AN2986 datasheet(13/18 Pages) STMICROELECTRONICS

13 / 18 page

AN2986

Gate current variation

Doc ID 15727 Rev 1

13/18

The MCU is able to provide sufficient gate current. An estimation of VOLmin has been

provided for one I/O port of STM32 (when 8 I/O ports are sunk with 8 mA each at the same

time).

2.4

Summary of calculations

This section summarizes the calculations for devices used in Example 1 to Example 6.

ΔIG describes the difference between the maximum and minimum gate current which has

an impact on the power supply rating.

Equation 10

Average current consumption is based on the hypothesis that the maximum value is

reached continuously during the half-cycle, i.e. as if the gate current were a squared form.

The maximum average gate current is then the average of the maximum values for positive

and negative half-wave:

Equation 11

where I+Gmax is the maximum gate current for positive half-wave according to Equation 7.

I–Gmax is the maximum gate current for negative half-wave and is calculated according to

Equation 7, using the VGTmin value estimated in Section 2.2 for negative half-wave. It means

that a VGTmin of 1 V for Z0103 and 0.7 V for ACS108-6T are used for calculating the

maximum gate current in the negative half-wave.

MCUs with negative 3.3 V supply voltage can be used for triggering ACS/TRIACs, but this

puts additional constraints on the power supply design. VGT peak variation is already around

30% of VDD, so it is mandatory to reduce VDD ripple by precise control. For example a ±5%

Table 2.

Current consumption of different devices and MCUs

RG (Ω) ±1%

IGmin (mA)

IGmax (mA)

ΔIG (mA)

IGavmax (mA)

STM8S + ACS108, 5 V

180

15.7

23.2 (1)

This value exceeds maximum allowed current capability for one high-sink I/0 port of STM8S with 5V power

supply (20 mA). Two pins must be used instead.

7.5

22.8

STM8S + ACS108, 3.3 V

110

15.1

20.8(2)

This value exceeds maximum allowed current capability for one high-sink I/0 port of STM8S with 3.3 V

power supply (12 mA). Two pins must be used instead.

5.7

19.4

STM8S + Z0103, 5 V

560

4.6

8.8

4.2

7.75

STM8S + Z0103, 3.3 V

300

4.6

9.6

5.0

7.6

STM32 + ACS108, 3.3 V

110

15.1

22.6(3)

This value exceeds maximum allowed current capability for one I/O port of STM32 (20 mA). Two pins must

be used instead.

7.5

22.15

STM32 + Z0103, 3.3 V

300

4.6

10.7

5.7

8.85

ΔI

Gma x

Gmin

–

Gavma x

--- I

Gma x

–

()

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