STCC05-B datasheet(4/13 Pages) STMICROELECTRONICS

4 / 13 page

STCC05-B

4/13

■ The buzzer driver with enable control

The MCU can excite a warning buzzer with a 50% PWM signal. The buzzer driver amplifies this signal in

current and translates it from the 5V MCU output to the VPS supply to produce the right sound level from

the buzzer.

The output stage is made of a NPN transistor, a PNP transistor and two 1k

Ω resistors.

The NPN transistor, referred to the power ground COM, is controlled by the input INBZ; its collector is con-

nected to the output BZ1. The input INBZ is driven by a simple push-pull MCU buffer.

The PNP transistor, referred to the VPS polarity, is controlled by the input ENBZ; and its collector is con-

nected to the output BZ2 through a 1k

Ω resistor. The input ENBZ is driven by a simple push-pull MCU

buffer.

The pin BZ2 is the supply terminal of the buzzer; and the circuit has a DC current rating of 9mA and the

PWM section runs from 10Hz up to 5kHz.

A 1k

Ω resistor RBZ is connected between the BZ1 and BZ2 pins to discharge the buzzer periodically. More-

over, the addition of an external capacitor-resistor network on BZ2 pin will allow the buzzer to turn on and

off smoothly when the pin ENBZ is toggling.

■ The speed sensor level shifter

The OUTS signal is generated by an electronic signal such as the indoor fan speed clock issued of a Hall

Effect sensor or a door switch signal and is transmitted to the MCU. As the INS input may be disturbed; a

spike suppressor and a simple EMI filter are added to increase the input robustness. The output signal

OUTS is not inverted with respect to the input signal INS.

Demagnetizing Diode

Relay

Transistor

10 k

Ω

OH =1kΩ

BZ= 1kΩ

10 k

Ω

OUTS

INS

EMI

Filter

50 k

Ω

500

Ω

50 k

Ω

50 k

Ω

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