ADPD1080 want to drive Vf=8V LED in 100mA

Hello support:

The Vf of UVC LED will be over 8V.

ADPD1080 can not drive high voltage LED.

I will make an external high voltage LED driver circuit.

But I want to sync LED driver ciuit.with ADPD1080.

How could I sync ?

Pull up a resistor on the LEDX1 pin.

When turn on LED, LEDX1 will go from high to low.

So use this signal to syn external LED driver circuit ?

Will there be some problem on doing this way?

Or there will be other good method to syn external LED driver circuit ?

best regards

  • 0
    •  Analog Employees 
    on Oct 23, 2020 1:12 PM 1 month ago

    Hi,

    The only requirement that the ADPD1080 has is that its LEDxx driver pins must not exceed 3.6 V. External driver circuits are typically noisier, so if possible, please refrain.

    So, let's say you have 10 V supply voltage and at 100 mA, the LED has 8 V of forward voltage. Then basically you get 2 V on LEDxx driver pin, which is perfectly acceptable for ADPD1080.

    Also you can add a series resistor between the supply voltage and the LED. For example, you add 20 ohms resistor. At 100 mA, it will have 2 V voltage drop on it. So, you can further reduce the voltage on the LEDxx driver pin.

    Another option is that you can add an NPN BJT in front of the LED driver pin. I attached a section from ADPD4100 datasheet, which is a much newer version of ADPD1080 (I recommend you to check out its datasheet as well: https://www.analog.com/media/en/technical-documentation/data-sheets/adpd4100-4101.pdf).

    This concept of BJT is the same though, you can apply it to ADPD1080 as well.

    If you still need help, you should open up a support ticket for design help. I dropped the link below, please fill out the form for that:

    https://form.analog.com/form_pages/support/integrated/techsupport.aspx

    Best regards,

    Yigit

  • Hi Yigit:

    The below picture is the UVC LED I-V curve.

    If I apply a series 20ohm to Vled 10V.

    Set 5mA to LED,the forward voltage is about 6V.

    The Vled is 10V so the voltage of LEDX will be 10-6-0.1=3.9V.

    This will damage ADPD1080.

    Use series resistance can not gurantee all the situation

    For the BJT solution, for small current of LED on 5mA,10mA.

    The Vce of BJT will also be small ?

    The LEDXX pin voltage is 10-6-Vce=4-Vce

    This BJT can garantee the voltage of LEDXX lower than 3.6V ?

    Best regards

  • 0
    •  Analog Employees 
    on Oct 24, 2020 5:45 PM 1 month ago in reply to future

    Hi,

    Yes, using series resistance limits the range of operation, it could be used if the current that you want to operate is well known. I gave 10 V and 20 ohms as just examples though, because I did not have the information that you gave me in your very last response and I assumed you decided to operate 100 mA and upwards based on your original post. But you're right, series resistance limits the range in this case.

    In terms of BJT, you have to check the datasheet of the BJT you choose. For very low currents(which I didn't think you would operate based on your original post), BJT has to be well chosen such that it ensures enough collector emitter voltage drop for your case. It could limit your range as well. If you do make some characterization of your system, BJT and series resistor options are still your least risky options.

    By the way, using a pull up resistor could be risky. It may suck the current by creating an alternative path for the LED driver current to choose. 

    If you have an option to use a GPIO to sync, it would be better. You could enable one GPIO that is not used as interrupt as an output, and set that GPIO output to pulse output by setting GPIOx_ALT_CFG bits in register 0x0B to 0x05,0x06 or 0x07. By that, you will get a sync with timeslot A, timeslot B or both timeslot LED pulses from a GPIO output. These settings will assert when the LED driver is turned on, therefore it will give indication of LED driver turning on and off. 

    If you need further help, you should open up a support ticket for design help by field engineers. Field application engineers could help your design process by helping you in person. For that, please fill out the form down below:

    https://form.analog.com/form_pages/support/integrated/techsupport.aspx

    Best regards,

    Yigit

  • By that, you will get a sync with timeslot A, timeslot B or both timeslot LED pulses from a GPIO output. These settings will assert when the LED driver is turned on, therefore it will give indication of LED driver turning on and off. 

    ...........

    The above method is just what I want.

    I will make a standalone high voltage LED driver circuit.

    Set the GPIO and use this GPIO to turn on the outside LED driver.

    Use this GPIO to sync the standalone LED driver with the sensor ADC circuit.

  • 0
    •  Analog Employees 
    on Oct 24, 2020 8:13 PM 1 month ago in reply to future

    Page 36-37 could help you with this.(OPTIONAL TIMING SIGNALS AVAILABLE ON GPIO0 AND GPIO1 section)

    Enable the GPIO pin you will use, configure it as output. Using 0x05 or 0x06 or 0x07 settings of GPIOx_ALT_CFG, you will get the sync signal you want. Basically connect a pull-up to the GPIOx pin you use to sync your circuit with the LED driver of the ADPD1080.