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A4984 Datasheet(PDF) 7 Page - Allegro MicroSystems |
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A4984 Datasheet(HTML) 7 Page - Allegro MicroSystems |
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7 / 22 page  DMOS Microstepping Driver with Translator And Overcurrent Protection A4984 7 Allegro MicroSystems, Inc. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 www.allegromicro.com Device Operation. The A4984 is a complete microstep- ping motor driver with a built-in translator for easy operation with minimal control lines. It is designed to operate bipolar stepper motors in full-, half-, quarter-, and eighth-step modes. The currents in each of the two output full-bridges and all of the N-channel DMOS FETs are regulated with fixed off-time PWM (pulse width modulated) control circuitry. At each step, the current for each full-bridge is set by the value of its external current-sense resistor (RS1 and RS2), a reference voltage (VREF), and the output voltage of its DAC (which in turn is controlled by the output of the translator). At power-on or reset, the translator sets the DACs and the phase current polarity to the initial Home state (shown in figures 8 through 11), and the current regulator to Mixed Decay Mode for both phases. When a step command signal occurs on the STEP input, the translator automatically sequences the DACs to the next level and current polarity. (See table 2 for the current-level sequence.) The microstep resolution is set by the combined effect of the MSx inputs, as shown in table 1. When stepping, if the new output levels of the DACs are lower than their previous output levels, then the decay mode for the active full-bridge is set to Mixed. If the new output levels of the DACs are higher than or equal to their previous levels, then the decay mode for the active full-bridge is set to Slow. This auto- matic current decay selection improves microstepping perfor- mance by reducing the distortion of the current waveform that results from the back EMF of the motor. Microstep Select (MSx). The microstep resolution is set by the voltage on logic inputs MSx, as shown in table 1. The MS1 pin has a 100 kΩ pull-down resistance, and the MS2 pin has a 50 kΩ pull-down resistance. When changing the step mode the change does not take effect until the next STEP rising edge. If the step mode is changed without a translator reset, and abso- lute position must be maintained, it is important to change the step mode at a step position that is common to both step modes in order to avoid missing steps. When the device is powered down, or reset due to TSD or an over current event the translator is set to the home position which is by default common to all step modes. Mixed Decay Operation. The bridge operates in Mixed decay mode, at power-on and reset, and during normal running according to the ROSC configuration and the step sequence, as shown in figures 8 through 11. During Mixed decay, when the trip point is reached, the A4984 initially goes into a fast decay mode for 31.25% of the off-time, tOFF. After that, it switches to Slow decay mode for the remainder of tOFF. A timing diagram for this feature appears on the next page. Typically, mixed decay is only necessary when the current in the winding is going from a higher value to a lower value as determined by the state of the translator. For most loads automatically-selected mixed decay is convenient because it minimizes ripple when the current is rising and prevents missed steps when the current is falling. For some applications where microstepping at very low speeds is necessary, the lack of back EMF in the winding causes the current to increase in the load quickly, resulting in missed steps. This is shown in figure 2. By pulling the ROSC pin to ground, mixed decay is set to be active 100% of the time, for both rising and falling currents, and prevents missed steps as shown in figure 3. If this is not an issue, it is recommended that automatically-selected mixed decay be used, because it will produce reduced ripple currents. Refer to the Fixed Off-Time section for details. Low Current Microstepping. Intended for applications where the minimum on-time prevents the output current from regulating to the programmed current level at low current steps. To prevent this, the device can be set to operate in Mixed decay mode on both rising and falling portions of the current waveform. This feature is implemented by shorting the ROSC pin to ground. In this state, the off-time is internally set to 30 μs. Reset Input (¯ R ¯¯ E ¯¯ S ¯¯ E ¯¯ T ¯). The ¯R¯¯E¯¯S¯¯E¯¯T¯ input sets the translator to a predefined Home state (shown in figures 8 through 11), and turns off all of the FET outputs. All STEP inputs are ignored until the ¯R¯¯E¯¯S¯¯E¯¯T¯ input is set to high. Step Input (STEP). A low-to-high transition on the STEP input sequences the translator and advances the motor one incre- ment. The translator controls the input to the DACs and the direc- tion of current flow in each winding. The size of the increment is determined by the combined state of the MSx inputs. Functional Description |
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