Other Parts Discussed in Thread: SN65HVD77, , THVD1550, THVD1439V, THVD1439, THVD1420
What is the minimum storage temperature for SN65HVD21D? Thanks in advance!
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Just a minor follow-up question from our customer. The SN65HVD77 device has a minimum storage temperature of -65°C, while its minimum junction temperature is rated at -40°C.
Is there a specific reason why wouldn't we see something similar for the SN65HVD21 part? Why its minimum storage temperature is rated only at -40°C and not -65°C?
The minimum junction temperature is a recommended operating condition and therefore is a factor used during characterisation of a device. Therefore, this operation condition is taken into account for all of the electrical parameters described on the datasheet that do not specify alternative conditions. If operational conditions go outside of the recommended range, such datasheet parameters can no longer be guaranteed.
The storage temperature is an absolute limiting value for the device, describing suitable conditions where the device will not experience damage. While operation is not guaranteed under such conditions, the device should not be effected after it is returned to suitable conditions. This is easy to perceive when talking about supply voltage for example, where if Vcc exceeds the recommended range, the device will not necessarily be damaged. However, if this voltage exceeds the absolute maximum rating, it is likely that it will experience permanent damage.
Because thermal characteristics, especially cold, mostly consider structural concerns for limiting values, such analysis is not always available to say when damage may occur. This is especially true for older devices. Therefore, it is common to use the electrical characterization minimum value for this limiting value as well.
We cannot guarantee device survivability in unspecified ranges, and recharacterizing an older device for this extended range is likely not possible.
I would recommend considering a newer device for your application that is already characterised in the temperature ranges you require. The THVD1550 for example is a similar pin-to-pin compatible device that specifies a minimum storage temperature of -65C.
So to clear up some misunderstandings - data-rate is based on the slew rate of the device so "slew rate limited" to 2.5Mbps is the same as a device rated to 2.5Mbps under RS-485 devices. Typically on our devices that we specify as "slew rate limited" they are at ~250kbps not 2.5Mbps
The closest devices we have that will meet your needs are:
THVD1439, THVD1439V, and THVD1420.
They are offered in D packages (and pin to pin to D package of this device). They are limited to 12Mbps (this is the slowest part we have that matches your application requirements of storage temperature min of -65C). They are not necessarily a 1:1 match but they should be able to be used in a similar applications and use case. They have some extra features but it shouldn't detract from the main need.
At this time these are the best options we have.
I shall look in to those devices.
FYI - From the datasheet
The SN65HVD21 device allows up to 256 connected nodes at moderate data rates (up to 5 Mbps). The "driver output slew rate" is controlled to provide reliable switching with shaped transitions which reduce high-frequency noise emissions.
Alright sounds good - please let me know if you have any other questions on the devices.
Yes - the device is rated up to 5Mbps but other devices at 5Mbps are going to have similar if not the same transition time ( and slew rate) regardless if there is a marketing term for slew rate limitation or not as if they don't one have similar specs of the datasheets is most likely being conservative on the data-rate. There may be additional internal circuits that help slow down the device - but in terms of actual functionality a part labeled "slew rate limited" doesn't have any additional specs - it is still just transition time (which directly impacts data-rate) .
Data-rate is typically derived from the formula: Data_rate = 1/(max_transition_time * 3) --> as the transition time should be no more than 1/3 of the bit time in the application. Slew rate limited is there to help really highlight the devices that are slower essentially. The slew rate is just the slope of the transition time.
Also note that is says slew rate limited - but doesn't actually spec a slew rate value (it specs transition time which is directly related to data-rate and is similar to slew rate - but its spec'd on all devices(its a quick conversion) ). All RS-485 transceivers are going to be slew rate limited due to the architecture of the device - some more modern transceivers may also support selectable data-rates which would include extra circuits to add slew rate limitations. My point here was that "slew rate limited" is not a very apt descriptor as all RS-485 transceivers are inherently slew rate limited (there is differential capacitance and output impedance which inherently slows the slew rate) - it is more often than not a marketing term (for RS-485 - other ICs slew rate is an important stand-alone spec and this is specifically RS-485) that isn't incorrect, but its less informative than it may first appear. General rule of thumb if slew rate limitations are of concern to you is to choose the slowest device that still meets the max data-rate required by the device as that device will inherently have more slew rate limitations than a device that is rated faster rather than choosing a device that says "slew rate limited" - as any feature/spec without hard numbers doesn't really mean much from an engineering point of view and are there to grab your attention.
Please let me know if you have any further questions!