SC419 datasheet(20/24 Pages) SEMTECH

20 / 24 page

SC419

Applications Information (continued)

Switching Frequency Variations

The switching frequency will vary depending on line and

load conditions. The line variations are a result of fixed

propagation delays in the on-time one-shot, as well as

unavoidable delays in the external MOSFET switching. As

IN increases, these factors make the actual DH on-time

slightly longer than the ideal on-time. The net effect is

that frequency tends to falls slightly with increasing input

voltage.

The switching frequency also varies with load current as a

result of the power losses in the MOSFETs and the induc-

tor. For a conventional PWM constant-frequency con-

verter, as load increases the duty cycle also increases

slightly to compensate for IR and switching losses in the

MOSFETs and inductor. A adaptive on-time converter

must also compensate for the same losses by increasing

the effective duty cycle (more time is spent drawing

energy from V

IN as losses increase). The on-time is essen-

tially constant for a given V

OUT/VIN combination, to offset

the losses the off-time will tend to reduce slightly as load

increases. The net effect is that switching frequency

increases slightly with increasing load.

PCB Layout Guidelines

As with any switch-mode converter, good PCB layout is

essential to achieving high performance. The following

guidelines will provide an optimum PCB layout.

The device layout can be considered in four parts;

Grounding for PGND and AGND

Power Components

Low-noise Analog Circuits

Bypass capacitors

Grounding

A ground plane layer for PGND is recommended

to minimize the effects of switching noise, resis-

tive losses, and to maximize heat removal from

the power components.

A separate ground plane or island should be used

for AGND and all associated components. The

AGND island should avoid overlapping the switch-

ing signals on other layers (DH/DL/BST/LX).

•

Connect PGND and AGND together with a zero

ohm resistor or copper trace. Make the connec-

tion near the AGND and PGND pins of the IC.

Power Components

Use short, wide connections between the power compo-

nents:

Input capacitors and high-side MOSFETs

High-side and Low-side MOSFETs and inductor

(LX connection). Use wide copper traces to

provide high current carrying capacity and for

heat dissipation.

Inductor and output capacitors

All PGND connections — the input capacitors,

low-side MOSFETs, output capacitors, and the

PGND pin of the SC419.

An inner layer ground plane is recom-

mended.

Each component should have a good,

low impedance connection to the ground

plane.

Place vias to the PGND plane directly near

the component pins.

Use short wide traces for the pin connections

from the SC419 (LX, DH, DL and BST). Do not

route these traces near the sensitive analog

signals (FB, TON, V

OUT).

Low-Noise Analog Circuits

Low-noise analog circuits are sensitive circuits that are

referenced to AGND. Due to their high impedance and

sensitivity to noise, it is important that these circuits be

kept away from the switching signals.

Use a ground plane or solid copper area for

AGND. Place all components connected to

AGND above this area.

Use short direct traces for the AGND con-

nections to all components.

Place vias to the AGND plane directly near

the component pins.

Proper routing of the V

OUT sense trace is essential

since it feeds into the FB resistor divider. Noise

on the FB waveform will cause instability and

multiple pulsing.

•



•



•

SC419 Datasheet(PDF) 20 Page - Semtech Corporation

SC419 Datasheet(HTML) 20 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

Datasheet Download

Link URL