Application of TL598 in PWM speed regulation of small DC motors

    Abstract: This article introduces the characteristics and usage of the TL598 chip, and proposes a method for designing a motor PWM control system based on the TL598 chip. The circuit diagram of the motor position follow-up system circuit and motor speed regulation system designed using TL598 is given, and the pulse generation program written with MCS51 command is given. The two circuits introduced in this article are cheap, highly reliable, and can easily realize the interface between TL598 and the computer.

    Keywords: TL598 PWM motor speed regulation

1 Introduction

TL598 is a switching power supply pulse width modulator integrated circuit produced by Texas Instruments. It can be used for pulse width modulation of DC motors to adjust current and speed. It can also use a chip to achieve T-shaped bipolar mode PWM control. Or use two chips to implement H-type bipolar mode PWM control. Compared with dedicated PWM integrated circuits for DC motors, TL598 is cheaper, easier to purchase, and has little functional difference. It solves the problems of speed regulator or current regulator, pulse width modulation, maximum current limit, etc. in one chip. In addition, some additional monitoring and protection functions are also set up in the TL598 chip, so that the TL598 has better working performance, further improves the anti-interference ability and reliability, and makes the system structure more concise and the space reduction is greater.

2 Features of TL598

Figure 1 shows the functional block diagram of LT598, and Figure 2 shows the pinout diagram of TL598 with the suffix P or N.

TL598 has a total of 16 pins. It integrates two error amplifiers, an internal oscillator (externally programmable), a PWM comparator, a dead-time control comparator, an edge pulse flip-flop, and a 5V precision voltage reference. Undervoltage self-locking control, output control circuit and output circuit. Pins 1, 2 and 16 and 15 are two pairs of non-inverting input terminals and inverting input terminals of the two error amplifiers respectively. The outputs of the two amplifiers appear on pin 3 (FEEDBACK) after an "OR" operation. In motor speed regulation, these five pins can be used to achieve feedback control.

Pin 6 (RT) and pin 5 (CT) are respectively connected to external resistor RT and capacitor CT. They can be used together to determine the frequency of the triangular wave on the CT pin, thereby determining the frequency of the output PWM: When the two output terminals are in parallel mode, the PWM The frequency is the triangular wave frequency; when the two output terminals are in push-pull mode, the PWM frequency is half of the triangular wave frequency. The relationship between the triangular wave frequency and RT and CT is fo=1/(2RT CT). The recommended operating frequency range of TL598 is 1kHz ~ 300kHz. This wide frequency range allows us to design the PWM control system of the motor with ease.

In order to increase the output power, multiple TL598 chips can be connected in parallel by connecting the RT pin of each chip to pin 14 (REF) and the CT pin to the common triangle wave signal source. 5V REF is generated by the internal voltage stabilizing circuit. On the one hand, it serves as the power supply for the internal control circuit (comparator, flip-flop, etc.). On the other hand, it can be derived from pin 14 as the reference voltage for the external control circuit. REF can provide 10mA current to the outside and has short-circuit protection to ground. The relationship between REF and Vcc is shown in Figure 3. Pin 13 of TL598 (output control terminal) is used to select whether the output of the two output terminals (8th pin OUT1 and 9th pin OUT2) is in parallel mode or push-pull mode. Its functions are listed in Table 1. When the output terminal is in push-pull mode, OUT1 and OUT2 are turned on in turn during two adjacent triangle wave cycles, and the maximum output pulse ratio is about 50%; when the output terminal is in parallel mode, the output of the two output terminals is exactly the same. Its maximum duty cycle is approximately 97%. As can be seen from Figure 1, the output port structure of TL598 is in the form of a totem pole. When in parallel mode, OUT1 and OUT2 are equivalent to a half bridge, so it is very convenient to use TL598 to design a PWM speed control system. Moreover, the internal output logic circuit of TL598 ensures that there will be no shoot-through problem (simultaneous conduction of two power tubes on the same side of the bridge arm will cause the power supply to be short-circuited to ground and damage the device). When Vcc=40V and the output current of each channel is 200mA, the TL598 can still work normally. That is to say, for a motor with an operating voltage of 36V, if both channels provide a maximum output of 400mA, the driving power can reach 14.4W. . If the power of the motor is greater than the output power of one chip, then two or more chips can be connected in parallel.

