點晶科技股份有限公司

SILICON TOUCH TECHNOLOGY INC.

ST1115C/ST1115D

SP-ST1115CD-A.005

Version

：A.005

Page

：8

3. Fan speed control by the PWM command

―Refer to Figure 5.

VSS

VSS

VSS

VSS

VSS

VSS

VSS

Q1

R4

R3

R1

PWM

R2

10K

75

1u

1u

IC

Hall

ST1115C

C1

RD

Digital Power

GND

Hin

VT

CF

C2

O2

O1

Analog Power

Z2

Z1

D1

RD

VDD

Figure 5. Fan speed control via the PWM command.

(1) Figure 5 shows the application circuit, where the fan speed is directly controlled by the

PWM command. The BJT Q1 together with resistors R1 and R2 constitutes an inverting

level shifter, which is responsible for translating the voltage level of the PWM command

to the VDD voltage level of the drive IC. In this way, the voltage level of the PWM

command can be very low, say, 2 Volt, for example.

(2) For this circuit, the PWM command is active high and its frequency range is 200 Hz ~

40 KHz. If low acoustic noise is desired, the 20 KHz~40 KHz of the PWM frequency

range is recommended.