5 Changeable Holding Current Circuit

The circuit in Figure 5-1 can realize changeable holding current function by using the CD74HC123. The CD74HC123 is a dual, retriggerable, monostable multivibrator, with reset. In this application, the input STEP comes from the step signal of the motor driver ICs, such as DRV8818. The output V_REF is applied to the V_REF pin of the motor driver ICs to achieve changeable holding current.

According to the principle of CD74HC123, when a pulse is applied to pin 1, there is a high output on pin 13, with a pulse width of 0.45 R_X × C_X starting at the falling edge of the signal on pin 1. When the signal frequency on pin 1 is bigger than 1 / (0.45 R_X × C_X), the high level on pin 13 maintains all the time. Figure 5-2 shows that by this time, the voltage on pin 13 stays high (Vcc), through a potential-divider network, an R4 × Vcc / (R4 + R2 // R3) output voltage is applied to the V_REF pin of the motor driver IC.

When the stepper motor is in holding state, there will be no pulse on the STEP, the level on pin 1 will be low so the output voltage on pin 13 will be driven low (GND), hence the VREF voltage becomes Vcc × (R2 // R4) / (R2 // R4 + R3), as shown in Figure 5-3. In this way, the holding current of the stepper motor can be set by choosing suitable resistors.

So the V_REF can be automatically changed when the stepper motor enters into holding state. Note: to get a good performance of normal operation, adjust the value of RX and CX based on a target STEP signal to achieve a constant high level on pin 13 when the stepper motor is in normal operation. For example, if the frequency of the step signal is 1/15 Hz, that is, there are only four steps in one minute, then 1/15 > 1 / (0.45 RX × CX), so RX × CX must be larger than 33.3, so 10 M can be selected for RX, 10 µF can be selected for C_X.

Figure 5-1 Changeable Holding Current Circuit

Figure 5-2 Output of Normal Operation

Figure 5-3 Output of Holding State