AN442 Datasheet, PDF (1/5 Pages) STMicroelectronics – The aim of this note is to show how to connect

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 mi-
crocontrollers (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
+VDD
µC
LINE
I_G
0V
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
+VDD
µC
0V
I_G
G _A1
LINE
A2
To save cost, manufacturers want to use fewer and fewer components and of course want to remove the
buffer transistor, but a problem arises.
April 2004
REV. 2
1/5