AN2986

Proposed solutions

Doc ID 15727 Rev 1

15/18

3

Proposed solutions

3.1

MCU current capability

The maximum current through the I/O port of the MCU can exceed the maximum current

capability of the MCU. This is a hazardous state for the MCU and has to be prevented.

There are two principal solutions. The first solution is to use a buffer transistor (refer to

Figure 6). This transistor takes over the gate current and the MCU I/O port controls only the

base with current that is 50 to 100 times lower than the ACS/TRIAC gate current.

Figure 6.

Schematics of using buffer transistor

The second solution is to put the I/O ports in parallel. The maximum number of I/O ports that

can be paralled is limited by the application and MCU pin count. The maximum current must

also not exceed the maximum current capability of the MCU through the GND pin (150 mA

for STM32, 80 mA for STM8S with 3.3 V power supply or 160 mA for STM8S with 5 V power

supply).

3.2

Average current consumption

The average consumption shown in Table 2 is given for gate control lasting an entire period.

The consumption is in this case high. Contrary to the use of relay, it is possible to decrease

current consumption by reducing the gate pulse length which is useful when several

ACS/TRIACs are controlled at the same time (for example several small pumps or valves).

Refer to AN302 and AN303 for more information on TRIAC pulse gate control.

AM00458

MCUI/O

port

R6

10 K

Ω

R7

10 K

Ω

C4

10 nF

RG

G

+ VDD

+ VDD

A1/COM

A2/OUT

Load