AN1172

ACS triggering mode

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1

ACS triggering mode

1.1

Negative gate current

The ACS silicon structure is different from the triac one. For instance, the gate embeds a

diode junction. Then the gate current can only circulate in one direction, from the COM pin

to the Gate one. A peak reverse voltage (VGM) of this junction is also defined in the ACS

data sheet.

In order to sink a current from the gate by a microcontroller output port, the supply voltage

positive terminal must be connected to the drive reference, i.e. the COM pin of ACSs (see

Figure 1.).

An interesting benefit of such a connection is that the ACS is not fired when the MCU is at

reset state. Indeed, in this case, all the MCU port pins are at high level. This means that the

gate resistors are all connected to the COM terminal. No spurious triggering can then occur.

It should be noticed that for a direct switch / MCU connection, the MCU current capability is

not the only point to check to decide if the buffer circuit can be removed. Actually, the

transistor, used to amplify the MCU current in order to control the gate, also play an

overvoltage protection role. Annex B gives the gate voltage limits between which the MCU

output port will be not stressed. It is also shown that with ACSs, the gate voltage remains

inside these limits even with worst cases of dI/dt gradients at turn-on.

Figure 1.

Gate / MCU connection

MCU

Vdd

Vss

M1

M2

D1

D2

R

I

R

Com

Vg

G

Valve / PUMP

etc.

AC MAINS

L

N

Iout

Vs