SC401B datasheet(29/32 Pages) SEMTECH

29 / 32 page

SC401B

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 IC 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 IC 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 wide, short

and routed away from noise sources, such as

BST, LX, VIN, and PGND between the input

capacitors, output capacitors, and the IC.

The feedback components for the switcher and

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

pins of the IC as possible to reduce the possibil-

ity of noise corrupting these analog signals.

BST, ILIM, TON, SS and LX

The connections for the boost capacitor

between the BST and LXBST must be short, wide

and directly connected.

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 IC and connected

between LXS (pin 28) and ILIM (pin 27) only.

TON should be close to the IC and connected

between TON (pin 31) and AGND (pin 30).

•

SOFT should be close to the IC and kept away

from the boost capacitor. Connect the AGND

end of C

SOFT to the AGND plane at pin 4.

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 on the LX PAD to

provide good thermals and connection to an

internal or bottom layer LX plane.

Capacitors and Current Loops

Figure 17 shows the placement of input/output

capacitors and inductor. This placement shows

the smallest current loops between the input/

output capacitors, the SC401B and the inductor

to reduce the IR drop across the copper.

•

Applications Information (continued)

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