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DRV8876: Driving LEDs

lu tan
Prodigy 160 points
Part Number: DRV8876
Other Parts Discussed in Thread: DRV8220, DRV8210

Hi,

I am trying to use the integrated H-bridge and integrated current sensing capabilities of the DRV8876 to forwards and backwards drive LEDs in parallel like this:

I read through the datasheet, but there are still a few things unclear to me. Thought that I create a post here on the forum to see if anyone can answer or direct me to a post with the answer.

1. Output current control: My thought is to either the PH/EN control mode or PWM control mode. Ae these two modes the same with the exception that PWM mode " allows for the H-bridge to enter the Hi-Z state without taking the nSLEEP pin logic low"? In either mode, I send out a PWM from my processor and feed to it the EN/IN1 pin. By changing the duty cycle of the PWM, the magnitude of the output current between OUT1 and OUT2 pins changes. Is my thought process correct?

2. My application requires that the max current through each LED branch to be 45mA (designated as 100% intensity), with ability to change intensity to 50%,25%,13%,7%,4%, and 2%. On datasheet page 4, I observe that IOUT between OUT1 and OUT2 can go up to 3.5A. My thought process is to observe the current shunt resistor RIPROPI with my processor's ADC. To increase/decrease the intensity, I will increase/decrease the PWM duty cycle until the ADC reads the correct current.   

    Question 2a: Looking at section 8.2.1.2.1 Current Sense and Regulation of the datasheet. Suppose I set ITRIP at 50mA ( a little higher than the max allowable of 45mA for my application) and use the same VADC of 2.5V, I      calculate RIPROPI as 50k and VREF as 2.5V.

    Question 2b: Is there another part number that has the same integrated features as this one, but has lower Iout max? Reason I ask is I was thinking that a lower Iout max may allow for higher resolution on the ADC reading       the detection current value.

3. My application requires that the voltage potential between OUT1 and OUT2 be 8V nominal and 10V max. If I input 8V to Vm, will this allows VOUT1-OUT2 to be 8V during forward drive and -8V during reverse drive? 

Thank you for looking at my concerns. Please let me know if anything is unclear.

Best regards,

Lu Tan

  • 0
    TI__Guru 55605 points

    Lu,

    1)  Your thought process is correct.

    2)  Your thought process is correct, but the accuracy at these current ratings is going to be pretty poor.  33% of your 45mA range.  

    3)  I would consider the DRV8210 or DRV8220 for these voltage ranges and a much lower current part.

    Please see figure 9-27 in the datasheet for an example of using a low-side shunt resistor in the ground path (DRV8210 datasheet).  With an external amplifier and gain stage, this would allow you to get much better accuracy.

    Regards,

    Ryan