LED driver to illuminate for video camera

The TPS6116x, TPS6118x and TPS6119x family of LED drivers can have "split" power rails, i.e., have the power stage input voltage different from the IC input voltage.  Please ensure that the IC input voltage is above the minimum and the power stage input voltage is below the SW pin maximum but is high enough not to result in input current's larger than the switch current limit.

Thanks Jeff,

The TPS61166 seems to be the best fit, but I had some questions about the split rail example implementation in the datasheet.  Am I correct in assuming I should pick R1 such that the resulting current (I=.2V/R1) is the max current I wish to drive the LEDs at?  For analog dimming, should I send the PWM signal to the PWM pin, as opposed to leaving it not connected as in figure 2? Should the Vin pin be connected to the signal power source? Are there any other alterations to the circuit I should make? 

I may have misunderstood your original question.  Can you explain to me, again, how the IC will be powered?  What is the battery voltage range?  Will it tie to the IC?  The TPS61166 has an internal regulator that steps down the voltage from a multi-cell battery.  

Also, just to clarify, the TPS61166 datasheet figures only show the device performing PWM dimming, internal  FET that is in series with the output.  The TPS61160/1/5 all take an input PWM signal and perform analog dimming by changing the reference voltage against which the boost converter regulates.  The TPS61166 can be modified to perform analog dimming but I suspect we have another IC that be a better fit.

Sorry, it’s more likely that my own inexperience is the problem.

The camera module, a few sensors, and the MCU will be powered by a 3V battery.

We’d like to use a separate battery (6V to 12V) with a higher amp hour rating to power the LED array as this will be the largest power draw in the device.

We’d like to use the MCU to control the dimming of the LED array; so I’d need an LED driver that could accept a 3v control signal (PWM, or i2c) while driving the array using the larger battery.

We plan to use LEDs in the neighborhood of 1A, so the driver would have to match that.

I suggested the TPS61166 because it seemed to offer what I was interested in, but according to your description I was mistaken, PWM dimming would work just as well as analog dimming (its not a high speed camera) but I was curious about analog dimming because it was my understanding that it was more efficient under most circumstances; correct me if I’m wrong.

The TPS61166 can only provide 300mA at most.  How many LEDs are in series and how many are parallel?  I am assuming that you have 4 or more LEDs in series and so will need a boost converter based LED driver (i.e., you need 4*3.5V/LED=14V from a 12Vmax input boost converter).  Otherwise, you do not need a boost converter based LED driver.  If you need a boost based driver, then I suggest the TPS61500.  You can power the TPS61500 IC and boost power stage from the 6Vto12V battery.  You can apply PWM signal to the EN pin and get analog dimming.  Analog dimming is slightly more efficient over the middle of the dimming range.  I have attached a draft of a paper that I wrote on WLED backlighting that might help explain things.  The final version was included in our latest 2009/2010 Portable Power Design Seminar.

Thanks Jeff,

Yes we need at least 4 LEDs that are rated at a 3.5v drop, my logic was that if I had a string of 5 LEDs in series and I wanted them operating at 1A then I would require a LED driver with a max output voltage of at least 17.5V and an Iout of 1A.

If the TPS61166 can only provide 300mA where do I find that value?

According to its description Iout(Max)(A) is 1, and I can’t find a 300mA value anywhere in the description. Am I supposed to calculate that value? If so, how would you suggest that I search the TI catalog, I found the TPS61166 by filtering the white LED drivers by Iout at >=1A.  And what does the Iout value signify?

The power FET of the LED driver's boost converter must be capable of sinking more current than the converter can provide as output current.  So, the switch current rating of the driver is always higher than the output current.  Our on line part database uses the Iout field for switch current limit for boost converter based devices.  The search engine is supposed to do the following computation to only provide LED driver that can power your application:

efficiency = Pout/Pin = ((sumofVLEDs+VFB) x ILED) / (Iin*Vin)

Assume an efficiency for a datasheet application near your applications input and output (somewhere around 87% is conservative but reasonable) and then solve for Iin.  Iin needs to be about 20% below the maximum switch current rating of the driver selected.

The TPS61166 is a bit unique in that it has a extra series FET that performs PWM dimming and also provides over protection.  The I(OL) specification in the datasheet is 300mA typical.