STCC05-B datasheet(3/13 Pages) STMICROELECTRONICS

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STCC05-B

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■ The reset circuit

This circuit ensures a Low Voltage Detection (LVD) of the output of the regulator. Most micro-controllers

have an active RESET pin in the low state: so, the /RST pin will be active at low state.

The reset comparator senses the regulator voltage VDD. The /RST pin goes high when VDD is higher than

the high threshold VH = 3.75V and after a delay time TUP; and is low when the VDD decreases below the

low threshold VL = 3.4V after the delay time TDW.

These delays are set by an external capacitor CUP connected to the /RST pin and depend on the trigger

thresholds of /RST: For CUP = 100nF, TUP= 400µs with VTH= VH/2; TDW= 200µs with VTH= VL/2.

■ The Zero Voltage Synchronization ZVS circuit

The Zero Voltage Synchronization ZVS circuit generates the signal ZVS that synchronizes the whole oper-

ation with the AC line cycle (20 ms on 50 Hz or 16.7 ms on 60 Hz). This signal allows the MCU to control

the AC loads and achieve the timing functions.

The input pin SYN is an image of the mains voltage. It is connected to either the power supply transformer

through a resistor RZV or an opto-coupler that is controlled directly by the AC line voltage. The circuit is

protected against fast line transient voltages: a robust ESD protection and a 30µs digital filter are imple-

mented to provide a higher immunity to the MCU operation. Its output signal ZVS is inverted respect to the

input signal VSYN.

■ The relay coil drivers

These robust circuits allow a DC relay coil to be driven by an MCU output. The relay coil has a minimum

resistance of 580

Ω and has a power up to 0.25W for VPS = 12 V. These characteristics are representative

of 3A relays such as FTR-F3AA-12V or JQ1A-12V series.

The output stage is made of a transistor and a demagnetization diode. The transistor is referred to the

ground COM, has a DC current rating of 50mA; and its collector is connected to the output RLI (I=1, 2, 3).

The diode is connected between the output pin RLI and the supply pin VPS.

Moreover, a fourth coil driver has an extended 150mA current capability to be able to drive the coil of a

relay having a 130

Ω minimum resistance and a 1.1W maximum power. These characteristics are repre-

sentative of 20A relays such as G4A-E-DC12, OMIF-S-112 or UKH12S series.

SYN

ZVS

30 µs FILTER

COM

70 k

Ω

25 k

Ω

30 k

Ω

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