48-Pin S12ZVCA Total Current on GPIO

[aaronlee]

Hi,

We have a LED project using 48-Pin S12ZVCA. The LED anode connect to external +5V power supply, and the cathode connect to S12ZVCA GPIO, the current is limit to 15mA by resistor.

We connect each GPIO(PAD0~9,  PS0,1,2,3,7 PT0,1,2,3,4,7 PP0,2,4,5,6 PL0,1) to control 28 pcs LED. How many LED we can driving at the same time?

Best Regards,

Aaron

[StanoA]

Hi Aaron,

As first I suggest you to use the external ballast transistor for VDDX voltage. The current consumption of LEDs is 28 * 0.015 = 0.42A. The ballast transistor must be properly selected due to power loss (12 – 5) * 0.42 = 3W.

The all LEDs current can increase the package temperature: 28 * 0.015 * 0.8 * 70 = 24°C. So you need take into account whole operating conditions of your application and added temperature rise of MCU from powered LEDs.

For the help please see the RM Section A.1.7. Power Dissipation.

I whish you many success in your design.

Best Regards,

Stano.

[aaronlee]

Hi Stano,

Thanks for your reply.

The external ballast transistor only supply 170mA, so we using external +5V power supply connect to LED +, and GPIO connect to LED -. Is it work? Do you have any recommend?

Best Regards,

Aaron

[StanoA]

Hi Aaron,

It is OK, just calculate the power dissipation and associated temperature rise of MCU.

Best Regards,

Stano.

[aaronlee]

Hi Stano,

Thanks for your reply.

We found the GPIO inject current  is -2.5 ~ +2.5mA except PP0,2,4,5,6.

Is it safe to drive the LED in 10~20mA?

Best Regards,

Aaron

[StanoA · Accepted solution]

Hi Aaron,

The injection current is current flowing through the internal protection diodes when port is not active. The max continuous current for GPIO pins is in Table A-2:

That’s the reason I suggest you to calculate max total power loss of the pins driving LEDs. It could cause overheating as I mentioned in my previous answer:

The all LEDs current can increase the package temperature: 28 * 0.015 * 0.8 * 70 = 24°C. So you need take into account whole operating conditions of your application and added temperature rise of MCU from powered LEDs.

The use of the external 5V power supply is OK, but please maintain the good GND connection on PCB between power sources.

Best Regards,

Stano.