Is it OK to connect capacitors to the output lines of a motor driver IC?

As a measure against EMI, it is possible to connect capacitors to the output lines of a motor driver IC.

Fig. 1 Example of a circuit with capacitors connected to the output lines
Fig. 1 Example of a circuit with capacitors connected to the output lines

EMI (Electromagnetic Interference) occurs when electromagnetic noise is emitted from electronic components such as semiconductors, causing malfunctions in external electronic parts or systems. Connecting capacitors to the output lines can help suppress the generation of electromagnetic noise.

Fig. 2 Example for slew rate waveform of TB67S539FTG
Fig. 2 Example for slew rate waveform of TB67S539FTG

One of the measures against EMI is shown in the Fig. 1. When a capacitor is connected to the output, an inrush current flows into the capacitor during switching. This inrush current is related to the capacity of the capacitor and the output slew rate of the IC.
Please select the capacity of the capacitor with a margin to ensure that the inrush current does not exceed the maximum rated current.
Please refer to the following example for selecting the capacity of the capacitor. Fig. 2 shows the output slew rate waveform of the stepping motor driver TB67S539FTG. The rise time [tr] is 65.0ns, and the fall time [tf] is 69.1ns.

・The inrush current [I] to the capacitor is estimated from the power supply voltage [Vm] and the capacitor’s impedance [ZC]

I=Vm/ZC……………………………………………………………(1)

・The impedance [ZC] of the output capacitor can be determined by the following formula from the slew rate [tr] and the capacitor capacity [C].

ZC =1/jωC　　ω＝2π f＝2π /tr…………………………………(2)

・Solving the above equation for [Imax], the maximum capacity [Cmax] of the output capacitor is as follow.

Cmax= Imax tr /2π Vm……………………………………………(3)

Here, the power supply voltage [Vm] is 24V, the maximum rated current [Imax] is 2A, and the slew rate [tr] is 65.0ns. From equation (3), the maximum capacity [Cmax] of the output capacitor can be calculated to be approximately 860pF. Please start testing with capacitors that have a capacity less than this value, allowing for a margin.

Please note that this calculation is only an example for reference and may differ from the actual situation depending on the characteristics of the components and the operating environment.

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