LTM8032EVPBF datasheet(13/20 Pages) LINER

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LTM8032

8032fa

APPLICATIONS INFORMATION

6. Use vias to connect the GND copper area to the board’s

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 LTM8032. However, these capacitors

can cause problems if the LTM8032 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

circuit, and the voltage at the VIN pin of the LTM8032 can

ring to twice the nominal input voltage, possibly exceed-

ing the LTM8032’s rating and damaging the part. A similar

phenomenon can occur inside the LTM8032 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 LTM8032 into an energized supply, the

input network should be designed to prevent this over-

shoot. Figure 5 shows the waveforms that result when

an LTM8032 circuit is connected to a 24V supply through

six feet of 24-gauge twisted pair. The ﬁrst plot (5a) 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. An alternative solution is shown in Figure 5b. A

0.7Ω resistor is added in series with the input to eliminate

the voltage overshoot (it also reduces the peak input cur-

rent). 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

5c, where an aluminum electrolytic capacitor has been

connected to FIN. 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. Figure 5c shows the capacitor added to the

VIN terminals, but placing the electrolytic capacitor at the

FIN terminals can improve the LTM8032’s EMI ﬁltering as

well as guard against overshoots caused by the Q of the

integrated ﬁlter.

Thermal Considerations

The LTM8032 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

LTM8032 mounted to a 36cm2 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.

GND

COUT

CIN

8032 F04

VIN

FIN

RUN/SS

SYNC

AUX

BIAS

VOUT

GND

OPTIONAL

FIN

CAPACITOR

Figure 4. Layout Showing Suggested External Components,

GND Plane and Thermal Vias

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