Table 1 Functions of OUTPUT CRTL

Pin 5 (dead time comparator input) is used to set the maximum possible duty cycle of the PWM signal, thereby setting the maximum input voltage of the motor. This pin can be used to implement overvoltage protection, overcurrent protection and soft start functions when the motor is working. When working, this management cannot be left floating, otherwise it will cause instability of the PWM signal.

3 applications

3.1 Motor position following system

The position follow-up system circuit composed of TL598 is shown in Figure 4. The driving power supply and control power supply share ±12V. OUT1 and OUT2 adopt parallel output form and form a T-shaped bipolar mode PWM control circuit together with +12V, -12V, ground wire, two fast diodes, and two energy storage capacitors. R1, R4, R2, R13, R7, R8, C4 and error amplifier 2 together form a differential amplifier circuit; R2 is used to set the ideal position of the motor; R13 is used to feedback the actual position of the motor; R1 and R4 are used to limit REF The output current; C4 is used to control the output and generate hysteresis so that the system has sufficient stability margin to avoid instability of the motor positioning. Error amplifier 1 is not used in this circuit. In order to avoid affecting the control output, the non-inverting input terminal is connected to -12V and the reverse input terminal is connected to REF. RT and CT are connected to external 10k Ω resistors and 0.1 μF capacitors respectively. This stabilizes the frequency of the PWM signal at approximately 1kHz.

During operation, if the motor deviates from the set position, the output value of R13 will also deviate from the balance value. Such a difference can change the pulse duty cycle of OUT1 and OUT2 after calculation by the differential amplifier circuit, thereby driving the motor to the correct position.

3.2 Motor speed control system

TL598 and microcontroller can form a motor speed control system. The speed control system shown in Figure 5 uses two integrated chips, GMS97C2051 and TL598. GMS97C2051 is a low-priced 8-bit microcontroller produced by South Korea's Samsung Electronics. Its instruction system is fully compatible with MCS51, and the I/O port has a current drive capability of 20mA. The microcontroller can generate a pulse width signal with an adjustable duty cycle through the interrupt of the internal timer 0, and the signal is input to the optocoupler. After the output signal of the optocoupler passes through the resistor-capacitor low-pass filter network, it is sent to the non-inverting input terminal of the TL598 comparator 1. The comparator is connected as a non-inverting follower. The pulse length emitted by the microcontroller is in the microsecond range, and the motor itself has electrical and mechanical inertia, so the filtered signal will not cause unbalanced fluctuations in the motor's speed. In addition to realizing the power supply isolation between the motor and the microcontroller, the role of the optocoupler is to convert the logic level "1" of the microcontroller (with randomness within a certain size range) into a level value Vref of a certain size. , which will make the motor speed more stable. The following is part of the pulse generation program written in MCS51 assembly language:

………

ORG 000BH

AJMP PULSE

………

PULSE: CLR TR0

………

JB F0,FAN

SETB F0

SETB P1.7

MOV TH0,#Z_V

AJMP PULSE1

FAN; CLR F0

CLR P1.7

MOV TH0,#F_V

PULSE1:SETB TR0

………

RETI

Timer 0 is set to timing mode 1, and TL0 is initialized with the value #00H. F0 is a flag bit used to record whether the previously sent pulse is a positive pulse or a negative pulse. Z_Y and F_V are the correct punch length and negative pulse length respectively.

4 Conclusion

The PWM speed control system of the DC motor designed with the TL598 produced by the American TI Company has the advantages of wide speed range, good speed, small dead time, and high power factor. For small motors with a power of about tens of watts, TL598 Wuying is a good way to implement this system. Due to the integrated ratio of the circuit, many capacity problems that arise in discrete component PWM systems will be easily solved here. Both circuits listed in this article have been verified and have good actual operation results.