14

LTC1649

Figure 9. Typical Schematic Showing Layout Considerations

1649 F09

Q1

COMP

SS

SHDN

GND

22

Ω

G1

G2

FB

LTC1649

SHDN

+

1

µF

1

µF

10

µF

+

10

µF

C1

Q2

0.1

µF

1k

L1

+

+

R2

R1

and output capacitors, the source of Q2, the LTC1649 GND

pin, the output return and the input supply return all

clustered at one point. Figure 9 is a modified schematic

showing the common connections in a proper layout. Note

that at 10A current levels or above, current density in the

PC board itself can become a concern; traces carrying high

currents should be as wide as possible.

Power Component Hook-Up/Heat Sinking

As current levels rise much above 1A, the power compo-

nents supporting the LTC1649 start to become physically

large (relative to the LTC1649, at least) and can require

special mounting considerations. Input and output ca-

pacitors need to carry high peak currents and must have

low ESR; this mandates that the leads be clipped as short

as possible and PC traces be kept wide and short. The

power inductor will generally be the most massive single

component on the board; it can require a mechanical hold-

down in addition to the solder on its leads, especially if it

is a surface mount type.

The power MOSFETs used require some care to ensure

proper operation and reliability. Depending on the current

levels and required efficiency, the MOSFETs chosen may

be as large as TO-220s or as small as SO-8s. High

efficiency circuits may be able to avoid heat sinking the

power devices, especially with TO-220 type MOSFETs. As

an example, a 90% efficient converter working at a steady

2.5V/10A output will dissipate only (25W/90%)10% =

2.8W. The power MOSFETs generally account for the

majority of the power lost in the converter; even assuming

that they consume 100% of the power used by the

converter, that’s only 2.8W spread over two or three

provide 90% efficiency in this design can commonly

dissipate 2W when soldered to an appropriately sized

piece of copper trace on a PC board. Slightly less efficient

or higher output current designs can often get by with

standing a TO-220 MOSFET straight up in an area with

some airflow; such an arrangement can dissipate as much

as 3W without a heat sink. Designs which must work in

high ambient temperatures or which will be routinely

overloaded will generally fare best with a heat sink.