STCC05-B datasheet(10/13 Pages) STMICROELECTRONICS

10 / 13 page

STCC05-B

10/13

the voltage to the buzzer, this circuit feeds the buzzer with the exponential voltage induced by the charge

and the discharge of the RS CS network.

The ROH and RS resistors contribute to reduce high harmonic sound distortions. Indeed, they limit the peak

current through the buzzer, feed the buzzer with the CS capacitor voltage, and limit the current through

the low side NPN transistor of the driver.

Therefore to set rising/falling durations of the sound shape, it is advised to adjust only the value of the CS

capacitor.

The integrated RBZ resistor is selected to discharge the buzzer when the low side transistor is off, espe-

cially at the maximum operating frequency. The buzzer is completely discharged within five times the time

constant of the resistor-buzzer with

τ = RBZ x CBUZZER.

Therefore, RBZ < 1 / (10 x FMAX x CBUZZER). Since the buzzer capacitance CBUZZER is about 20nF at the

maximum operating frequency of driver is 5kHz, this RBZ resistance is set at 1k

Ω.

■ ZERO CROSSING DETECTION CIRCUITS

The detection of the zero crossing of the AC line voltage can be achieved at least on two ways with the

STCC05, depending of the power supply unit.

When the power supply uses a magnetic 50/60Hz transformer, the input pin SYN is connected to a trans-

former output through a resistor RZV, the electrical path being closed by the low side bridge diodes.

Figure 15: Buzzer terminal voltages VBZ1 & VBZ2

and buzzer current IBZ

Figure 16: Buzzer terminal voltage VBZ2 with

buzzer enable and input circuit signals

Figure 17: ZVS circuit operation using the AC

secondary of a transformer

Time : 100µs/div , V

& V

: 4V/div , I

: 20mA/div

BZ1

BZ2

VBZ1

VBZ2

IBZ

Time : 100ms/div , V

, EN

& IN

: 5V/div

BZ2

VBZ2

INBZ

ENBZ

SYN

ZVS

SYN

ZVS

SYN

ZVS

LINE

COM

RZV

100 k

Ω

ZVS

25 kW

15 k

Ω

SYN

20µs FILTER

The delay between the real Zero Crossing event

and the falling edge of ZVS depends on the inter-

nal filtering time, the resistance RZV, the rectifier

drop voltage VF, the VPS supply load and the tem-

perature. The STCC05 contribution to this delay

can be evaluated by measuring the delay between

its input voltage VTF and its output voltage VZVS.

When using VF = 0.8V, RZV = 15k

Ω, VPS = 15V,

ICC = 20mA, it is about 50 µs on rising voltage VTF

and 115 µs on falling voltage VTF.

When the power supply uses a switch mode power

supply, the input pin SYN is synchronized by an

opto-coupler, which is connected to the mains ter-

minals through high resistances. The isolator out-

put is on all the time except during the zero

crossing where no more current feeds the input

and the output transistor switches off.

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