DS26LV31QML: Short circuit limiting behavior/protection

Hi Jesse,

This device does have current limits on the differential driver output current (under short to GND will source up to 160mA) but it doesn't have a thermal shutdown feature.

For your direct questions:

1. Shorting to GND indefinitely is dependent on what power is being dissipated over the device and the ambient temperature. 

Where the max Power is approximated by the following formula: P_max(T) = P_max(25C) - (T-25)*DeratingFactor --> P_max(T) = 1119mW - (T - 25C)*7.5mW/C.

The short circuit resistance between output and ground should be approximately be between 90 Ohms and ~16Ohms corresponding with typical and max current respectively 

So you would be expecting to consume between 90 * 30mA^2 = 81mW to 16 * 160mA^2  ~ 410mW for the output shorted to ground - this is going to be in addition to idle current of the device and any AC losses due to switching will be added onto this.

An indefinite short to GND could be okay if you are not violating P_max(T) which will be dependent on ambient temperature. This is just an approximation as the P_max(T) is derived from thermal impedance which is partially dependent on the system it is placed into.

2. We don't directly guarantee that specification on this device - most likely it will output less current than a short to GND as ideally the VO will be somewhat close to VCC/2 so there will be less current requested from the active driver. Its going to have the same considerations however of power consumption and you have to consider that the current will exit one pin and enter another increasing the power consumption on the device. 

3. No thermal shutdown - if power rating is violated and die temp goes too high the device could be damaged/destroyed. This will also apply to multiple shorts - one short most likely won't damage the device - multiple will - so if this part is chosen there needs to be some way at a system level to detect shorts and have a way to mitigate them to prevent more than one at any time. 

Please let me know if you have any other questions!

Best,

Parker Dodson

Hi Jesse,

From an operational point of view - the output acts a resistance just as in any RS-422 or RS-485 device - the mechanism that actually works internal to the device is irrelevant for application design with the device  but generally can be assumed to be VCC -> Diode/Transistor --> output where the transistor is the variable resistance  - during a short circuit to GND the output resistance will be between 16 and 90 ohms - assuming VCC = 3V and a diode drop between VCC and Output of  appx. 0.3V  which implies 2.7V across a variable resistance  - and with the IOS spec'd at sourcing typically 30 mA  - it will be typically around 90 ohms - but it could get down to ~16 Ohms for highest short circuit (realistically it would be closer to 17 - but a little under - however this is a pretty quick approximation).

The device self-limits its current by varying its output resistance - so no what I said in absolutely  no way implies that you need to put an external resistor  to keep the device in its IOS spec - it is very plainly stated in the datasheet that device will not source no more than 160mA when shorted to GND - all I did was give the approximate output resistance when a short circuit to GND occurs as it gives a high level overview of how to think about how the part works in application (as if you modeled the device like that it will act very similarly to real device - these types of devices are generally very linear). Series resistors on outputs will degrade SNR of system greatly as the bus should be terminated at the receiver so any series resistance will attenuate output . While some applications will put series resistors here (which most of the time they shouldn't unless they are pulse proof resistors added with external surge protection - otherwise their costs greatly outweigh the benefits as this effectively weakens the drive strength of the bus) 

From a system level you really only need to know what current to expect to approximate as well as voltage supply level - because power during short can be roughly approximated to be VCC * IOS - so worst case 3.6V * 160mA = 576mW just from the short circuit - not including other losses (switching, idle, etc...) - the resistances I had in my first post where there to show conceptually what is going on because ultimately that is what is happening - the output impedance increases to limit output current when output is shorted to GND. 

Best,

Parker Dodson