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  • TI Thinks Resolved

SN65HVD101: SN65HVD101 limits LDO at 80mA

Part Number: SN65HVD101

Hello,

we have a new prototype sensor, where we use the SN65HVD101 for communication and supply.

As we are testing, we recognized occasionally drops on our supply line. Further test showed, that these correspond with activities of another sensor component, which increases the system load in specific operation modes to 100mA.

After that, we checked reactions on load changes of the SN65HVD101 and had to realize, that its limits the current to 80mA and afterwards decreases the voltage.

Is there any part in the datasheet, where this behavior is scripted?

Also is it possible to change this behavior?

Best Regards

Daniel Roy

  • Hi Daniel,

    First off, and just to clarify, the LDO output current is rated for a maximum of 20mA, so I assume the current you are referring to is the CQ Driver output current which is specified as Io(lim) parameter in table 7.3 on page 5 of the datasheet and has an adjustable current limit up to 450 mA. 

    Pin 4 of the SN65HVD101 is the ILIMADJ pin (defined in the pin function table on page 4 of the datasheet) and is used to set the current limit using an external resistor to GND.  Figure 2 of the datasheet (on page 8) shows the typical curve that relates the resistor value (Rset) and the current limit on Io(lim).  Figure 7 on page 9 of the datasheet shows the Rset resistor connected to the ILIM_ADJ pin.

    Sections 9.3.2 through 9.3.4 and 9.3.6 on pages 10 and 11 discuss the CQ current-limit functions in further detail.  If an over current condition is observed, the CUR_OK pin will be driven to a logic low until the CQ pin is no longer in a current-limit condition.  Table 4 on page 12 of the datasheet also specifies the pin behaviors for various current  and voltage states.

    Section 10.2.2.1 on page 15 of the datasheet also instructs how to configure the SN65HVD101 device.  In it's example, it instructs using a 4k resistor for Rset to set the Io(lim) current limit to 250mA.  The larger this resistance is, the lower the current limit will be set.  For maximum current capability use a very low resistor value or connect directly to GND.  However, I would recommend using a 0-ohm resistor instead of a direct connection to GND just in case you need to adjust this in the future.  Furthermore, leaving this pin open is not recommended either, but it would result in the lowest current limit possible because it would appear to be an infinitely large resistance to GND.

    From your description of the current limit situation, I believe that you have either left the ILIM_ADJ pin floating, or are using too high of a resistor value.  Adding a lower value resistor should increase the current limit and prevent this voltage drop.

    Regards,

    Jonathan

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