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Application Note

DE-COUPLING NOISE SOURCES IN THE CS1601

1. Introduction

The CS1501 & CS1601 are high-performance digital power factor correction (PFC) controllers designed for switching mode pow-

er supply (SMPS) & lighting applications. This paper deals with the issue of coupled noise to the controller, how to minimize it,

and how to filter it. Although only the CS1601 is referenced in this application note, all content is applicable to the CS1501 as well.

2. The Noise Issue

SMPS produce electronic noise — high-frequency spikes on DC voltages caused by the switching of current through parasitic

inductive & capacitive impedances. This noise can interfere with normal operation of the IC, causing false triggers and erratic

operation.

2.1 Practical Methods to Reduce Noise

The first step in minimizing noise is good layout. AN350 — CS1601 PCB Layout Guidelines gives instruction on how to optimize

layout for the CS1501/CS1601. The basic goal is to minimize trace impedances on the PCB for traces that switch high voltage

or current.

2.2 Basic Guidelines

All power traces should be as short & wide as possible (low impedance).

The trace between the GND pin of CS1601 and the V

link

capacitor should be as short as possible, and explicitly used for CS1601

GND return current.

2.3 Specific Problems

Factors such as design or topology choice, space limitations, or preferred process technology can limit the designer’s ability to

optimize the layout. Under such circumstances, further attention must be paid regarding de-coupling on the IC. Described below

are several specific circumstances that may cause certain performance issues. The problems that are caused are also described

along with suggested solutions.

Issue 1: GND cannot be linked directly to GND of V

link

cap.

Problem: CS1601 V

link

measurement may be prone to AC errors. The GND for CS1601 is now inductively coupled to

the “real” GND, the GND of the V

link

capacitor.

Solution: Place a capacitor from the IFB pin to the GND pin of CS1601, very close to the IC.

Issue 2: V

comes from a noisy auxiliary winding.

Problem: Noise on V

can couple to both the IAC & IFB pins.

Solution: Add decoupling caps between both the IAC & IFB pins and V

, close to IC.

Issue 3: Driver requires instantaneous current to turn FET on.

Problem: Instantaneous energy requirement can cause V

to droop at the IC pin.

Solution: Add a 2.2

µF minimum ceramic capacitor directly at pin 8 to act as storage device.

Issue 4: Current Sense pin (CS) “sees” a voltage waveform with noise.

Problem: The current sense pin can be sensitive to noise. Care needs to be exercised to make the “GND” of the sense

resistor the same as the “GND” of the IC.

Solution: Add a small capacitor at the CS pin. (Cont.)

AN353

JUL‘11

AN353REV2

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Application Note 1

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Strona 1 - Application Note

Strona 2

AN3532It is a general recommendation that a resistor (RCS) is used in series between the CS pin & the sense resistor (RSense). The timeconstant (T

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AN3533This page intentionally blank.

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AN3534Contacting Cirrus Logic SupportFor all product questions and inquiries contact a Cirrus Logic Sales Representative. To find one nearest you go t

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