LTC4008 datasheet(20/24 Pages) LINER

20 / 24 page

LTC4008

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APPLICATIO S I FOR ATIO

PCB Layout Considerations

For maximum efficiency, the switch node rise and fall

times should be minimized. To prevent magnetic and

electrical field radiation and high frequency resonant prob-

lems, proper layout of the components connected to the IC

is essential. (See Figure 11.) Here is a PCB layout priority

list for proper layout. Layout the PCB using this specific

order.

1. Input capacitors need to be placed as close as possible

to switching FET’s supply and ground connections.

Shortest copper trace connections possible. These

parts must be on the same layer of copper. Vias must

not be used to make this connection.

2. The control IC needs to be close to the switching FET’s

gate terminals. Keep the gate drive signals short for a

clean FET drive. This includes IC supply pins that con-

nect to the switching FET source pins. The IC can be

placed on the opposite side of the PCB relative to above.

3. Place inductor input as close as possible to switching

FET’s output connection. Minimize the surface area of

this trace. Make the trace width the minimum amount

needed to support current—no copper fills or pours.

Avoid running the connection using multiple layers in

parallel. Minimize capacitance from this node to any

other trace or plane.

4. Place the output current sense resistor right next to

the inductor output but oriented such that the IC’s

current sense feedback traces going to resistor are not

long. The feedback traces need to be routed together

as a single pair on the same layer at any given time with

smallest trace spacing possible. Locate any filter

component on these traces next to the IC and not at the

sense resistor location.

5. Place output capacitors next to the sense resistor

output and ground.

6. Output capacitor ground connections need to feed

into same copper that connects to the input capacitor

ground before tying back into system ground.

LTC4008 Datasheet(PDF) 20 Page - Linear Technology