Variable speed DC fan motor driver THMC40/41 and its application

    Abstract: THMC40/41 is a special driver that can drive and control two-phase adjustable speed 12V brushless DC motors. The article introduces the functional characteristics of THMC40/41, describes in detail the structure, pin functions and limit parameters of the driver, and finally gives their application circuits.

    Keywords: Variable speed fan motor drive THMC40/41

1 Functional features of THMC40/41

THMC40/41 is a two-phase DC fan brushless motor driver; it can be used to control 12V cooling fans that require PRM rate control. This drive has the following features:

●The DC fan can be speed controlled without external power drive, and can reduce the number of components, save space and reduce costs.

●Adopts a unique method of adjusting the motor winding power level to control the fan speed.

●By changing the operating cycle of the VPWM voltage, the fan speed can be adjusted by PWM control within the range of 11% to 100%.

●Low power consumption, the typical current value in sleep mode is only 300μA.

●With shutdown and automatic retry functions, it can effectively protect the locked rotor.

●With speed signal or locked rotor detection signal output.

●Hall sensor signal conditioning with low noise.

●With thermal shutdown protection function, it will automatically shut down when the chip junction temperature exceeds 165℃.

2 Internal structure and pin functions

2.1 Internal structure

The THMC40/41 driver internally consists of a 23kHz PWM oscillator, PWM generator, tachometer, high-side gate driver, low-side gate driver, Hall sensing comparator, start-up and sleep detection circuit, locked rotor detection and automatic restart circuit , thermal management circuit, high and low-side switches, synchronous switching circuit and power supply circuit, etc. The internal structure block diagram is shown in Figure 1.

The difference between the two chips of THMC40/41 is that the function of pin 2 is different. The pin 2 (TACH) output of THMC40 is an open-drain tachometer signal output. It can be used to monitor and measure the actual speed of the fan; the pin 2 (RD) output of THMC41 is an open-drain motor rotor lock detection output, which can be used as an alarm when the motor is locked. However, both drivers set up high- and low-end PW with RDS=0.4Ω and a rated current of 1A for the motor winding, which is the same as the DMOS drive circuit, and are connected from pin 5 (VOUT), pin 11 (PHA) and pin 10 (PHB). output, therefore, no external power drive circuit is required. The THMC40/41 controls fan speed by adjusting the power level of the motor windings.

2.2 Pin arrangement and functions

THMC40/41 adopts a dual-row 14-pin package. Its pin arrangement is shown in Figure 2. The function description of each pin is as follows:

Pin 1 (Cosc): External oscillation capacitor connection terminal, which may be used to design the frequency of the PWM oscillator. The internal 180μA current source and current sink connected to pin 1 are used to charge and discharge the oscillator capacitor to generate a triangular wave voltage signal from 0.5 to 2.3V on pin 1.

Pin 2 (TACH): Open-drain tachometer signal output (THMC40 only), which can be used to monitor and measure the actual speed of the fan. For THMC41, its pin 2 (RD) is the open-drain motor rotor lock detection output, which can be used to alarm when the motor is locked. The corresponding relationship between the pin 2 (TACH) status of THMC40 and the input status of pin 13 (H+), pin 12 (H-), pin 11 (PHA) and pin 10 (PHB) is as listed in Table 1.

Table 1 Correspondence between pin 2 and pins 10 to 13 of THMC40

Pin 3 (CP): External charge storage capacitor connection terminal, used to generate the gate drive voltage required for the VOUT output high-end DMOS tube.

Pin 4 (VPWR): Driver supply voltage input terminal.

Pin 5 (VOUT): High-end PWM drive output of the motor winding.

Pin 6 (NC) and pin 8 (NC) are empty ends.

Pin 7 (PGND) and pin (AGND): are the power ground and module ground of the driver respectively.

Pin 10 (PHB), pin 11 (PHA): are the low-end drive pins of the motor B-phase and A-phase windings respectively. Can be used to drive commutation (controllable based on rotor position monitored by a Hall effect position sensor).

Pin 13 (H+), pin 12 (H-): They are the positive and negative input pins of the sensing comparator respectively.

Pin 14 (VPWM): PWM duty cycle control voltage input terminal. The control signal can be a DC voltage signal or a digital PWM control signal from the super I/O chip on the PC motherboard. But under the latter condition, an RC filter consisting of a 10kΩ resistor and a 0.1μF capacitor should be inserted between the speed control line and VPWM, as shown in Figure 3.

3 Electrical parameters

The main electrical parameters of THMC40/41 are as follows:

●Maximum input voltage VPWM: 18V;

●High-end PWM drive output maximum voltage VOUT: 18V;

●Low-side driving voltage VPHA, VPHB: 40V;

●Sensor comparator sensing input voltage VH+, VH-: 7V;

●PWM duty cycle control input voltage VPWM: 7V;

●Open-drain tachometer signal output voltage V7ACH (THMC40 only): 7V;

●Open-drain motor rotor lock detection output voltage VRD (THMC41 only): 7C;

●Oscillation capacitor voltage Vcosc: 7V;

●Charge pump capacitor voltage VCP: 30V;

●High-end PWM drive output current IOUT: 1.5A;

●Low-end PWM drive output current IPHA, IPHB: 1.5A;

●Power consumption PD: 1022mW;

●Working temperature range Tc: -30～+80℃;

●Storage temperature range Tstg: -55～+150℃;

●Maximum junction temperature Tj: 150℃.

●Welding temperature (10s) TLEAD: 300℃. 4 applications

THMC40/41 is mainly used for driving variable speed brushless DC fan motors. Its application areas include desktop computers, workstations, servers, printers, laboratory testing instruments, and laboratory power supplies.

Figure 4 is the application circuit schematic diagram of THMC40/41. In the figure, the 12V DC voltage is added to pin 4 (VPWR) of THMC40/41 through diode CR1 as its operating voltage, and its operating current is about 5μA. The capacitor C4 connected to pin 4 is a bypass capacitor, and the function of C5 is to eliminate the current spike that appears on the input power supply when the motor commutates. C2 is the PWM oscillator frequency setting capacitor, and C3 is the charge pump energy storage capacitor.

By applying the high-end drive output of pin 5 (VOUT) of THMC40/41 to the common terminal L of the motor winding, the PWM signal generated within the chip can be used to provide a controllable power to the motor, thereby controlling the motor's speed. When the PWM drive signal is on, the motor current increases; during the PWM drive signal off period, the motor current decreases. When the rate control input on pin 14 (VPWM) of THMC40/41 is open, the fan operation can be controlled by a 1~10MΩ resistor. The VPWM voltage can be used to determine the duty cycle of the PWM waveform output on VOUT. VPWM can be either a DC control voltage input or a digital control input. Compare the VPWM voltage with the triangular wave voltage generated on pin 1 (Cosc), so that the output duty cycle of the PWM comparator inside the chip is proportional to the voltage VPWM. When the voltage VPW on pin 14 is <0.7V, THMC40/41 is in sleep state; when VPWM changes between 0.7~2.5V, its PWM duty cycle will also change accordingly in the range of 11%~100%. As a result, the fan speed changes accordingly.