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# DRV8837: Capacitance between OUTA-OUTB for reducing Back EMF

Part Number: DRV8837

Hi team,

Customer will use DRV8837 in the condition of VM=9V. They will connect a capacitor between OUTA and OUTB for reducing back EMF.

Could you check the maximum capacitance that can be connected between OUTA-OUTB?

I think there is a concern that the device can't turn on because OCP is asserted in the startup due to the inrush current if the capacitance is too large.

So, if you know how to decide the maximum capacitance between OUTA-OUTB, could you also explain it to me?

Regards,

Saito

• Hi team,

Customer is also going to add differential capacitance in motor line like Cdiff1 and Cdiff2 to reduce EMI noise.

Could you also check the maximum differential capacitance on OUTA and OUTB line?

Regards,

Saito

• In reply to Takahiro Saito:

Saito,

Maximum output capacitance (Ccommon + Cdiffx) is limited by:

1. OCP protection:

(Ccommon + Cdiffx) < Iocp_min * tdeg_min / VM_max = 1.9A * (1us *70%)/ (9V * 110%) = 0.13uF

2. IC thermal performance: each PWM cycle, the capacitor is charged and discharged once. If we assume the capacitor energy is not spread to the air and all dissipated in the DRV8837 circuit. The power loss = fpwm * 0.5*(Ccommon + Cdiffx)* VM^2 . With 0.1uF output terminal capacitor and 9V input 50kHz PWM, the loss is about 50kHz * 0.5 * 0.1uF * 81VV = 200mW. I would try to reduce the (Ccommon + Cdiffx) value if there is no EMI issue.

Best Regards,

Wang Li

Motor Drive Solutions

Hi Wang-san,