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capacitor selection for LED's

Category: Software
Product Number: ADPD1080

Hi I am building a vital signs monitor using the ADPD1080. I have a question in regards to the selection of the capacitor CVLED on page 33 of the datasheet.

I am trying to understand what is meant by the VFB_LED_MAX and VLED_MIN values and how do i determine them. I am using a dual RED/IR LED (APW-MW2-1210-010) and using an ADP150 3.3V power regulator to power them.

Is the VFB_LED_MAX just the max forward voltage of the LED'd in the datasheet?

Is the VLED_MIN value just the voltage output of the ADP150?

If i pulse the LED's for 2us the capacitor values using formula 2 on page 33 would be

RED LED = (2us*200mA)/(3.3V - (2.3+0.6)) = 1uF

IR LED = (2us*200mA)/(3.3V - (1.65 + 0.6)) = 0.38uF

Do these figures look ok??

The datasheet also states that if using a capacitor value that is too small it can issues.

Do i also need to have a separate cap for each of the LED's? is is possible to just use the one cap with the highest value for both the LED's?

Any advise would be much appreciated.

Regards

Jaime

  • VFB_LED_MAX is the worst case LED forward bias voltage at the highest driver current that will be used.   The LED forward bias voltage has part to part variation and it varies with current.  So if you have a million units you will want all of them to have enough compliance room (also called headroom) on the voltage driver for the maximum current.  If you design for the LED maximum of the average of your million LEDs, then half of your units will not have enough compliance room.

    The VLED voltage will not be perfectly constant in the circuit.  When the LED driver fires charge will be removed from the CVLED capacitor and the voltage will drop.  At the same time the power management IC will try to refill this capacitor, but this refil is not perfectly instantaneous.  The lowest level at which VLED drops while the LED driver fires is called VLED_MIN.  This has to do with the LED supply voltage in your circuit.  Ideally it would be the output voltage for the ADP150, but it is not going to source 500mA in 2us.  Also your PCB routing lines have some real world inductance and resistance.  The safest assumption would be to ignore the contribution of the ADP150 while the LED driver is on and assume that all of the charge is coming out of CVLED, and that CVLED is then refilled in-between LED pulses.

    Your capacitor calculations look good.  now just guard band them a bit to account for any part to part variation.  Putting a 1uF on the IR and a 2.2uF on the red would be a reasonable guard band.