LTM8031

13

8031fa

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 LTM8031. However, these capacitors

can cause problems if the LTM8031 is plugged into a live

or fast rising or falling 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 under-damped tank

ring to twice the nominal input voltage, possibly exceed-

ing the LTM8031’s rating and damaging the part. A similar

phenomenon can occur inside the LTM8031 module, at the

output of the integrated EMI filter, with the same potential

of damaging the part.

If the input supply is poorly controlled or the user will be

plugging the LTM8031 into an energized supply, the input

network should be designed to prevent this overshoot. Fig-

ure 4 shows the waveforms that result when an LTM8031

circuit is connected to a 24V supply through six feet of 24-

gauge twisted pair. The first plot (4a) is the response with

a 2.2μF ceramic capacitor at the input. The input voltage

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

required to operate in a high ambient temperature or deliver

a large amount of continuous power. The amount of current

derating is dependent upon the input voltage, output power

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

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.

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

thermal coefficients are determined per JESD 51-9 (JEDEC

standard, test boards for area array surface mount package

thermal measurements) through analysis and physical

Figure 3. Layout Showing Suggested External Components,

GND Plane and Thermal Vias

GND

8031 F03

FIN

RUN/SS

SYNC

AUX

BIAS

GND

OPTIONAL

FIN

CAPACITOR