Stepping Into The Future

Bob Christie examines the EasyStepper concept for driving bipolar stepper motors using an integrated step and direction translator interface.

A built-in translator makes devices incorporating EasyStepper very easy to use. The user has simply to input one pulse on the device's "step" input and the motor will take one microstep. There are no phase sequence tables, high-frequency control lines ,or complex interfaces to program. This simple, two-input interface is ideal for applications involving multiple motors.

Most microstepping motor drivers require control lines for digital-to-analogue converters (DACs) to set the reference for the pulse-width-modulated (PWM) current regulator and phase inputs for current polarity control. In more sophisticated drivers, there are also inputs required for the PWM current-control mode to operate in slow-, fast-, or mixed-decay modes. Thus, up to eight to 12 inputs might be involved, all of which are typically supplied by the system microprocessor. The EasyStepper concept solves this problem by combining a simple two-line step and direction interface and an efficient DMOS output in a single integrated circuit (Figs. 1 and 2). For each transition in the step input, the driver sequences one microstep.

These new ICs also include circuitry that automatically sets the current-decay mode between slow and mixed-decay PWM operation. This eliminates the need to provide additional control lines and will result in reduced audible noise.

The DMOS outputs are n-channel devices with low onresistance rated at ±2.5A and 35V, which means that they will satisfy high-end applications requiring low power dissipation. DMOS outputs also have the ability to implement synchronous rectification by turning on the appropriate output DMOS device during the current decay period and effectively shorting out the body diodes with the low on-resistance driver. This results in lower power dissipation and eliminates the need for external Schottky diodes in most applications.

EasyStepper is based on the translator circuit, which can be seen on the left-hand side of Figure 1. This circuit converts the step and direction inputs into the control signals required to sequence the current in each of the two H-bridge outputs for full, half, quarter, and eighth-step microstepping operation of a bipolar stepper motor.

At power-up or reset, the translator sets the DACs and phase current polarity to the initial "home" state conditions, and sets the current regulator for both phases to mixed-decay mode. When a step command signal (logic "low" to "high" transition of the step input) occurs, the translator automatically sequences the DACs to the next level and current polarity. For the reverse operation, the direction input is set to logic "high" and the translator reverses the sequence. The DAC outputs are used by the PWM current regulator to set the trip point of the current output of each phase. The microstep resolution is set by inputs MS1 and MS2.

Each H-bridge is controlled by a fixed "off" time PWM current-control circuit which limits the load current to a desired value (ITRIP). Initially, a diagonal pair of source and sink DMOS outputs are enabled, and current flows through the motor winding and the current-sense resistor (RS) as shown in Fig 3. When the voltage across RS equals the DAC output voltage, the current-sense comparator resets the PWM latch, which turns off the source drivers (slow-decay mode) or both the source and sink drivers. The current then recirculates. During this recirculation, the current decreases until the fixed "off" time expires.

The maximum value of current limiting is set by the selection of the current-sense resistor and the voltage at the reference input. The DAC output reduces the reference output voltage to the current-sense comparator in precise steps which are dependent ITRIP. The internal PWM current-control circuitry uses a one-shot circuit to control the time the drivers remain off.

In addition to setting the fixed "off" time of the PWM control circuit, the external capacitor sets the comparator blanking time. This function blanks the output of the current-sense comparator when the outputs are switched by the internal current-control circuitry. The comparator output is blanked to prevent false overcurrent detections caused by reverserecovery currents in the clamp diodes, as well as switching transients related to the capacitance of the load. This blanking feature eliminates the need for a low-pass filter between the current-sense resistor and the current-sense comparator that is required on most PWM current regulators.

Automatic mixed decay-operation optimises the current chopping mode in order to achieve the best sinusoidal current waveform for microstepping. Slow decay has the advantage of minimum current ripple. However, when microstepping at higher step rates, slow-decay chopping may fail to properly regulate current on the falling slope of the sine wave when current is decreasing. This is a result of motor back EMF overriding the voltage applied to the motor, forcing the current to increase during the decay period. Fast decay solves the current-regulation problem of slow decay. With almost the full supply across the motor winding, it has the ability to get the current out of the winding quickly. The disadvantage of fast decay is increased current ripple, which in turn causes increased motor heating.

When the current reaches ITRIP, the device will go into fastdecay mode until the voltage on the RC terminal decays to the voltage on the PFD terminal. After this fast-decay portion, the device will switch to slow-decay mode for the remainder of the fixed "off" time period.

Although mixed decay improves microstepping performance, it will still have higher current ripple than slow decay. The best solution is to use a slow decay on the increasing slope of the sine wave and mixed decay on the falling slope of the sine-wave output, which the EasyStepper devices do automatically. When a step-command signal occurs on the step input, the translator automatically sequences the DACs to the next level. If the new DAC output level is lower than the previous level, the decay mode for that H-bridge will be set by the voltage level on the PFD input. If the new DAC level is equal to or higher than the previous level, the decay mode for that Hbridge will be slow decay.

When a PWM "off" cycle is triggered—by a bridge disable command or an internal fixed "off" time cycle—load current will recirculate according to the decay mode selected by the control logic. Synchronous rectification will turn on the appropriate DMOS devices during the current decay and effectively short out the body diodes with the low 'on' resistance driver (Fig. 4). In fast-decay synchronous rectification mode, the voltage across RS is monitored to prevent reverse conduction. Just before the recirculation current reaches zero, all of the DMOS devices are turned off, and current flows through the body diodes. In a typical stepper-motor application, the motor-driver IC is in current-decay (recirculation) mode for a higher percentage of the PWM cycle compared to the on time. This means that most of the power dissipation is a result of the forward-voltage drop of the internal body diode of the power DMOS. The reduction in power dissipation resulting from the use of synchronous rectification can eliminate the need for external Schottky diodes in most steppermotor applications.

In EasyStepper devices, a "sleep" input is used. This disables much of the internal circuitry, including the output DMOS device, regulator, and charge pump. A logic "low" will put the device into sleep mode, while a logic "high" will allow normal operation and will start up the device in the "home" position. The total current consumption in sleep mode (including logic and the motor supply current) is less than 40mA.

The "enable" and "reset" inputs turn the DMOS outputs on or off. The translator inputs are independent of the "enable" input state, so that the outputs can be disabled, stepped to a defined microstep state, and then re-enabled in this position. The "reset" input resets the translator to the home state.

The "home" output is a logic output indicator of the initial state of the translator. At power-up, the translator is reset to the home state. The home output-current level is common to all four microstepping levels. It can be used as a control input to indicate that the microstepping resolution is able to be changed at this step without causing a current (and, therefore, torque) disturbance to the motor.

The EasyStepper design has an undervoltage lockout circuit that protects the IC from potential shoot-through currents when the motor supply voltage is applied before the logic supply voltage. All outputs are disabled until the logic supply voltage is above 2.7V. Thermal protection circuitry turns off all of the power outputs if the junction temperature exceeds 165°C. Normal operation is resumed when the junction temperature has decreased by about 15°C.

Microstepping-motor-driver ICs using the EasyStepper integrated step and direction translator interface offer several features that result in benefits for the application. When this simple interface is combined with low power dissipation, high-current outputs, and efficient mixed-mode current control, the result is a high-performance and cost-effective solution for the next generation of stepper-motor drivers.

TAGS: Components
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