Stroboscope 3.0: High Power LEDs and Driver

by tunafishsandwich

I received my Rebel LEDs and driver board yesterday.

I. Optimizing Performance (I.e., A lot of trouble to figure out I needed to buy 3 9v batteries)

The heart of the driver is a ZXLD1360.  The key performance attributes I want from the drive are current and turn-on/turn-off time.  Maximal current translates to maximal brightness.  Minimal turn-on and turn-off time translate to minimal blur during operation.

Source material:

[Note: As far as I can tell, the schematic for the sparkfun driver board is incorrect.  A similar driver board schematic has significantly different parameters for key circuit components that make a lot more sense given the application of these boards.  I’m going forward under the assumption that the component values on the second schematic are correct.  To the my ability to measure tenths of ohms, it would seem this assumption is correct.]

The ZXLD1360 is generally used in a circuit like so:

Current: The parameter that determines current output is the value of a sensing resistor (Rs) external to the ZXLD1360.  A value of 0.1 Ω should result in a drive current of 1000 mA, which would push the Rebel LEDs to the limit according to the datasheet.  According the the sparkfun datasheet, this value is 0.15 Ω.  This should result in a drive current of 667 mA per this table from the ZXLD1360 datasheet:

The plot below shows that 667 mA will get most of the performance out of the Rebels.  I might try to modify the board later, but Rs is an SMD component, which will make things difficult.  For now I’m going to accept Rs as a constraint beyond my control.

Ton/Toff: Several parameters influence the switching time for the driver.  The datasheet provides equations for both transients.



Refer to the datasheet for definitions of all of the parameters.  The parameters of interest to me are Vin, Vled, L, and Rs.  As I would expect, inductance value of L1 has a key effect on Ton.  This value is also a constraint courtesy of sparkfun.  They have chosen a value of 68 μH.  Rs is already fixed at 0.15 Ω.  I’m using 3 Rebels, in series.  A value of current of 660 mA corresponds to a voltage drop of  3.4 v, according to the Rebel datasheet, so Vled would 10.2 v.

The only parameter I really have under my control, absent modding the board, is Vin.  Since Vled is 10.2 v, using my 9 v wall wart is out of the question.  Using batteries, the obvious choices are 12 v, 18 v, 24 v, and 27 v.  Using the parameters discussed above, and plugging these voltage options into the ZXLD1360 performance calculator, I get the following for switching times:

  • 12 v -> Ton= 12.0 μs and Toff=1.5 μs
  • 18 v -> Ton= 2.4 μs and Toff=1.6 μs
  • 24 v -> Ton= 1.4 μs and Toff=1.7 μs
  • 27 v -> Ton=1.2  μs and Toff=1.8 μs

Based on this, I’m going to use 3  9 v batteries in series.  Now, several of these parameters are interrelated, e.g., supply voltage will have some effect on drive current.  From the performance calculator, with a supply voltage of 27 v, my average drive current will be closer to 680 mA, which means I’ll get a little more out of the Rebels, brightness-wise.

II. Interfacing With Existing Hardware

The ZXLD1360 can be pulse controlled via the Vadj pin as follows: Vadj floating -> LEDs on, Vadj low -> LEDs off.  This means it should not be driven from an arduino digital pin (in the normal sense at least).  The datasheet recommends using an open collector stage on the Vadj line, which would require more components to  make this work.  I found this novel solution on the arduino forum thanks to google.  Code is here.  My lord, these LEDs are bright!