ITS724G/BTS724G replacing fuse

[User22768]

Hi,

In the datasheet for ITS724G/BTS724G it says "replaces electromechanical relays, fuses and discrete circuits". I'm wondering what it practically means that it replaces fuses.

I'm using the device to drive lamps and solenoids. I have fuse on the incomming power cable but there is no fuse on the outputs after the  ITS724G/BTS724G device. 

I have seen that the device limits the current by turning output off and on when it gets overheated. It behaves as a pwm signal with the frequency of about 10-100 Hz. Frequency is high if "over current" is high. 

Do you suggest having a PTC fuse or other resettable fuses on the output or can I relay on the built in protection in the ITS724G/BTS724G device? I do not have a uP that can read the diagnostic signal. 

(In one of my other designs I have used the 428L2 device which have same protection features. This device did not hold for exessive over current. The device was driving a vibrator motor which consumed around 8 -13 A which is too much for the 428L2 device. I hade a 5A PPTC fuse (MC33165 ) on the output but that did not trip. Vibrator was only on for  5-20 seconds at the time)

[Vishnu_Nambrath]

Hello, 

ITS724G/BTS724G is a smart FET with Gate Driver, Protection, and Diagnostics features integrated. As part of diagnostics, it has a current limitation feature.  This feature has a strong dependency on the temperature as shown below. It takes some time to active the current limitation features as shown below.

Integrated protection functions are designed to prevent IC destruction under fault conditions described in the
data sheet. Fault conditions are considered as "outside" normal operating range. Protection functions are not
designed for continuous repetitive operation.

Hence , our strong recommendation is to have ST monitoring for the faults so that SC is detected and removed by switch off the output stage. 

Device current limitation feature need certain amount of current and time to activate the feature. If it doesn't fit to your  requirement, Please plan to have an additional protection by fuse. 

Coming to your second scenario,  I hope your saying the device damaged permanently. 
IT428L2 typical current is 7A, and you were operating it above the limits. And also device cannot trigger the current limitation since the current is not reaching the I_L(lim). Hence the device operated out of specification.  This can lead to the permanent damage of the device. 

Hope my explanation is clear for you. 

If you have any further questions, please let me know. Happy to support 🙂 

Regards 
Vishnu Nambrath

[User22768]

Hi Vishnu,

Thank you for your quick and good answer. 

I don't monitor the diagnostic signal and don't have the possibility to turn off the output in case of a short circuit (SC). I have an 5 Amp slow blowing automotive fuse on the incomming power.  I don't think the fuse will blow in case of an SC to ground on one of the outputs. When current reaches I_L(lim) it starts turning output on and off as shown in figure 3a, preventing a high current and thus the fuse will not blow. I have not tested this so this is just an theory based on previous tests and observations. If the fuse does not blow the ITS724G might be in this "short circuit state" for a long time (many hours). Is that ok? Would it help adding ptc fuses on the outputs? 

[Vishnu_Nambrath]

Hello, 

I would like to have a look at the 5A fuse datasheet. Basically, I want to check the operating time for I_L(SCr) current. 
Do you have any PTC fuse under consideration, if yes please share its datasheet also?

Please let me know your load current for each channel and operating temperature range. 

Regards 

Vishnu N 

[User22768]

Hello,

The fuse is an ATOF automotive blade fuse:

https://www.farnell.com/datasheets/1633611.pdf

The PPTC fuse that I have in mind is MC011393

https://www.farnell.com/datasheets/2793111.pdf

The nominal load current is 1.6A for two of the channels and 0.1A on one channel. The forth cannel is unconnected.   

I did a test with the ITS724G and these components listed above. Input voltage 13V and output connected to GND. The 5A balde fuse did not blow and the 3A PPTC fuse did not trip. The ITS724G got hot which was expected. The current was limited by ITS724G to around 1.5A which decread to around 1 A after a few seconds (measured by handheld current clamp)

[Vishnu_Nambrath]

Hello, 

ATOF automotive blade fuse is operating very low speed as you said.

Can you please let me know your ambient temperature requirement?
Even if you use 3A fuse, for 80C it is only 1.5A due to temperature derating. 
I am just trying to see any low current  PTC can be selected based on ambient temperature requirements and load current such that it will trip for the short circuit condition?

Can you also plot the current waveform during short circuit.  Short circuit current in your test bench seems to be very low for typical conditions. Please double check your DC source current is not limited to low values.

Thank you 

Vishnu N

[User22768]

Hello,

The ambient teperature is noramlly around 0 C (outside wintertime). Both fuses are very slow. Seems that ITS724G is reacting much faster than  both these fuse types and thus is the component that will "suffer" the most from the short circuit. Is this a short citcuit situation that is withing the specification of what the component shall sustain without breaking? Or do I need to find an external fuse that protects the ITS724G in case of an short sircuit?

[Vishnu_Nambrath · Accepted solution]

Hello, 

The short circuit protection features are not for continuous and repeated operation.  It is expected to have a microcontroller monitoring on the ST pin to switch off the output stage. Since you don't have that, please plan to have a fast acting fuse which can protect the device. 

Thank you 

Vishnu N