ST AN442 Application note

AN442

APPLICATION NOTE

TRIAC & MICROCONTROLLERS: THE EASY CONNECTION

INTRODUCTION

The aim of this note is to show how to connect an ST triac and an ST microcontroller.

CONVENTIONAL SOLUTION

For many years the triac has been used to switch load on the AC mains and thanks to the low cost of mi­crocontrollers (µC) this solution is widely used in the appliance market.

All the system use a buffer transistor between the output port of the microcontroller and the triac as shown
in the Figure 1.

Figure 1. Drive in the 1st and 4th quadrants

+V

DD

µC

0V

I

G

LINE

Because of the low sensivity of the triac in the 4th quadrant this type of drive is often unpractical, and is
replaced by the topology of the Figure 2.

Figure 2. Conventional drive in the 2nd and 3rd quadrants

+V

DD

G

A1

LINE

A2

µC

0V

I

G

To save cost, manufacturers want to use fewer and fewer components and of course want to remove the
buffer transistor, but a problem arises.

REV. 2

1/5April 2004

AN442 APPLICATION NOTE

Due to the low output current of the microcontroller, the triac had to be very sensitive and consequently
was not able to withstand for example the static dv/dt, and the commutation.

NEW ST SOLUTION

Two parameters have been improved:

– The sensitivity of the triacs.

– The output capability of the microcontrollers in terms of sunk current.

A microcontroller is now able to drive one standard triac or several sensitive triacs, without buffer transis­tors (see Figure 3).

Figure 3. An easy connection!

+V

DD

V

G

ST

TRIAC

G

A1

LINE

A2

ST
µC

I

G

R

G

V

OL

0V

Table 1 shows the output capability of a range of controllers and the sensitivity of the triacs.

Table 1. Triac and microcontroller characteristics

MICROCONTROLLERS &

OUTPUT CAPABILITIES

ST621x SERIES

ST622x SERIES

= 20mA

I

OL

at

V

= 1.3V

OL

I

=100mA

VSS

TRIAC SENSITIVITY

T & TW SERIES

Tx05 SERIES

S & SW SERIES

Tx10 SERIES

C SERIES

CW SERIES

B & BW SERIES

I

I

GT

I

GT

I

GT

I

GT

= 5mA

GT

= 10mA

= 25mA

= 35mA

= 50mA

GATE

PARAMETERS

V

=1.5V

G

at

= 10mA

I

G

=1.5V

V

G

at

I

= 20mA

G

=1.5V

V

G

at

I

= 50mA

G

= 2V

V

G

at

I

= 70mA

G

= 2V

V

G

at

I

= 100mA

G

CONNECTION

1 PORT/TRIAC

1 PORT/TRIAC

2 PORTS

IN PARALLEL/TRIAC

3 PORTS

IN PARALLEL/TRIAC

4 PORTS

IN PARALLEL/TRIAC

2/5