram may also be

used to estimate the LTM8031 internal temperature. These

thermalcoefficientsaredeterminedperJESD51-9(JEDEC

standard,testboardsforareaarraysurfacemountpackage

thermal measurements) through analysis and physical

correlation. Bear in mind that the actual thermal resistance

of the LTM8031 to the printed circuit board depends upon

the design of the circuit board. The die temperature of

the LTM8031 must be lower than the maximum rating of

125°C, so care should be taken in the layout of the circuit

to ensure good heat sinking of the LTM8031.

The bulk of the heat flow out of the LTM8031 is through the

bottom of the module and the LGA pads into the printed

circuit board. Consequently a poor printed circuit board

design can cause excessive heating, resulting in impaired

performance or reliability. Please refer to the PCB Layout

section for printed circuit board design suggestions.

Finally, be aware that at high ambient temperatures the

internalSchottkydiodewillhavesignificantleakagecurrent

increasing the quiescent current of the LTM8031.

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