LTM8023MPV datasheet(13/20 Pages) LINER

13 / 20 page

LTM8023

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APPLICATIONS INFORMATION

4. Place the CIN and COUT capacitors such that their

ground current ﬂow directly adjacent or underneath

the LTM8023.

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

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

breaking the ground connection between the external

components and the LTM8023.

6. Use vias to connect the GND copper area to the boards

internal ground plane. Liberally distribute these GND

vias to provide both a good ground connection and

thermal path to the internal planes of the printed circuit

board.

Hot-Plugging Safely

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of LTM8023. However, these capacitors

can cause problems if the LTM8023 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 volt-

age at the VIN pin of the LTM8023 can ring to twice the

nominal input voltage, possibly exceeding the LTM8023’s

rating and damaging the part. If the input supply is poorly

controlled or the user will be plugging the LTM8023 into

an energized supply, the input network should be designed

to prevent this overshoot. Figure 6 shows the waveforms

that result when an LTM8023 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 6b 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 6c. 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. 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 LTM8023 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, out-

put 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 a

LTM8023 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 LTM8023 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 LTM8023. To estimate the junction temperature,

approximate the power dissipation within the LTM8023 by

applying the typical efﬁciency stated in this data sheet to

the desired output power, or, if you have an actual module,

by taking a power measurement. Then calculate the tem-

perature rise of the LTM8023 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 LTM8023 to the printed circuit

board depends upon the layout of the circuit board, but

the thermal resistance given with the Pin Conﬁguration,

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 LTM8023.

LTM8023MPV Datasheet(PDF) 13 Page - Linear Technology

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