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DRV8871: Output does not reflect the input

Intellectual 585 points

Replies: 2

Views: 192

Part Number: DRV8871

Hi,

We are using the DRV8871 as a motor driver for a small motor (R=10 Ohms). Its nominal current is well within 300mA.

The problem seems to be that when we are changing the duty cycle, the output somehow shows a distorted voltage. We verified with a couple of chips and they all show the same issue.

The inputs are as follows:

VM=12.3V ,  PWM Duty cycle = 50% , IN(X) Voltage = 3.3V , PWM Frequncy = 500Hz.

IN2 is shorted to GND and we have more than adequate decoupling using 5 100uF Capacitors.

Below is an oscilloscope shot with the motor connected and running at no load condition: CH1:- DRV8871 Output to the ground (OUT1), CH2:- DRV8871 Input (IN1).

Below is an oscilloscope shot with the motor connected and running at no load condition : CH1:-  (OUT2-GND ), CH2:- (OUT1- GND).

Below is an oscilloscope shot of the output with nothing connected : CH1:-  (OUT2-GND ), CH2:- (OUT1- GND).

Below is an oscilloscope shot of the output with a 33Ohm Resistor Alone  : CH1:-  (OUT2-GND ), CH2:- (OUT1- GND).

Can you help me out? I can't understand why the output is not able to follow the input square wave even at no load.

Regards,

Tony

  • Tony,

    1. When IN1 is high and IN2 is low, the winding current goes forward like (1) in the picture. OUT2 follows the VM as your first waveform

    2. When  IN1 and IN2 are low, all FETs are off; the winding current goes coast mode like (3) in the picture. The winding current cannot immediately change the direction. So, it will first make the current goes through OUT1 low side FET body diode and OUT2 high side FET body diode. And then, LC resonance is happened between winding inductor and FET's parasitic Coss. That is what you see in the first picture when IN1 is low.

    The above explanation can also explain other waveforms.  You may try to turn on IN2  when IN1 is turned off to get the output square wave.


    Best Regards,

    Wang Li

    Motor Drive Solutions

  • In reply to Wang5577:

    Hi Wang,

    Thanks for that, we have resolved the issue.

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