SN65LBC180: Termination resistor

Hi Eric,

I understood. Thanks for your advice.

I'd like to confirm one more question.

What is the Vod(max) when the customer apply the 5.25V?

Is the Vod(max) have possibility to achieve 5.25V?

Regards,

Yoshi

Hi Yoshi,

The value of Vod depends on several operational conditions. However, the real expected value of Vod will never achieve the specified maximum. I'll explain why:

In an ideal case where there is driver has no internal resistance (no voltage drop when driving bus to Vcc or GND), Vod will always be equal to Vcc. In this model, the driver can perfectly short the high-driven bus to Vcc and the low-driven bus to GND. This is where the Vod Max specification comes from and why it is equal to nominal Vcc. In the case where Vcc = 5.25v, the ideal Vod (or Vod Max) would also be 5.25v. However, because of the nature of real components, there is inherent loss in the system. This means our driver's internal resistance is some value greater than zero. So over any finite bus load where current is sourced from the driver, we will see a voltage drop from supply voltage to high-driven bus voltage (and low-driven bus voltage to ground). This drop will be proportional to the current source by the driver and will thus increase as our bus load RL decreases.

The upper bound for the voltage drop through the driver is defined in the datasheet as our Min Vod over different load conditions. The lower bound for the voltage drop specifies Max Vod at Vcc to show that the device is not capable of driving a differential magnitude greater than the supply voltage. However, there is no real case in which this device could drive a bus differential with the same magnitude as the supply voltage - it can only drive a lower magnitude. 

Simply put: When Vcc = 5.25v, Vod(Max) = 5.25v. SN65LBC180 will not achieve Vod(Max). 

Let me know if this explanation makes sense.

Regards,
Eric

Hi Eric,

Thanks for your explanation.

I understood. Do we have the graph which has X-axis is VOD and Y-axis is Vcc? The customer considering how much VODD will change when the Vcc is change.

Also, do we have the result of open, short test as pin-FMEA?

Regards,

Yoshi

Hi Yoshi,

Differential output voltage will be proportional to driver output current. Figure 13 describes this current and its relationship with supply voltage Vcc. As you would expect, driver current and the resulting bus voltage increases when Vcc increases. Let me know if this is not enough information for the customer.

I'm not sure if we have such failure mode analysis. This device internally limits output current in the event of a short to avoid causing device damage. Open circuit conditions would impact data but would not cause damage do device driver or receiver. Could I ask what the interest is in this?

Regards,
Eric

Hi Eric,

Thanks for your comment. It is enough information.

The customer concern about the behavior of each pin shot condition for all pattern. If we have would it be possible to share?

Anyway, the customer considering to use SN65LBC180 for driver, SN65LBC179 for receiver and termination resistor is 100 ohm.

Could you tell me whether there is issue or not?

Regards,

Yoshi

Hi Yoshi,

I don't think I understand "pin shot condition". Could you elaborate on the customer's concern? Would they like to know more about the current limiting feature of the device during a bus fault? Is this an ESD concern? 

SN65LBC180 and SN65LBC179 are both compliant with the standard for RS-485 and are therefore compatible. The standard specifies driver capabilities to drive a total load of 54-ohms (two 120-ohm terminations plus transceiver loads). 100-ohm termination goes slightly outside of this spec, but should not present an issue if other variables are not nearing the limits of the spec. It is also best to match the impedance of the cable being used; if this value is closer to 100-ohms, this is likely preferable. 

Regards,
Eric

Hi Eric,

Sorry for lack of my explain. The customer would like to know the analysis result for Pin short-circuit to adjacent pin.

Also, Do we have the data when RL is using 100 ohm same as Figure 13?

Regards,

Yoshi

Hi Yoshi,

The customer would like to know the analysis result for Pin short-circuit to adjacent pin.
This device has an internal current limit on the driver. In the event of a short circuit, the current through the driver will not exceed the specified Short-circuit output current (Ios) while specified conditions are met. Depending on the strength of the short, it is likely that communication during this fault will not be possible. The current limit is in place to offer protection for the device until the short is removed.

Also, Do we have the data when RL is using 100 ohm same as Figure 13?
I don't think we have such a figure. It can be assumed that the current being supplied from the device will be inversely proportional to the resistance between the differential lines. That is to say, with a higher resistance value of 100-ohms, there will be less current through the output. Use Figure 13 as a reference for where the linear regions lie. Let me know which values you are interested in if you would like help with this.

Regards,
Eric

H Eric,

I don't think we have such a figure. It can be assumed that the current being supplied from the device will be inversely proportional to the resistance between the differential lines. That is to say, with a higher resistance value of 100-ohms, there will be less current through the output. Use Figure 13 as a reference for where the linear regions lie. Let me know which values you are interested in if you would like help with this.

→ Ok, I understood. The customer would like to know the max output current when the Vcc = 5.25V with 100 ohm resistor.

Would it be possible to estimate?

Regards,

Yoshi

Hi Yoshi,

Using our previous ideal assumption where the device is able to pull a differential of the full Vcc across any load, the output current would be Vod / RL = 5.25v / 100-ohm = 52.5mA. In a real system, this current would be lower due to the resistance of the driver. Note that if more load was presented to the bus or a short condition were applied, this current would increase up to the internal current limit of the device.

Regards,
Eric