I have multiple LED circuits that with varying voltage and current specifications. I would like to use a common driver and have tentatively selected the LM3409HV. I am using a microprocessor controlled PWM circuit sending a signal to the EN pin for dimming. I have build all of these to date, using values from the TI calculator, however I have seen varied results.
My most immediate need is the product specified in the second to last column. Per the calculations in the datasheet, I have selected a switch frequency of 500kHz, Roff=12.5kOhm, L=26uH, Rsns=155mOhms and Cin=6uF. When I power up the circuit with the load of 36 volts, the LEDs light up for a few seconds then turn off, then start to strobe at a variable rate. Any suggestions on where to look first?
| Vin | 18 | 36 | 18 | 36 | 18 | 36 | 42 | 42 |
| Vin-max | 36 | 42 | 36 | 42 | 36 | 42 | 43 | 43 |
| Average Vf | 3.1 | 3.1 | 3.1 | 2.9 | 3 | 2.9 | 2.5 | 2.5 |
| #LEDs in Series | 5 | 10 | 5 | 10 | 5 | 10 | 15 | 11 |
| Vo | 16.5 | 31 | 15.5 | 29 | 15 | 29 | 37.5 | 27.5 |
| ILED | 0.35 | 0.35 | 0.35 | 0.35 | 0.35 | 0.35 | 0.5 | 0.5 |
| #LEDs in Parallel | 1 | 1 | 3 | 2 | 5 | 3 | 3 | 3 |
| I-LED String | 0.35 | 0.35 | 1.05 | 0.7 | 1.75 | 1.05 | 1.5 | 2 |
| dILED-PP | 0.014 | 0.014 | 0.014 | 0.028 | 0.06 | 0.042 | 0.0576 | 0.0576 |
| dVIn-PP | 0.54 | 1.44 | 1.44 | 1.08 | 0.84 | 1.08 | 0.84 | 0.84 |
| VTurn-On | 5 | 10 | 5 | 10 | 5 | 10 | 10 | 10 |
| rD | 1.63 | 3.25 | 1.63 | 3.25 | 1.63 | 3.25 | 4.88 | 3.58 |
| VHYS | 1.1 | 1.1 | 1.1 | 1.1 | 1.1 | 1.1 | 1.1 | 1.1 |
| Eff | 0.96 | 0.96 | 0.96 | 0.96 | 0.96 | 0.96 | 0.96 | 0.96 |
