Where in the FRDM-KL25z Datasheet is the maximum current that can be sourced from the P5V_USB pin?

[MelonM]

I'm using the FRDM-KL25z board for a project, but it appears that I am drawing too much current. As a result, the voltage is dropping to ~2.7V from the expected 3.3V.

I was able to find I_OHT on page 8 of the attached datasheet, which indicates that the output high current total for all ports is 100 mA. On page 6 of the datasheet, I_D specifies that the maximum current to/from a single port pin is 25 mA.

I have three circuits connected to the board, but I don't believe I'm exceeding the 25 mA limit for any individual IO pin. Each circuit is connected to either the 3.3V pin or the 5V pin which gets its power from the USB connection to my computer.

Where in the datasheet can I find the maximum output current for the 3.3V and 5V pins? Are these pins part of a port? Could the current be limited in the USB port of my computer?

[Pablo_Ramos]

Hi @MelonM 

The Output high current total for all ports (IOHT) of 100mA and the Instantaneous maximum current single pin limit (applies to all port pins) (ID) of 25mA applies for the GPIOs, not for the 3.3V and 5V from the arduino pins.

The 3.3V and 5V pins are limited for the external regulator and the USB port (please refer to the schematic).

If you are getting a voltage dropping, we recommend you use an external supply for your external circuits.

Hope it helps you!

[MelonM]

I don't see a schematic in your reply. Are you referring to Figure 3 on page 6 of the attached document?

For context, my goal with the board is to use sensor input to switch another electrical device on and off using a mechanical relay. The relay is part of a relay module (https://www.digikey.com/en/products/detail/sunfounder/TS0010D/18668618). The relay itself has the model SRD-05VDC-SL-C.

I tried testing by powering the KL25z and the relay module in parallel using a 5V (2A) AC adaptor, but I think my adaptor might not be working correctly. I don't have a bench power supply, but I do have some 12V AC adaptors and some linear regulators. I've never used linear regulators before, so it might take a while for me to get them working properly (I've read that they generate a lot of heat and I'm not sure if I have anything that can act as a good enough heat sink). I'll respond to this thread again when that happens.

I did a bit of troubleshooting today with the board being powered through the SDA USB port. Using the connections in the attached picture I have gathered the following information using my multimeter:

Before Connecting the Relay to the KL25z:

• P3V3 Pin Voltage: 3.29V
• P5V_USB Voltage: 5.01V

Relay Connected to P5V_USB and GND, Relay Input Shorted to P3V3:

• P3V3 Pin Voltage: 3.29V
• P5V_USB Pin Voltage: 5.01V
• Current Drawn by Relay Module Vcc from (P5V_USB): 0 mA
• Current Drawn by Relay Module In1 (from GND): 0 mA

Relay Connected to P5V_USB and GND, Relay Input Shorted to GND (relay coil engaged):

• P3V3 Pin Voltage: 3.13V
• P5V_USB Pin Voltage: 4.25 - 4.5 V (Wouldn't Stabilize)
• Current Drawn by Relay Module Vcc from (P5V_USB): 62 mA
• Current Drawn by Relay Module In1 (From GND): -1.2 mA 

From these measurements, it seems like drawing even ~60 mA from the P5V_USB pin causes a voltage drop on the P3V3 pin.

I'm not having an easy time understanding the table on pages 10-12 of the datasheet I attached in my first post. It seems like the board itself only draws ~6.5 mA when running normally? There's also mention of "Analog Supply Current" in the first row of that table and the note says to "see each module's specification for its supply current". I'm not sure what counts as an analog module.

From googling, it seems like a USB port can supply either 500 mA or 900 mA, so the 6.5 mA from the board plus the ~60 mA from the relay doesn't seem like it should cause any issues.

Side Question: My main reason for wanting to figure this out is that I'm still actively working on the code. If I wasn't able to source enough current from the USB port, it would be annoying to unplug the board from the AC adaptor and plug in the USB cable every time I wanted to modify the code, then do the reverse in order to test the code. On the top of page 6 of the attached document, it is stated that "protection circuitry is in place to allow multiple sources to be powered at once". Does this mean that I can have the board connected to my computer through the OpenSDA port while simultaneously providing power to it with an AC adaptor through the P5-9V_VIN pin?

[Pablo_Ramos]

Hi @MelonM 
Yes, you can have the board connected to your computer through the OpenSDA port while providing power through the P5-9V_VIN pin, this happens because the SDA port have a Schottky diode that provides back drive protection Table 3. Page 7 on the user manual.
Also, here is the schematic that I was referring before, you can see how the board is powered in the top side of page 5.

Hope it helps you!