Thursday, July 7, 2016

Motor Driver - DRV8834

I am designing a motor driver to drive a stepper motor AM1020-V-6-65. At first, I thought to use A4988 from Allegro. But its supply voltage of 8V - 35V is not suitable for a 6V motor, its chip size 5 mm x 5 mm is bigger than what I want. DRV8835 from TI has appealing 2 mm x 3 mm size but it is meant for DC motor and there is no indexer to support convenient control of stepper motors. Finally, DRV8834 with 4 mm x 4 mm size, motor supply voltage 2.5V - 10.8V, and current 1.5A per coil is chosen.

An example schematic to control DRV8834 in indexer mode with Arduino (using ATMEGA328P-MM) is shown below.

Figure. DRV8834 schematic

Motor supply pin, VM, is recommended to connect with at least 10 μF capacitor. Charge pump pin, VCP is to connect with VM through 0.01 μF capacitor. VREFO is a referenc voltage pin that provides 2V reference. It can be normally used with a potentiometer to provide control voltage to AVREF and BVREF to limit current. In this example, current limit for coil A and coil B is set by connecting AVREF and BVREF pin to 2V VREFO directly. The relationship of voltage Vr at AVREF and BVREF pin, resistance value at AISEN and BISEN pin called Rs, and current limit, Ilim is described below. $$ I_{lim} = \frac{V_r}{5 . R_s} $$ The current Ilim flowing through Rs is large and as a consequence, the power of resistor must be large enough to allow it.

Another thing to note is that the percentage of fast decay of PWM cycle is determined by resistance values at ADECAY and BDECAY pins. In this example, for 25% fast decay, 20 kΩ is used. In indexer mode, only ADECAY pin is used.

The motor will step according to the number of pulses given to step pin. DIR pin controls direction of stepping.

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