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ADP8140 - current sink headroom

Hello Team,

I am planning to use ADP8140 ELD controller to drive backlight LED module with operating voltage of 26V - 29V with max 50mA current. How do I estimate the current sink headroom voltage range for 50mA? In datasheet specs, I couldn't find the specs for 50mA current. Please let me know.

Also, I am planning to use PWM dimming method, in that case out of two available options which will be better - ADP8140ACPZ-1-R7 or, ADP8140ACPZ-2-R7? 

If I use only one SINK pin, can we still detect FAULT condition?

Thanks & Regards,

Rajesh Kumar

  • Hi 

    Please see tables 7 to 9 for  the headroom voltage. 

    Using  ADP8140ACPZ-1-R or ADP8140ACPZ-2-R7 would depend on your application. e.g minimum voltage, number or sink used for FB_Out and operating current.

    If you are to use a single sink pin and using FB_OUT to control a DC-DC or PMOS the system would experience a fault because it will detect invalid conditions on the unused sink pins 

    Regards,

  • Hi,

    Thanks for responding back.

    I would like to summarize our application as below:

    1. LED operating current is typ 50mA and can be reduced to 5mA using PWM dimming preferrably.

    2. We have only one LED module, so require only one sink pin to be used.

    3. Preferred mode of operation will be PWM EA operation by connecting mode pin to REG.

    With respect to above, can we leave unused sink pin floating. We don't want to use FB_OUT and FAULT pin of the device. We have external control mechanism to control the power stage. We are planning to use this device as sinking the LED current.

    Our requirement for LED current sinking is as follows:

    For day light visibility, sinking current typical is 50mA and operating voltage is 28.5V+/-0.5V. However in night mode, sink current reduces to about 5mA, and operating voltage will be changed to 26.5V +/-0.5V. We want to use PWM dimming either using DIM pin or VT pin to reduce the current to 5mA in less brightness mode.

    Please let me know if you require further details.

    Regards,

    Rajesh

  • Hi 

    If you are to leave the other sink pins floating it might cause problems on with ADP8140. Try to use ADP8140ACPZ-2-R7 and the tie ch1-3 together then leave 4 open

    I would suggest looking on the some of our LED drivers.

    https://www.analog.com/en/parametricsearch/11429#/p5351=1|1&p0=LT&d=5351|5574|5358|5362|5382|5357|s3|s5||

    Regards

  • Thanks for the clarifications.

    We can consider using -2 version of ADP8140. In that case to meet total 50mA current requirement, we have to set the Rset value to 16.9K ohm to set 50mA/3 = 16.7mA of sink current. To set lower current shall we limit to Rset value to 75k ohm, i.e., 34mA as per datasheet, Figure 14. 

    In datasheet, first page under features section "4 current sink channels with an adjustable current from
    125 μA to 500 mA". So how to set current of say 2mA, for this Rset value should be 1.28M ohm which is outside the range specified in figure 14. Please clarify.

    While using ADP8140-2 version, can we leave FB_OUT unconnected? We will be using PWM dimming mode using DIM pin, so in that case can we leave VT and MIN pin unconnected? Please clarify.

    Regards,

    Rajesh