MAX7221 experiencing 40C temperature rise above ambient

Each MAX7221 (2) in my current design is getting very hot. I used the FLIR One to get a general measurement. I observed a 40C rise above ambient. I'm not sure what the root cause is. Perhaps someone has experienced this as well. I have attached the thermal image, see the cross-hair in the y-direction which represents one of the MAX7221. In addition, I also included the schematic for reference. Thanks, Brian

Max7221 FLIR Image

• Have you tried taking the data sheet and some measurements and calculate if everything is normal? I think that max chip is regulating linearly, try configuring a lower current Commented Mar 29, 2016 at 14:02
• ambient+40 may feel hotter than you like, but it's a way yet to too hot. If it's meant to be that hot (calculate power dissipation and thermal resistance) that's just fine. If it's not, find the error in your circuit that's loading it more than you expect. Commented Mar 29, 2016 at 14:12

You have set the segment current at the maximum recommended, which results in 40mA/segment. So the chip is drawing as much as 320mA (depending on how many segments are on). The red LEDs will drop about 2.1V at 40mA according to the data sheet.

So, if all the LEDs were on, the maximum power dissipation in the chip would be (5-2.1)*0.320 = 0.93W. If you are using the SO package, the thermal resistance is 85°C/W so the die will be running at about 105°C with a 25°C ambient (assuming nominal power supply voltage etc.).

As the temperature rises the LED drop will decrease and the supply voltage could be on the high side so this is only a conservative calculation if you assume that all the segments will likely never be on at once.

The root cause is P= E*I.

• Just to close the loop on Sphero's very good answer, a die running at 105 deg C is fine for this chip. Commented Mar 29, 2016 at 14:37
• Thank you very much Spehro and Scott, that helped me a lot. I'll post a thermal image later to show you the improvement. Thank you again! Commented Mar 29, 2016 at 14:47

The root cause is that the MAX7221 limits the currents to the LEDs and dissipates the excess energy. This makes it hot.

Possible solutions are:

• lower the LED current (there is a resistor to program this current, ISET, pin 18)

• lower the supply voltage is possible

• add series resistors to drop the voltage so that the MAX7221 needs to drop less voltage

• accept that the MAX7221 gets hot, as long as you do not exceed the maximum dissipation and temperature (see datasheet) this is OK. If the you know that the ambient temperature will never be above 60 - 80 degrees C this might be an acceptable solution.

• glue a heatsink on the MAX7221