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MLX90805

Intelligent Triac Controller

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MLX90805

Intelligent Triac Controller

Description of Block Diagram

Voltage Regulator

The chip is supplied from the AC line voltage, by a

half wave rectifier. The voltage at pin VDDA is limited

to ~ 15.5V. The digital part and some of the periph-

eral blocks are supplied by internally generated VDD

~ 5V.

Analog Power on Reset

This block tracks the voltage at VDDA, and permits

generation of firing pulses for the triac only if VDDA >

~13V. It is considered otherwise that the motor is not

properly supplied by the mains.

Oscillator

There is an on chip oscillator. All timing constraints

inside the chip are derived from this clock.

FLL

A frequency locked loop circuit is implemented to

obtain a clock frequency from a current controlled

oscillator, by using the mains frequency as a refer-

ence. A successive approximation algorithm is used

at start up to minimize the time for the oscillator ad-

justment.

Reference Voltage

This voltage is used to supply the external potenti-

ometer for the definition of different speed settings.

Logic

This block performs all control functions to realize

time synchronization, smooth soft start, and proper

triac firing, so that motor runs at a defined speed.

Triac Driver

This output is able to drive directly a triac. It defines

the triac gate current and operates as current gen-

erator. There is no need of external resistor for cur-

rent limitation.

Auto Retriggering

This block tracks if the triac is on after each firing

pulse. If the triac is off 20us after a firing pulse, a

new pulse is generated.

Thermal Protection

The chip is able to supply an external protection cir-

cuitry, typically an NTC resistor with reference resis-

tor, to track the ambient temperature. If the voltage at

THP equals Vref/2 the protection is activated and

the chip sets the firing angle defined by the value in

ROM address 1. A resistor connected to pin FB can

introduce hysteresis in the detection level.

Options

This block defines different modes of the chip opera-

tion.

ADC

The analog signal from the potentiometer, which de-

fines the speed setting, is transferred into digital by a

4-bit ADC. The reference for the converter is the volt-

age used to supply the potentiometer.

ROM

The digital words from the ADC act as the address of

a ROM table in which the different firing angles are

programmed. This means that 16 different firing an-

gles can be selected.

Zero Cross

This block detects the moments when mains voltage

crosses zero level. An accurate detection allows

good synchronization, so firing pulses driving the

triac can be generated at the right moment.

90805

ZCD

SET

ADC

Vref

Vref

Vss

Vdda

Voltage

regulator

zero

crossing

detector

Lookup

ROM

Vref/2

COMP

Power monitoring

and reset

Logic

Triac driver

Auto

retriggering

Options

MLX90805 Intelligent Triac Controller

Page 2

THP

FB

Rev 1.2 17/May/00

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