LTM8021

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8021fb

APPLICATIONS INFORMATION

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GND

RUN/SS

PLANE

8021 F04

Figure 4. Layout Showing Suggested External Components,

GND Plane and Thermal Vias

PCB Layout

Most of the problems associated with the PCB layout

have been alleviated or eliminated by the high level of

integration of the LTM8021. The LTM8021 is nevertheless

a switching power supply, and care must be taken to

minimize EMI and ensure proper operation. Even with the

high level of integration, one may fail to achieve a specified

operation with a haphazard or poor layout. See Figure 4

for a suggested layout.

Ensure that the grounding and heatsinking are acceptable.

A few rules to keep in mind are:

and GND connection of the LTM8021.

currents flow directly adjacent to, or underneath, the

LTM8021.

4. 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 LTM8021.

Hot-Plugging Safely

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of LTM8021. However, these capacitors

can cause problems if the LTM8021 is plugged into a live

supply (see the 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 volt-

nominal input voltage, possibly exceeding the LTM8021’s

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

controlled or the user will be plugging the LTM8021 into

an energized supply, the input network should be designed

to prevent this overshoot. Figure 5 shows the waveforms

that result when an LTM8021 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.