MIC22705 datasheet(21/28 Pages) MICREL

21 / 28 page

Micrel, Inc.

MIC22705

December 2010

M9999-121710-A

PCB Layout Guidelines

Warning!!! To minimize EMI and output noise, follow

these layout recommendations.

PCB Layout is critical to achieve reliable, stable and

efficient performance. A ground plane is required to

control EMI and minimize the inductance in power,

signal and return paths.

The following guidelines should be followed to insure

proper operation of the MIC22705 converter:

•

The 2.2µF ceramic capacitor, which is connected to

the SVIN pin, must be located right at the IC. The

SVIN pin is very noise sensitive and placement of

the capacitor is very critical. Use wide traces to

connect to the SVIN and SGND pins.

•

The signal ground pin (SGND) must be connected

directly to the ground planes. Do not route the

SGND pin to the PGND Pad on the top layer.

•

Place the IC close to the point of load (POL).

•

Use fat traces to route the input and output power

lines.

•

Signal and power grounds should be kept separate

and connected at only one location.

Input Capacitor

•

A 22µF X5R or X7R dielectrics ceramic capacitor is

recommended on each of the PVIN pins for

bypassing.

•

Place the input capacitors on the same side of the

board and as close to the IC as possible.

•

Keep both the PVIN pin and PGND connections

short.

•

Place several vias to the ground plane close to the

input capacitor ground terminal.

•

Use either X7R or X5R dielectric input capacitors.

Do not use Y5V or Z5U type capacitors.

•

Do not replace the ceramic input capacitor with any

other type of capacitor. Any type of capacitor can be

placed in parallel with the input capacitor.

•

If a Tantalum input capacitor is placed in parallel

with the input capacitor, it must be recommended for

switching regulator applications and the operating

voltage must be derated by 50%.

•

In “Hot-Plug” applications, a Tantalum or Electrolytic

bypass capacitor must be used to limit the over-

voltage spike seen on the input supply with power is

suddenly applied.

Inductor

•

Keep the inductor connection to the switch node

(SW) short.

•

Do not route any digital lines underneath or close to

the inductor.

•

Keep the switch node (SW) away from the feedback

(FB) pin.

•

To minimize noise, place a ground plane underneath

the inductor.

•

The inductor can be placed on the opposite side of

the PCB with respect to the IC. It does not matter

whether the IC or inductor is on the top or bottom as

long as there is enough air flow to keep the power

components within their temperature limits. The

input and output capacitors must be placed on the

same side of the board as the IC.

Output Capacitor

•

Use a wide trace to connect the output capacitor

ground terminal to the input capacitor ground

terminal.

•

Phase margin will change as the output capacitor

value and ESR changes. Contact the factory if the

output capacitor is different from what is shown in

the BOM.

•

The feedback divider network must be place close to

the IC with the bottom of R2 connected to SGND.

•

The feedback trace should be separate from the

power trace and connected as close as possible to

the output capacitor. Sensing a long high-current

load trace can degrade the DC load regulation.

RC Snubber

•

Place the RC snubber on either side of the board

and as close to the SW pin as possible.

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