AM26C31M: Power dissipation

Asaf,

Please excuse the delay in the response while our team is on the holidays. We will get back to you by this week.

Regards,

Asaf,

For power dissipation, you can check out this blog. Back to your specific question, the current need to multiply 4 if all channels are in short.

Regards,

Hao

Hello Hao,

Thanks for your reply.

According to the article you sent, it seems that in normal operation there is a heat problem with the device.

When using 5V+/-5% supply, 100 ohm differential resistor @ 20Mhz, the power dissipation is:

PDic = 16.8mW

PDdc= 66.65mW

PDac = 130.2mW

Total power dissipation for 4 outputs = 4 * (16.8mW + 66.54mW + 130.2mW) = 854.6mW

Thermal resistance = 75.6 Deg/watt

Temp rise = 66 deg

This is pretty high for normal operation.

Now, when one output is shorted,

 I belive that the only parameter changes is PDdc:

PDdc = (5.25V - 170mA*100 ohm)*170mA = 0.89W (for the shorted output)

Total dissipation = 3 normal operating outputs + shorted output = 1.54W

At this situation, the max amb temp is 30deg...

Am I wrong, or missing something?

Because it seems that we have power dissipation problem.

Thanks and best regards,

Asaf

Hello Hao,

According to the article, when all outputs are shorted, the power dissipation will be:

Pdissipation all channels = 4 * 5.25V * 0.17 = 3.6W.

At this power the temperature rise will be (at soic) 276deg...

Can I put a series resistor in order to limit the current?

How to calculate its' value?

Best regards,

Asaf

Asaf,

Your calculation might be a bit pessimistic. Please refer to Figure 1 in page 7 on the datasheet for the power dissipation in typical use case.

In the short condition, you can surely add series resistor to limit the output current. Another note is that the device's abs max current on supply is 200mA.

Regards,

Hao

Hi Hao,

Thanks for your reply.

Sorry for nagging, but I've read the datasheet and the article you sent few times and still I'm not sure how to calculate the temperature rise when all outputs are shorted.

This calculation is critical since I need to know whether to put heat sink for the device.

When all outputsare shorted, the ICC current is about 170mA, when F=20Mhz. In this situation the output voltage is 0V.

So, when calculating the poer dissiaption all I need to use is the ICC and VCC?

Meanning that the power dissipation is: 5V * 0.17mA = 0.85W.

And this is the power used to calculate the temperature rise?

No need to use the switching losses or simething else?

Thanks, and best regards (and happy new year :) )

Asaf Abraham

Asaf,

Happy new year.

I'm sorry I didn't make it clear. The maximum current the device can supply is 200mA (the maximum power is 200mA*5V=1W). So in the design, we may have to make sure the condition is guaranteed in either normal operation or short condition.

Regards,

Hao