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Original question:

DRV8432: Controlling Overcurrent Protection

DRV8432: DC motor oc-adjust resistor values

Francesco Ramunno
Prodigy 20 points

Part Number: DRV8432

Hi everyone.

I would like to have some information about the overcurrent limit of FULL BRIDGE MODE OPERATION (figure 8 of the data-sheet), in which two DC motors are connected to the driver. In particular, I would like to know whether the OC-ADJUST RESISTOR is referred to the over-current value of EACH motor, or to the total amount of the current provided by the driver. For example, I want to set an overcurrent value of 4.1 A for EACH motor. In this case, is the OC-ADJUST RESISTOR of 68 kOhm (shown in the data-sheet) correct?

Thank you in advance

Francesco

  • 0
    TI__Guru** 114905 points

    Hi Francesco,

    The OC_ADJ is per output. An OC_ADJ resistor of 68kOhm will set the current at 4.1A per motor.

    Please note the following from section 7.3.2.2 of the datasheet:

    The values in Table 2 show typical OC thresholds for a given resistor. Assuming a fixed resistance on the OC_ADJ pin across multiple devices, a 20% device-to-device variation in OC threshold measurements is possible. Therefore, this feature is designed for system protection and not for precise current control.

    Also please note the following from section 7.4 of the datasheet:

    In operation modes 1, 2, and 4 (CBC current limit is used), once the CBC current limit is hit, the driver will be deactivated until the next PWM cycle starts. However, in order for the output to be recovered, the PWM input corresponding to that driver in CBC must be toggled. Because of this, CBC mode does not support operation when one half-bridge PWM input is tied to dc logic level.

  • 0 in reply to Rick Duncan
    Prodigy 20 points

    Hi Rick, 

    thank you for your prompt response.

    Would you mind explaining me how to set the CBC current limit for my situation (4.1 A per motor maximum)?

    Thank you in advance

  • 0 in reply to Francesco Ramunno
    TI__Guru** 114905 points

    Hi Francesco,

    For your situation, setting the OC_ADJ to 4.1A will result in CBC as low as 3.28A and as high as 4.92A. You will have to decide whether the w minimum current is enough or the maximum current is current is too much. Depending on how fast the rises, there may not be much difference. 

    As mentioned above, the CBC has some variation from device to device. It is not intended for precise control of the current.

    If you want precise control of the current, sense resistors should be added in the GND_x paths. The voltage across the resistor can be amplified and measured with an ADC.

    See the link http://e2e.ti.com/cfs-file/__key/telligent-evolution-components-attachments/00-171-01-00-00-59-27-56/DRV8412EVM_5F00_RevG_2D00_Schematic.pdf for an example.