UM0277 datasheet(9/32 Pages) STMICROELECTRONICS

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UM0277

Functional description

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2.3.2

ST7LITE39F2 microcontroller

The MCU used in the thermostat kit is the ST7LITE39F2. It belongs to the ST7LITE39F2

MCU family. It embeds a large number of features at the minimum cost. The peripheral

hardware requirements are then reduced to the minimum:

●

No crystal oscillator or external resonator is used. Indeed, the internal RC-oscillator of

the ST7LITE39F2 is used to generate the clock.

●

No external RESET circuit is used, due to the internal circuit of the ST7LITE39F2.

In case of MCU firmware change, four options have to be set:

●

Software watchdog activation

●

RC oscillator selection

●

PLL disabled

●

Low Voltage Detection selection

2.3.3

Capacitive power supply

To reduce the board price as much as possible, a capacitive power supply is used on the

board. The maximum average current sunk by the board is about 30mA for 4ms gate current

pulse widths (see Table 2). In this case, a 1µF C3 capacitor value should be used in order

for the board to work correctly under typical application conditions (nominal capacitor value,

230V RMS line voltage).

For higher or lower currents, the C3 capacitor can be changed. Table 3 lists the maximum

allowed average current versus the different C3 capacitor values for typical application

conditions (nominal capacitor value, 230 VRMS line voltage) and worst application case

(nominal C3 capacitor value -10%, 198 VRMS line voltage), for a 50Hz mains frequency.

For more information about the design of the capacitive power supply please refer to

Application Note AN1476.

Table 2.

Maximum average current sunk by the board

Devices

Average current

consumption

Comments

MCU

2.5mA

Maximum supply current in Run Mode.

fCPU = 1MHz and VDD = 5.5V

LEDs

2.7mA

LEDs switch ON/OFF alternatively on each AC mains

polarity in order to reduce the current consumption.

VF = 1.6V @ IF = 4mA, VDD_Max = 5.5V and

R9 = R10 = R11 = 750Ohms±1%

Gate average

current necessary

to control AC

switches

21mA

Switches Q1 (Compressor) and Q2 (Defrost) cannot be

switched ON at the same time. The gate average current is

only defined according to the Q2 (Defrost) and Q3 (Light

Bulb) gate current.

VDD_Max = 5.5V, RDSON_MCU = 25Ω, R4 = 130Ω±5%,

R5 = 390Ω±5% and 4ms gate currents pulses widths

Others (Push

Button, NTC

supply, Buzzer)

4.3mA

VDD_Max = 5.5V, RNTC@0°C = 16Ω, R14 = 10kΩ±5% and

R15 = 200Ω±5%

(IBuzzer supply current = 4mA)

Total

30.5mA

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