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A4979 Datasheet(PDF) 10 Page - Allegro MicroSystems |
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A4979 Datasheet(HTML) 10 Page - Allegro MicroSystems |
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10 / 44 page  Microstepping Programmable Stepper Motor Driver With Stall Detect and Short Circuit Protection A4979 10 Allegro MicroSystems, LLC 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com SCK Serial interface clock. Data is latched in from SDI on the rising edge of the SCK clock signal. There must be 16 rising edges per write and SCK must be held high when STRn changes. STRn Serial data strobe and serial access enable. When STRn is high any activity on SCK or SDI is ignored, and SDO is high impedance allowing multiple SDI slaves to have common SDI, SCK, and SDO connections. DIAG Diagnostic output. Function selected via the serial inter- face, setting Configuration Register 1. Default is Fault output. OSC With bit 13 in Configuration Register 1 set to 0, either con- nect this pin to AGND to use the internal oscillator running at the default frequency of 4 MHz, or connect a resistor to VDD to set the internal oscillator frequency. (The approximate frequency is calculated from: fOSC = 10 000 / (48 ROSC – 20) where fOSC is the internal oscillator frequency in MHz, and ROSC is the value, in kΩ of the resistor between OSC and VDD.) If bit 13 in Configuration Register 1 is set to 1, then OSC is the input for an external system clock, which must have a frequency between 3 and 5 MHz. In this mode a watchdog is provided to detect loss of the system clock. If the OSC pin remains high or low for more than the watchdog time, tWD, 1 μs typical, then the Fault Register flag (bit 15 in the diagnostic registers) is set and the outputs are disabled until the clock restarts. Driving a Stepper Motor A two-phase stepper motor is made to rotate by sequencing the relative currents in each phase. In its simplest form, each phase is simply fully energized in turn by applying a voltage to the winding. For more precise control of the motor torque through temperature and voltage ranges, current control is required. For efficiency this is usually accomplished using pulse width modula- tion (PWM) techniques. In addition current control also allows the relative current in each phase to be controlled, providing more precise control over the motor movement and hence improve- ments in torque ripple and mechanical noise. Further details of stepper motor control are provided in Appendix A. For bipolar stepper motors the current direction is significant, so the voltage applied to each phase must be reversible. This requires the use of a full bridge (also known as an H-bridge) which can switch each phase connection to supply or to ground. Phase Current Control In the A4979, current to each phase of the two-phase bipolar stepper motor is controlled through a low impedance N-channel DMOS full bridge. This allows efficient and precise control of the phase current using PWM switching. The full-bridge con- figuration provides full control over the current direction during the PWM on-time, and over the current decay mode during the PWM off-time. Due to the flexibility of the A4979 these control techniques can be completely transparent to the user or can be partially- or fully-programmed through the serial interface. Each leg (high-side, low-side pair) of a bridge is protected from shoot-through by a fixed dead time. This is the time between switching off one FET and switching on the complementary FET. Cross-conduction is prevented by lock-out logic in each driver pair. The phase currents and in particular the relative phase currents are defined in the Phase Current table (table 7). This table defines the two phase currents at each microstep position. For each of the two phases, the currents are measured using a sense resistor, RS, with voltage feedback to the respective SENSx pin. The target current level is defined by the voltage from the digital-to-analog converter (DAC) for that phase. The sense voltage is amplified by a fixed gain and compared to the output of the DAC. There are two types of maximum current: the absolute maximum, ISMAX, the maximum possible current defined by the sense resis- tor and the reference input; and the phase maximum, IPMAX, the maximum current delivered to a motor phase. The absolute maximum current, ISMAX, is defined as: ISMAX = VREF / (16 × RS) where VREF is the voltage at the REF pin, and RS is the sense resistor value. The phase maximum, IPMAX, is the 100% reference level for the phase current table and may be a fraction of the absolute maxi- mum current, ISMAX, depending on the value of the MXI0 and MXI1 bits in Configuration Register 0. |
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