LTM4663 datasheet(28/30 Pages) AD

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LT8722

Rev. 0

For more information www.analog.com

To ensure that the LT8722 operates below the maximum

junction temperature, even at high load, careful atten-

tion must be paid to provide a lower θJA value for the

device. Typical techniques for enhancing heat dissipation

include using larger copper layers and more vias on the

printed circuit board (PCB) and possibly adding a heat

sink when needed.

The LT8722 LQFN package has a large exposed pad

(EPAD) at the bottom that must be soldered to the ana-

log ground plane on the board. Most of the device’s heat

dissipates through the EPAD. Therefore, the copper layer

connected to the EPAD as well as the vias on it must

be optimized to conduct the heat effectively. It is recom-

mended to use a large via array and distribute them evenly

on the EPAD. Generally, it is more effective to increase the

number of vias than to increase the diameter of the via

within a limited area.

LOW EMI PCB LAYOUT AND INPUT CAPACITOR

SELECTION

The LT8722 is specifically designed to minimize EMI

emissions and maximize efficiency when switching at

high frequencies. For optimal performance the LT8722

requires the use of multiple VIN bypass capacitors. Two

small ceramic 0.1µF capacitors should be placed as close

as possible to the LT8722: One of these capacitors should

be tied to VIN/GND (pins 4 and 5 respectively); a sec-

ond capacitor should be tied to VIN/GND (pins 11 and 10

respectively). Two ceramic 4.7μF capacitors should also

be used as bypass capacitors—one of these capacitors

should be placed close to pins 4 and 5 and one of these

capacitors should be placed close to pins 11 and 10. See

Figure 15 for a recommended PCB layout. For more detail

and PCB design files refer to the demo board guide for

the LT8722. Note that large, switched currents flow in the

LT8722 VIN and GND pins and the input bypass capaci-

tors. The loops formed by the input capacitors should be

as small as possible by placing the capacitors adjacent

to the VIN and GND pins on either side of the LT8722.

APPLICATIONS INFORMATION

Step-down regulators draw current from the input sup-

ply in pulses with very fast rise and fall times. The input

capacitor is required to reduce the resulting voltage rip-

ple at the LT8722 and to force this very high frequency

switching current into a tight local loop, minimizing EMI.

Capacitors with small case size such as 0402 and 0603

are optimal due to their low parasitic inductance. It is best

to use ceramic capacitors of type X7R or X5R. Y5V types

have poor performance over temperature and applied

voltage and should not be used. The input capacitors

should be placed on the same side of the circuit board,

and their connections should be made on that layer. The

SW and BOOST nodes should be as small as possible.

To keep thermal resistance low, extend the ground plane

from GND as much as possible, and add thermal vias to

additional ground planes within the circuit board and on

the bottom side.

8722 F15

GND

VIN

LDR

TEC+

TEC–

SFB

AREA OF THE HOT LOOPS (SHOWN IN RED) SHOULD BE MINIMIZED

BY PLACING THE CAPACITORS AS CLOSE TO VIN/GND PINS AS POSSIBLE.

Figure 15. Recommended PCB Layout

LTM4663 Datasheet(PDF) 28 Page - Analog Devices

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