SC417 datasheet(26/29 Pages) SEMTECH

26 / 29 page

SC417/SC427

current handling (including the chip power ground con-

nections). Power components should be placed to mini-

mize current loops and reduce losses. Make all the power

connections on one side of the PCB using wide copper

areas if possible. Do not use minimum land patterns for

power components.

Current Limit

Obtaining an accurate current limit for the SC417/SC427

requires careful PCB layout. The device uses the RDS

(ON)

the low-side MOSFET for current sensing. The ILIM com-

parator that performs the current limit function monitors

the ILIM pin with respect to AGND, but the low-side

MOSFET is internally connected to PGND. The PCB layout

needs to following these guidelines.

AGND and PGND must be connected together

directly at the pins of the IC and not anywhere

else. This can be done through PCB copper or a

zero ohm resistor. Keep the traces short and

direct. The preferred connection for PGND is at

pin 19. The preferred connection for AGND is at

the PAD1.

The PGND path from the device to the input and

output capacitors requires a wide copper areas.

Use multiple vias and copper areas if available.

Do not break up these copper areas with inter-

vening components. The goal is to minimize

the IR drop between all PGND connections.

Provide multiple vias and multiple copper areas

between the LX pins and the inductor. The goal

is to minimize any IR drop that might contrib-

uted to the RDS

(ON)

. This is also good to carry

heat away from the device.

For the connection from the RLIM resistor to the

LX node, use a direct Kelvin connection to pin 28

(LX), as near to the pin as possible. Do not

connect to a place further in the copper between

the LX pins and the inductor — the intervening

copper will appear as increased RDS

(ON)

and will

reduce the operating current limit.

The layout can be considered in two parts; the control

section referenced to AGND, and the switcher power

section referenced to GND.

•

Control Section

Locate all components referenced to AGND on the sche-

matic and place these components near the device and

on the same side if possible. Connect AGND to the AGND

pad and pins using a plane or island if possible.

The device supply decoupling capacitor (V5V to PGND)

should be located as close as possible to the pins. The V5V

decoupling capacitor preferred placement is on the oppo-

site side of the PCB. It should be routed with traces as

short as possible, using at least two vias when connecting

through the PCB.

There are two sensitive feedback-related pins at the device

— VOUT and FB. Proper routing is essential to keep noise

away from these signals. All components connected to FB

should be located directly at the chip, and the copper area

of the FB node must be minimized. The VOUT trace that

feeds into the VOUT pin, which also feeds the FB resistor

divider, must be kept as far away as possible from noise

sources such as all switching signals (LX, DH, DL, BST) and

the inductor. Route the VOUT trace in a quiet layer if pos-

sible, from the output capacitor back to the chip.

Power Section

The switcher power section key guidelines are as follows.

There should be a very small input loop between

the input capacitors, inductor, and output

capacitors. Locate the input decoupling capaci-

tors directly at the VIN pad on the device.

The LX phase node should be a large enough to

carry the required current. Careful sizing is

required since this is the noisiest node.

The PGND connection between the input capaci-

tors, low-side MOSFET, and output capacitors

should be as small as possible, with wide traces

or planes and multiple vias.

The impedance of the PGND connection

between the low-side MOSFET and the PGND

pin should be minimized. This connection must

carry the DL drive current, which has high peaks

at both rising and falling edges. Use multiple

layers and multiple vias to minimize impedance

and keep the distance as short as practical.

•

Applications Information (continued)

SC417 Datasheet(PDF) 26 Page - Semtech Corporation

SC417 Datasheet(HTML) 26 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Datasheet Download

Link URL