STCC05-B Datasheet, PDF (10/13 Pages) STMicroelectronics – HOME APPLIANCE CONTROL CIRCUIT

STCC05-B
the voltage to the buzzer, this circuit feeds the buzzer with the exponential voltage induced by the charge
and the discharge of the R_S C_S network.
The R_OH and R_S resistors contribute to reduce high harmonic sound distortions. Indeed, they limit the peak
current through the buzzer, feed the buzzer with the C_S 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 C_S
capacitor.
The integrated R_BZ 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 = R_BZ x C_BUZZER.
Therefore, R_BZ < 1 / (10 x F_MAX x C_BUZZER). Since the buzzer capacitance C_BUZZER is about 20nF at the
maximum operating frequency of driver is 5kHz, this R_BZ resistance is set at 1kΩ.
Figure 15: Buzzer terminal voltages V_BZ1 & V_BZ2
and buzzer current I_BZ
Figure 16: Buzzer terminal voltage V_BZ2 with
buzzer enable and input circuit signals
VBZ1
VBZ2
INBZ
VBZ2
IBZ
ENBZ
Time : 100µs/div , VBZ1 & VBZ2 : 4V/div , IBZ : 20mA/div
Time : 100ms/div , VBZ2 , ENBZ & INBZ : 5V/div
■ 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 R_ZV, the electrical path being closed by the low side bridge diodes.
Figure 17: ZVS circuit operation using the AC
secondary of a transformer
V_TF
V_SYN
V_ZVS
V_AC
VDD
20µs FILTER
15 kΩ ^RZV
SYN
25 kW
AC
LINE
V_SYN
100 kΩ
S₁
Q
S2
ZVS
V_ZVS
V_TF
COM
The delay between the real Zero Crossing event
and the falling edge of ZVS depends on the inter-
nal filtering time, the resistance R_ZV, the rectifier
drop voltage V_F, the V_PS supply load and the tem-
perature. The STCC05 contribution to this delay
can be evaluated by measuring the delay between
its input voltage V_TF and its output voltage V_ZVS.
When using V_F = 0.8V, R_ZV = 15kΩ, V_PS = 15V,
I_CC = 20mA, it is about 50 µs on rising voltage V_TF
and 115 µs on falling voltage V_TF.
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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