AN2986 datasheet(7/18 Pages) STMICROELECTRONICS

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AN2986

Gate resistor choice

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As an example, Table 1 shows data for STM8S and STM32™. The data for STM8S are

taken from the high-sink pin. The data for STM32 are taken from the GPIO pin.

It is possible to put the pins in parallel to achieve bigger current capability. In that case the

maximum current through the MCU ground pin (VSS) has to be checked as it is limited to

a maximum value (refer to IVSS parameter on the MCU datasheet, for example:

STM8S103xx).

The gate resistor (RG) resistance varies with the accuracy of the resistor used. Standard

resistor accuracy is ±5% or ±1% with a preference for ±1% accuracy. The worst case has to

be taken into account for calculations.

The voltage drop of the gate junction (VGT) is a parameter that varies with the load current

flowing through the TRIAC/ACS and with temperature. Its value increases with a

temperature drop at a rate of 2 mV / °C. Load current dependency is discussed in

Section 2.1.

1.2.2

Gate resistor calculation

The value of the gate resistor has to be calculated in order to deliver the proper amount of

gate current under the worst condition which are:

●

minimum ambient temperature (0 °C),

●

minimum supply voltage (VDD min),

●

maximum voltage drop of the gate junction (VGTmax),

●

maximum output low voltage (VOLmax) for one high-sink I/O pin of the MCU

●

maximum resistance of the gate resistor for ±1% resistor accuracy (1.01 x RG).

The following equation summarizes all conditions for proper RG selection:

Equation 1

Two examples of calculation are shown:

Example 1: MCU: STM8S103K2 with ACS108-6T for power supply 5 V ±10%

Specified values:

IGT (25 °C) = 10 mA, VDD = 5 V, VGT(25 °C) = 1 V

Calculated values: IGT (0 °C) = 1.5 x IGT (25 °C) = 15 mA,

VDDmin = 0.9 x 5 V = 4.5 V,

Table 1.

Output low level voltage (VOLmax) for STM8S and STM32

IG = 8 mA

IG = 12 mA

IG = 20 mA

STM8S (VDD = 5 V) HS pin

0.35 V

0.6 V

1 V

STM8S (VDD = 3.3 V) HS pin

0.55 V

0.9 V

N/A(1)

Maximum current capability of high-sink I/O port for STM8S with 3.3 V power supply is 12 mA as specified

in the datasheet STM8S103xx.

STM32 (VDD = 3.3 V) GPIO pin

0.4 V

NC(2)

Information is not specified in the datasheet STM32103xx.

1.3 V

DD min

GTma x

–

OL ma x

–

GT 0° C

() 1.01

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