Bidirectional current sampling
In the conventional stepping drive, an external sense resistor is connected between the source and the ground of each H-bridge, and only the forward voltage on the sense resistor when the PWM is turned on is measured. In slow decay mode, current loops through the internal MOSFET and does not pass through the sense resistor, so current cannot be measured. In fast decay mode, the current through the resistor is reversed, producing a negative voltage. For current power IC processes, negative voltages are difficult to sample with simple processing.
If we can monitor the winding current during the current decay period, many stepper motor driven current regulation problems can be solved. However, as described above, it is difficult to achieve by an external sense resistor, and a better choice is to try internal current detection. Internal current sensing allows current to be monitored at any time, such as PWM on-time, as well as fast decay and slow decay. Although it increases the complexity of the driver IC, internal current sensing greatly reduces system cost because external sampling resistors are not required. These resistors are very large and expensive, and the price is usually about the same as the driver IC!
MP6500 stepper driver IC
MP6500 bipolar stepper motor driver chip, integrated internal current detection, is a good replacement for the traditional low-cost peak current control bipolar stepper motor driver IC. The internal circuit block diagram of the MP6500 is shown in Figure 5.
The MP6500 has a maximum drive current peak of 2.5A (depending on package and PCB design); the supply voltage ranges from 4.5V to 35V. Supports full step, half step, quarter step, and eighth step drive mode. No external current sense resistor is required, only a small, low-power resistor with grounding is required to set the peak current of the winding.
The internal current detection relies on the precise matching design of the power tube and related circuits to ensure that the winding current is always accurately sampled, thereby improving the running quality of the stepper motor.
Normally, the MP6500 works in slow decay mode. However, when a fixed off time is over and the slow decay is over, if the current winding current is still above the expected level, the fast decay mode is turned on to quickly reduce the drive current to the desired value. This hybrid control mode allows the drive current to quickly drop to zero while ensuring that the average current is as close as possible to the setpoint. When the step jumps, fast decay is used so that the current current is quickly adjusted to zero, as shown in Figure 6.
If the supply voltage is high, the inductance is low, or the required peak current amplitude is low, the current is likely to be higher than the set value. Due to blankingtime, there is a minimum on-time for each PWM cycle, and many conventional stepper motor drivers cannot control the winding current. If this happens, the MP6500 will continue to use the fast decay mode to ensure that the winding current never exceeds the set value (see Figure 7).
