Regulator question.

Hello,

I have a 3.3V regulator which is driven by 5V from a DC power supply, and the DC PSU regularly outputs 50mA to operate the regulator, which supplies voltage to MCU and other devices.

The MCU has a GPIO pin which can push 20mA, and this pin drives a 20mA LED.

When LED is turned OFF, PSU outputs 50mA, but when I toggle the LED, so now the MCU drives the LED, the PSU suddenly outputs 110mA instead of the expected 70mA (50mA regularly + 20mA LED).

Do you know why is that?

I assumed that if MCU outputs addintional 20mA current, then the 3.3V regulator would output addintional 20mA current, and therefore the PSU would output addintional 20mA current.
Where is my mistake?



Thanks alot guys.

Does the processor go idle, or stop executing instructions when the LED is off?

Current isn't pushed, it's pulled. Conversely, voltage is pushed and not pulled.

So the 110mA is what the circuit is pulling when the LED is turned on. Something is causing it to pull that much.

Please post a schematic as right now there is too little information to go on.

What value of resistor are you using for current limiting on the LED?


Torben

To your questions.
The MCU keeps working with no change after the LED has been turned ON.

Its a 2.2V LED and i'm using a 61ohm resistor in series - meaning (3.3-2.2)/61 = 18mA.

I'm sure that the MCU pin itself cant output more then 20mA because 20mA is the maximun output current of it according to datasheet.

So where do the 110mA come from?

hi alpha,
The correct terms for 'pushing/pulling' are source and sink.

So the PIC pin can source 20mA.

http://support.motioneng.com/Hardware/IO/Slice-IO/topic_sink_source.htm

Ok thanks for the correction.

Do you know how to explain the 110mA?

Are you measuring the current draw of the entire chip? What else is loading your other I/O lines. It has to be coming from somewhere. Perhaps you've inadvertantly grounded other I/O lines?

A PIC output can sink or source up to 25mA when its supply is 5V. But then the output voltage will have a loss of about 0.5V.
It might be able to sink or source 20mA when its supply is only 3.3V but its voltage loss might be 1V.

The OP isn't using a PIC it's a TI MCU. I couldn't find a TI MCU forum.

How about that schematic?


Torben

From where are you taking the measurement?

Try to take the measurement on the voltage regulator output.