SC403 datasheet(29/32 Pages) SEMTECH

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SC403

Applications Information (continued)

AGND Island

AGND should have its own island of copper with

no other signal traces routed on this layer that

connects the AGND pins and pad of the device

to the analog control components.

All of the components for the analog control cir-

cuitry should be located so that the connections

to AGND are done by wide copper traces or vias

down to AGND.

Connect PGND to AGND with a short trace or 0Ω

resistor. This connection should be as close to

the device as possible.

FB, VOUT, and Other Analog Control Signals

The connection from the V

OUT power to the

analog control circuitry must be routed from the

output capacitors and located on a quiet layer.

The traces between VOUT and the analog control

circuitry (VOUT, and FB pins) must be short and

routed away from noise sources, such as BST, LX,

VIN, and PGND between the input capacitors,

output capacitors, and the device.

ILIM and TON nodes must be as short as possible

to ensure the best accuracy in current limit and

on time.

ILIM should be close to the device and connected

to LX with a Kelvin trace to pin 28 on the device.

All of the LX pins are connected to the LX PAD

on the device, which should be a sufficient con-

nection and will prevent the need to connect

the resistor further into the LX plane.

The feedback components for the switcher and

the LDO need to be as close to the FB and FBL

pins of the device as possible to reduce the pos-

sibility of noise corrupting these analog signals.

•

BST, ILIM and LX

LX and BST are very noisy nodes and must be

routed to minimized the PCB area that is exposed

to these signals.

The connections for the boost capacitor

between the device and LX must be short and

directly connected to the LXBST (pin 13).

The connections for the current limit resistor

between the ILIM pin and LX must be as short as

possible and directly connected to pin 28 (LXS).

The LX node between the IC and the inductor

should be wide enough to handle the inductor

current and short enough to eliminate the pos-

sibility of LX noise corrupting other signals.

Multiple vias should be used to provide a good

connection to LX between the device and the

inductor.

Capacitors and Current Loops

The current loops between the input capacitors,

the device, the inductor, and the output capaci-

tors must be as close as possible to each other to

reduce IR drop across the copper.

All bypass and output capacitors must be con-

nected as close as possible to the pin on the

device.

Soft-Start Capacitor

The capacitor used for soft-start should be

located away from the BST pin and its capacitor.

If possible locate the boost capacitor on the

opposite side of the board form the IC and soft-

start capacitor.

•

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