Features: 750 mA continuous output current; 45 V output holding voltage; internal clamp diode; internal PWM current control; low output saturation voltage; internal thermal shutdown circuit; half-step or quarter-step for bipolar stepper motors Step operated dual full bridge pwm motor driver.

describe

The A2919 motor driver is designed to drive two windings of a bipolar stepper motor or bidirectionally control two DC motors. Both bridges are capable of sustaining 45v and include internal pulse width modulation (pwm) to control the output current to 750ma. The output has been optimized for low output saturation voltage drop (less than 1.8v total supply plus 500mA sink).

For pwm current control, the maximum output current is determined by the user-selected reference voltage and sense resistor. Two logic level inputs select output current limit of 0%, 41%, 67% or 100 % of maximum level. The phase input of each bridge determines the direction of the load current.

These bridges include ground clamps and flyback diodes to prevent induced transients. When switching current directions, an internally generated delay prevents cross currents. No special power-up sequence is required. Thermal protection circuitry disables the output if the die temperature exceeds the safe operating limit.

The A2919 is available in a 24-pin surface mount SOICW with as little loss as possible. A2919 is a heat sink, which improves power dissipation capability. Operating in temperatures from -20°C to 85°C, as well as +125°C, this batwing configuration provides maximum unit power.

application information

pwm current control

The A2919 dual bridge is designed to drive both windings of a bipolar stepper motor. The output current is sensed and controlled independently in each bridge by an external sense resistor (R), an internal comparator, and a monostable multivibrator.

When the bridge is turned on, the current in the motor windings increases and flows through the external sense resistor until the sensed voltage (V) reaches the level set by the comparator input: ITRIP = VREF/10 RS

The comparator then triggers the monostable, which shuts down the bridge's source driver. Due to internal logic and switching delays, the actual load current peak will be slightly above the trip point (especially for low inductance loads). This delay (t) is typically 2 μs. After shutdown, the motor current decays and circulates through the ground clamp diode and receiver transistor. The source driver off time (and therefore the magnitude of the current reduction) is determined by the monostable external RC timing element, where t is in the range of 20 kΩ to 100 kΩ and 100 pf to 1000 pf.

The fixed off time should be short enough to keep the current chopping above the audible range (<46 μs) and long enough to properly regulate the current. Since only slow decay current control is available, short off-times (<10 μs) require extra effort to ensure proper current regulation. Factors that can negatively impact the ability to properly regulate current when using short circuit time include: higher motor supply voltages, light loads, and longer than necessary blank times.

When the source driver is re-enabled, the winding current (induced voltage) is again allowed to rise to the comparator's threshold. This cycle repeats to keep the average current in the motor windings at the desired level.

Loads with high distributed capacitance can cause current spikes that can trip the comparator, resulting in erroneous current control. An external RC delay is applied to delay the action of the comparator. Depending on the load type, many applications do not require these external components (sensing connected to E).

Logic control of output current

Two logic level inputs (L and I) allow digital selection of motor winding current at 100%, 67%, 41% or 0% of the maximum level in each meter. The 0% output current condition turns off all drivers in the bridge and can be used as an output enable function.

These logic level inputs greatly enhance the implementation of the µp control drive format.

During half-step operation, L and L allow the μP to control the motor with constant torque between all positions in an eight-step sequence. This is done by digitally selecting 100% drive current when only one phase is on and 67% drive current when both phases are on. A logic high on L and L turns off all drivers to allow fast current decay.

During quarter-step operation, I and I allowed the µP to control the motor position in a sixteen-step sequence. This is accomplished by digitally selecting the drive current as shown in the table (for one operating quadrant). Me and I have all drivers turned off to allow for fast current decay.

A logic control input can also be used to select a reduced current level (and reduced power consumption) for "hold" conditions and/or increased current (and available torque) for start-up conditions.

generally

The phase input of each bridge determines the direction of the motor winding current. An internally generated dead time (~2µs) prevents cross currents when switching phase inputs.

All four drivers in the bridge output can be turned off between steps (L=l_2.4V), resulting in fast current decay through internal output clamps and feedback diodes. In half-step and high-speed applications, fast current decay is required. Phase, L, and I inputs are high.

Varying the reference voltage (V) provides continuous control of peak load current for microstepping applications. Thermal protection circuitry shuts down all drivers when the junction temperature reaches +170°C. It is only used to protect the device from faults caused by excessive junction temperature and should not be meant to allow output short circuits. The output drivers are re-enabled when the junction temperature drops to +145°C.

The A2919 output driver is optimized for low output saturation voltages less than 1.8V total (source plus sink) at 500mA. Under normal operating conditions, when combined with the excellent thermal performance of the batwing assembly design, this allows both bridges to operate simultaneously and continuously at 500 mA.