A4401 datasheet(13/17 Pages) ALLEGRO

13 / 17 page

Automotive Quasi-Resonant Flyback Control IC

A4401

Allegro MicroSystems, Inc.

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

www.allegromicro.com

filtering, it is imperative that the “power loop” formed

by the input filter, primary winding, MOSFET, Q1,

and sense resistor is as short and “tight” as possible.

This means components should be placed as close

together as possible, and the loop area should be

minimized, in order to reduce the effects of magnetic

pickup and noise generation at higher frequencies. A

ground plane is not necessary, however, a good star

ground connection should be formed between the

input filter capacitor and the sense resistor. Circuit

traces should be as wide as possible, in order to mini-

mize leakage inductance. Figure 3 illustrates the main

power loop.

A local ground plane around the A4401 can be used to

minimize ground bounce issues. This can be done with

a simple connection from the ground pin of the A4401

to the star ground, being careful to avoid any connec-

tion between this local ground plane and the power

loop.

The compensation components connected to the

COMP pin, the filter capacitor connected to VIN, the

feedback resistors connected to VA, and the resonant

capacitor connected to LX should be located as close

to their respective pins as possible. In addition, the

length of the traces between those components and

ground should be as short as possible.

Secondary Block

Each output power circuit should

be laid out with identical principles relative to the pri-

mary side power circuit, that is, with each secondary

winding, rectifying diode, and output capacitor posi-

tioned close together and forming a tight loop.

The high voltage output circuits should be kept well

away from all the control circuitry. It is recommended

that good connections be made between each of the

local output grounds and the star ground. In addition,

all of the output grounds should be connected together

via wide traces and not ground planes. Figure 4 illus-

trates one of the output loops.

The feedback resistor connected to the regulated

output rail should be located as close to the VA pin as

possible. The PCB trace that connects the top of this

resistor to the output rail should not be located near

any of the output power loops or ground connections.

Electromagnetic Interference

Some of the previous sections provide information

on reducing EMI in terms of input filter capacitance

selection, magnetics design (to achieve zero voltage

switching at nominal input voltage), and board lay-

out. This section provides some additional advice on

reducing EMI.

Effects of Magnetics Design on 0 V Switching

Fig-

ure 5 illustrates a converter running at full power with

VBAT = 13.5 V. The upper trace is the voltage across

LX and the lower trace is the voltage across the sense

resistor (essentially the current through the primary

winding and the MOSFET, Q1). The magnetic set in

this case was designed to achieve 0 V switching at

a VBAT of 7 V.

LSEC

COUT

Minimize this loop area

Figure 4. One of three output power loops

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