STSPIN250 overcurrent

[GHoll.1]

I have two questions about the STSPIN250:

• What is the overcurrent threshold? It's not listed anywhere in the datasheet
• After what interval is the overcurrent threshold triggered? The datasheet mentiones a Toff time, but that's for the PWM current control if I read it correctly.

I did some tests myself, and I found out that above 2.5A the fault pin (so an overcurrent happened) is triggerd, while it's listed in the datasheet that it should handle 2.65A RMS. Am I measuring wrong or is something else not right?

[Cristiana SCARAMEL]

​Hi,

in STSPIN250 datasheet at page 9 you can find the parameter IOC Overcurrent threshold (last row of Table 5), moreover at page 22 is shown the graph "Overcurrent threshold versus supply voltage" (pay attention the graph report a normalized curve at Vs = 5V).

The intervention time of the overcurrent shutdown from protection triggering is about one hundred of nanoseconds.

When the overcurrent or the short-circuit protection is triggered, the power stage is disabled and the EN\FAULT input is forced low through the integrated open-drain MOSFET discharging the external CEN capacitor.

[GHoll.1]

Hi Christiana,

thanks for your answer. I overlooked that paramater, so thats clear now.

However, why do I get a fault indication when I have a current a little above 2.5A? I am using the X-NUCLEO-IHM13A1 evaluation board, with a 5V reference. For load I use a 2 ohm power resistor so I can control the current.

[Cristiana SCARAMEL · Accepted solution]

If I understand well you are using our X-NUCLEO-IHM13A1 evaluation board with VS = 5V.

In this case if you turn on the diagonal with PWM input (the PH input set only the current direction) the current goes to 5V/2ohm = 2.5 A immediately.

Which kind of signal you are applying to PWM input?

If you turn on the bridge in continuous, you are near the absolute maximum ratings of 2.6 Arms and the it is possible that the FAULT indication is a thermal shut down due to a power dissipation. 

Also, I wish to remind you that the absolute maximum ratings section specifies the stress levels that, if exceeded, may cause permanent damage to the device.

[GHoll.1]

If I use a 2 ohm resistor, I measure a voltage drop over the H-bridge of +/- 1V. So my power supply needs to have at least 6V in order to have 2.5A current. A voltage drop of 1V with 2.5A is a power loss of 2.5W. I think you are right that it's a thermal shutdown, because it's not happening instantaneously.

But according to the datasheet it has a junction to board of 23.3 C/W which should mean it should only rise to around 58C above ambient. Or can't I calculate it like this?

But I have another issue. When I connect a 3VDC motor to it, it also triggers the fault when the DC motor is starting under a load. Perhaps it could be the overcurrent due to a small amount of stall condition? But if I measure the sense resistor of the X-NUCLEO-IHM13A1 evaluation board, I am not measuring an overcurrent. See below the voltage what I measure.

What could be the reason that it triggers the fault in this case?

[Cristiana SCARAMEL]

Hi @GHoll.1​,

concerning the​ first point the estimation of power dissipation is right. But for the temperature calculation you must consider the junction to ambient thermal resistance (57.1 °C/W), that means about 140 °C over the ambient temperature.

The result is in line with thermal shutdown temperature (typ value 160 °C).

About the second point at power-on it is normal to have a FAULT indication (refer to board user manual UM2148 at page 6 point number 3 of the list).

In general at power-up, power-down and when leaving the standby condition, the EN/FAULT pin is forced low until the internal circuitry stabilize.

[GHoll.1]

Hi @Cristiana SCARAMEL​ ,

The FAULT indication happens when a 3VDC motor is started, after powerup.

See below the oscilloscope image.

The blue line is the FAULT led (which triggers when pulled low). The yellow line is the sense resistor. As you can see there is a current of about 1,47A (500mv/0,34ohm). But then suddenly the voltage drops to -500mv (so -1.47A) and then shortly after it the FAULT is triggered. Does this happen because of the sudden voltage drop to negative? Because 1.17A should not be enough to trigger the overvoltage.

The strange thing is, this happens only between 3V-4V with a 3V nominal DC motor. When I load a 7.4V nominal DC motor with the same power I have no problem with this. Because the current is half it's not triggering anything. But I just need to know what exactly is causing this, because like I said with a 3V motor I can't get the full power out if it (which is 2.1A at full load) because the STSPIN250 triggers a fault.

[Cristiana SCARAMEL]

​Hi @GHoll.1​,

try to investigate the root cause of sense voltage inversion to understand if it can be related to FAULT trigger.

There are two hypothesis that can cause the sense voltage inversion:

1. PH signal change (the opposite diagonal is switched on)
2. STBY/REST signal low (the device enters in the standby status and the outputs are in high impedance, the current flows through the freewheeling diodes)

Could you make some oscilloscope screenshot monitoring the PH and STBY/RESET pin signal triggering on the sense voltage inversion?

In this way we can verify if some noise causes the status change of one of this two pins.

[GHoll.1]

Hi @Cristiana SCARAMEL​ ,

I did measurements on the PH and STBY/REST pins, but there is no change on those channels.

The voltage inversion what you can see in the oscilloscope screenshot, is the back EMF because of the sudden stop of the motor.

So for me it's still unknown why this is happening, because I can't measure any overcurrent whatsoever, and like I said before this only happens when I use it between 3-4V. When I use a 7V motor it operates normal.

[Cristiana SCARAMEL]

Hi @GHoll.1​,

to better understand what is happening, could you give me more details?

Are you using internal PWM current control or provide the PWM input directly?

The voltage inversion on the sense resistor is consistent with the transition from diagonal on and current recirculation in fast decay.

With this device the fast decay (on freewheeling diodes) is possible only when the device is in high impedance: EN set low by external or internally pulled low after a failure.

More oscilloscope screenshot should be useful showing VS voltage and OUT voltages.