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TCA6507: Schematic Review request

Part Number: TCA6507

Hi Team,

I would like to get your input in the schematic below of the customer which uses TCA6507:

Please confirm if the pull-up resistor values (180k ohms) are okay in the design?

Please let me know if you have any questions for the customer to confirm this design.

Thanks,

Jonathan

  • Thank you Jonathan for posting!

    For anyone reading, this is my project's flash function for image capture for diagnosis via cnn. 

    There are 6 LEDs:

    3 white 20mA

    3 amber 20mA

    The TCA6507 is being driven by a GPIO pin on the Quark carrier board attached to the Jetson Nano microcontroller.

    The GPIO pin can provide up to 5 volts.

    I will be defending my resistor value choice next week.


    The BJT NPN simulation I performed to illustrate the open drain structure of the LED pins (P0-P6) gave me good results with much lower resistor values.

    Thanks for reading and any advice is greatly appreciated!

  • Hi Christine,

    Can you provide a datasheet for the LEDs?

    Why was 82k chosen as the pull up values for the I2C bus? This seems like it would violate the rise time requirement.

    I don't see any local decoupling on the Vcc lines either.

    The enable pin's 82k pull up resistor is quite weak as well. Noise on the pin could easily disable and reset the device. No local decoupling here either.

    -Bobby

  • Good morning Bobby and thanks for your interest!

    I am attaching a few items below:

    The first shows the LED circuit GPIO connections to the Quark carrier for the Jetson Nano.

    The second is a reference schematic I located on the Texas Instruments TCA6507 page.

    And the last is my schematic updated last night to match it.

    I circled 3 areas on the reference schematic that correspond to the values I am working with.

    Can you help me validate the resistor values in those areas?

    I am also attaching part list and links:

    (3) white leds

    (3) pull up resistors

    https://www.mouser.com/ProductDetail/ROHM-Semiconductor/SLD430WBD2PT3?qs=FwrL%252BevIYAaff5ZA%2FUNPNg%3D%3D

    https://www.digikey.com/en/products/detail/bourns-inc/CR1206-FX-2000ELF/2562845

    (3) amber leds

    (3)pull up resistors

    https://www.mouser.com/ProductDetail/Cree-Inc/C503B-ACN-CY0Z0252-030?qs=D3QG1iK7FqismRdKJdNk0A%3D%3D

    https://www.digikey.com/en/products/detail/bourns-inc/CR1206-FX-1501ELF/2562773

    (2) SCL/SDA resistors 

    https://www.digikey.com/en/products/detail/bourns-inc/CR1206-FX-2211ELF/2562870

     

  • Good morning Bobby and thank you for your help!

    Parts list:

    (3) white LEDs

    (3) pull up resistors

    www.mouser.com/.../SLD430WBD2PT3

    www.digikey.com/.../2562773

    (3) amber LEDs

    (3) pull up resistors

    www.mouser.com/.../C503B-ACN-CY0Z0252-030

    www.digikey.com/.../2562845

    (2) SDL/SCA resistors

    www.digikey.com/.../2562870

    I am attaching a few items below:

    A diagram illustrating the GPIO connections we are connecting to on the Quark carrier board for the microcontroller.

    The second image is a reference schematic I found on the TCA6507 product page.

    The third image is my schematic updated to contain the resistor values from the reference schematic.

    Thank you for your time on this, I really can't say that enough.

    -Christine

  • Hi Christine,

    Typically you would pick pull up resistors on the SDA/SCL lines of the I2C bus based on the expected bus capacitance and max frequency you need to support.

    We have an app note for that here: 

    LED resistors are a bit more straight forward. You need to look at the Vforward of the LED and subtract that from the Vcc you supply. Then divide by the forward current you are expecting to support. For the 

    It has a Vforward of 3.2V typical and a 30mA forward current. If you are using 3.6V as your Vcc then the calculation for the resistor is (3.6V-3.2V)/0.030A = ~13 ohms. This ofcourse would probably be a very bright. The luminous intenfity is fairly linear (you can see this in  figure 3 of the datasheet) so if you were to choose a 130 ohm resistance, you would see about 10x weaker luminouesity. Though keep in mind your Vforward would also drop if you were to lower your Iforward. (Figure 1 shows this to be 3V at 4mA for example)

    The same calculations can be applied to the other LED/resistors you've used. I suspect 1.5k ohms to provide a  dim white light if you were to use 3.3V as your supply since the typical Vforwarfd is 3.2, your voltage drop across the resistor is very low. 

    -Bobby

  • Thank you so much for your feedback Bobby! I am attaching my updated schematic.

    Can you offer any advice on de-coupling capacitors? I saw the note in the datasheet, and am wondering if it is strongly suggested when attaching to a microcontroller.

    Thanks again, and I hope you had a good thanksgiving.

  • Hi Christine,

    Sorry for the delay, I may be able to help while Bobby is out.

    Decoupling capacitors are typically small .1uF caps placed very close to an IC's supply pin. These capacitors filter out high-frequencies to ensure that the IC does has a clean and stable supply voltage and does not introduce switching noise to the supply rail itself. It's generally a good idea to include these for all ICs to protect them - and other ICs sharing the same power rail - from supply noise. 

    Let me know if you have any more questions.

    Regards,
    Eric Schott