LTM8022MPV datasheet(13/20 Pages) LINER

13 / 20 page

LTM8022

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Hot-Plugging Safely

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of LTM8022. However, these capacitors

can cause problems if the LTM8022 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. In this case,

the voltage at the VIN pin of the LTM8022 can ring to

twice the nominal input voltage, possibly exceeding the

LTM8022’s rating and damaging the part. If the input

supply is poorly controlled or the user will be plug-

ging the LTM8022 into an energized supply, the input

network should be designed to prevent this overshoot.

Figure 5 shows the waveforms that result when an

LTM8022 circuit is connected to a 24V supply through six

feet of 24-gauge twisted pair. The ﬁrst plot 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. In Figure 5b

an aluminum electrolytic capacitor has been added. This

capacitor’s 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. An

alternative solution is shown in Figure 5c. A 0.7

Ω 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. This solution

is smaller and less expensive than the electrolytic capacitor.

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.

Thermal Considerations

The LTM8022 output current may need to be derated if it

is required to operate in a high ambient temperature or

deliver a large amount of power. The amount of current

derating is dependent upon the input voltage, output

power and ambient temperature. The derating curves

in the Typical Performance Characteristics section can

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

LTM8022 mounted to a 33cm2 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.

The die temperature of the LTM8022 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

LTM8022. To estimate the junction temperature, approxi-

mate the power dissipation within the LTM8022 by applying

the typical efﬁciency stated in this datasheet to the desired

output power, or, if you have an actual module, by taking a

power measurement. Then calculate the temperature rise

of the LTM8022 junction above the surface of the printed

circuit board by multiplying the module’s power dissipation

by the thermal resistance. The actual thermal resistance

of the LTM8022 to the printed circuit board depends upon

the layout of the circuit board, but the thermal resistance

given on page 2, which is based upon a 33cm2 4-layer

FR4 PC board, can be used a guide.

Finally, be aware that at high ambient temperatures the

internal Schottky diode will have signiﬁcant leakage current

(see Typical Performance Characteristics) increasing the

quiescent current of the LTM8022.

APPLICATIONS INFORMATION

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