I recently designed a PCB for the TPS61169 LED driver. I used the sample layout from the datasheet, as it is a tiny SMD package and I couldn't breadboard it. I also followed the Typical Application in section 9.2.
However, it doesn't function properly.
Referring to 9.2 figure 4,
- Supply voltage is 5.0V
- CIN is 100uF tantalum + 0.1uF ceramic
- L is 6.8uH inductor
- D is an SS24 schottky diode
- COUT is 2x 1uF ceramic
- RSET is 2x 1.6R resistors in parallel (0.8R equivalent)
In testing, I have been trying to drive 3 green 3W LEDs in series.
Using the equation in 8.3.4, RSET = 204mV/250mA = 0.816R.
I calculated the inductor peak to peak current using equation 4 to be 3.84A, and the inductor peak current using equation 5 to be 2.17A. I don't think these are correct, however, as they seem high compared to the values in table 2 of the datasheet - Recommended Inductors.
Here is a schematic of the board:
A few notes: the two inductors and the two diodes are for different components - through hole and SMD - as I didn't know what components I had or could get at the time. Also, only two of the resistors R2 - R5 are populated.
Here is the board done in Eagle. Layout is per the datasheet.
- unexpectedly high current - my power supply reports a current of whatever its limit is set at, and will go up as high as I dare to set it. The current measured by my DMM in series with the LEDs is about 1/4th, and scales with the input current. I have no clue where the extra current is going.
- voltage drop - along with the high current, my power supply goes into CC mode, and the voltage drops to 4.2V at 250mA. As I increase the current, the voltage decreases.
- CTRL pin - the CTRL pin is used to dim the LEDs. However, feeding it a 20KHz square wave from my function generator has no effect on LED current. If I ground the CTRL pin, give it a voltage under 0.7V, or leave it floating, the LEDs will not light.
Here are some screenshots from my scope...
What I think is going wrong:
- I had trouble soldering the TPS61169, as I stupidly didn't put thermals on the connected copper planes, therefore, I had to use excessive heat. I don't think I destroyed the TPS, as I made two of the boards, and they both display the same problems. It's less likely that I destroyed both.
- more likely is that my inductor is saturating. Unfortunately, I don't have a different one yet, and the one I used was from aliexpress, and didn't have specs on current handling or saturation current.
As soon as I get a proper sized inductor, I will try that.
Am I correct in suspecting that the inductor is saturating? Are the symptoms consistent with a saturating inductor? Is there anything else that could be causing these problems that I am missing?
Thank you all for your help.