Datasheet A4988 (Allegro) - 7

DMOS Microstepping Driver A4988 with Translator and Overcurrent Protection FUNCTIONAL DESCRIPTION Device Operation.
The A4988 is a complete microstepping
Mixed Decay Operation.
The bridge operates in Mixed motor driver with a built-in translator for easy operation with decay mode, at power-on and reset, and during normal running minimal control lines. It is designed to operate bipolar stepper according to the ROSC configuration and the step sequence, as motors in full-, half-, quarter-, eighth, and sixteenth-step modes. shown in Figures 9 through 13. During Mixed decay mode, when The currents in each of the two output full-bridges and all of the the trip point is reached, the A4988 initially goes into a fast decay N-channel DMOS FETs are regulated with fixed off-time PWM interval for 31.25% of the off-time, tOFF . After that, it switches (pulse-width modulated) control circuitry. At each step, the cur- to slow decay for the remainder of tOFF. A timing diagram for this rent for each full-bridge is set by the value of its external current- feature appears on the next page. sense resistor (RS1 and RS2), a reference voltage (VREF), and the output voltage of its DAC (which in turn is controlled by the Typically, mixed decay is only necessary when the current in the output of the translator). winding is going from a higher value to a lower value as determined by the state of the translator. For most loads, automatically selected At power-on or reset, the translator sets the DACs and the phase mixed decay is convenient because it minimizes ripple when the current polarity to the initial Home state (shown in Figures 9 current is rising and prevents missed steps when the current is falling. through 13), and the current regulator to Mixed decay mode for For some applications where microstepping at very low speeds is both phases. When a step command signal occurs on the STEP necessary, the lack of back EMF in the winding causes the current to input, the translator automatically sequences the DACs to the increase in the load quickly, resulting in missed steps. This is shown next level and current polarity. (See Table 2 for the current-level in Figure 2. By pulling the ROSC pin to ground, mixed decay is set sequence.) The microstep resolution is set by the combined effect to be active 100% of the time, for both rising and falling currents, and of the MSx inputs, as shown in Table 1. prevents missed steps as shown in Figure 3. If this is not an issue, it When stepping, if the new output levels of the DACs are lower is recommended that automatically selected mixed decay be used, than their previous output levels, then the decay mode for the because it will produce reduced ripple currents. Refer to the Fixed active full-bridge is set to Mixed. If the new output levels of the Off-Time section for details. DACs are higher than or equal to their previous levels, then the
Low Current Microstepping.
Intended for applications decay mode for the active full-bridge is set to Slow. This auto- where the minimum on-time prevents the output current from matic current decay selection improves microstepping perfor- regulating to the programmed current level at low current steps. mance by reducing the distortion of the current waveform that To prevent this, the device can be set to operate in Mixed decay results from the back EMF of the motor. mode on both rising and falling portions of the current waveform.
Microstep Select (MSx).
The microstep resolution is set by This feature is implemented by shorting the ROSC pin to ground. the voltage on logic inputs MSx, as shown in Table 1. The MS1 and In this state, the off-time is internally set to 30 µs. MS3 pins have a 100 kΩ pull-down resistance, and the MS2 pin has a 50 kΩ pull-down resistance. When changing the step mode,
Reset Input (¯R ¯E ¯S ¯E ¯T ).
The ¯R ¯E ¯S ¯E ¯T input sets the translator the change does not take effect until the next STEP rising edge. to a predefined Home state (shown in Figures 9 through 13), and If the step mode is changed without a translator reset, and abso- turns off all of the FET outputs. All STEP inputs are ignored until lute position must be maintained, it is important to change the the ¯R ¯E ¯S ¯E ¯T input is set to high. 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,
Step Input (STEP).
A low-to-high transition on the STEP or reset due to TSD or an overcurrent event, the translator is set to input sequences the translator and advances the motor one incre- the home position which is by default common to all step modes. ment. The translator controls the input to the DACs and the direc- 7 Allegro MicroSystems 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com