This is the schematic TI gave me.
- Vin 24-36v
- Vout 18.6v
- Iout .89A
- Ripple 10%
If you provide me a link to the tool you use to configure this driver, I would like to take a look at it.
You have Iadj (pin 6) pulled high. So the driver is going to try and output the maximum current of 2.5 Amp. The PWM from your micro has to do all the dimming.
If for some reason your PWM fails, the current goes to 2.5 Amp and you will likely lose all your LEDs.
Iadj should throttle the current back to 0.89A and if you want it to stay at 0.89A you do not have to do any PWM.
Always set the max output current to your maximum value. Then use the PWM only to make and adjustments to dim below the max current.
The inductor you had that you replaced. It was a much better inductor than the Bourns you are now using. The Bourns has about 10x more DC resistance.
I do not know why you are only using a 15µH inductor it seems too low. If the TI app suggested a 15µH inductor you fed the app the wrong parameters or there is a bug in the app.
You want the driver to operate in the continuous conductive mode. In the discontinuous mode your inductor can produce ringing, your LEDs may flicker. If you can see flicker there is a serious problem in the design. The human eye and mind cannot perceive 100-200hz flicker, it is measured not seen.
If the amplitude of the ringing in the inductor is so high you can hear the ringing, there is a problem in your design, not the manufacturer of the inductor.
To operate in DCM it is better to have a low value inductor. The ripple current is not good for the life of the LED.
The inductor is what keeps the current flowing when the switching voltage swings.
The TI documentation is very good and very detailed. They go into detail on each and every component. You need to understand why they do that and do it.
This is not simple stuff. There are resources.
You need to take a look at your UVLO too. The following link will explain that.
LINK: Design Challenges of Switching LED Drivers
The following is from TI's app note Understanding Buck Power Stages in Switchmode Power Supplies
Section 5, Component Selection is an easy read. Well, compared to the prior sections.
In switching power supply power stages, the function of inductors is
to store energy. The energy is stored in their magnetic field due to
the current flowing. Thus, qualitatively, the function of an inductor
is usually to attempt to maintain a constant current or sometimes
to limit the rate of change of current flow.
The value of output inductance of a buck power stage is generally
selected to limit the peak-to-peak ripple current flowing in it. In
doing so, the power stage’s mode of operation, continuous or
discontinuous, is determined. The inductor ripple current is directly
proportional to the applied voltage and the time that the voltage is
applied, and it is inversely proportional to its inductance.
In addition to the inductance, other important factors to be
considered when selecting the inductor are its maximum dc or peak
current and maximum operating frequency. Using the inductor within its
dc current rating is important to insure that it does not overheat or
saturate. Operating the inductor at less than its maximum frequency
rating insures that the maximum core loss is not exceeded, resulting
in overheating or saturation.
There are many types of inductors available; the most popular core
materials are ferrites and powdered iron. Bobbin or rod-core inductors
are readily available and inexpensive, but care must be exercised in
using them because they are more likely to cause noise problems than
are other shapes. Custom designs are also feasible, provided the
volumes are sufficiently high.
Current flowing through an inductor causes power dissipation due to
the inductor’s dc resistance; the power dissipation is easily
calculated. Power is also dissipated in the inductor’s core due to the
flux swing caused by the ac voltage applied across it but this
information is rarely directly given in manufacturer’s data sheets.
Occasionally, the inductor’s maximum operating frequency and/or
applied volt-seconds ratings give the designer some guidance regarding
core loss. The power dissipation causes a temperature increase in the
inductor. Excessive temperature can cause degradation in the
insulation of the winding and also cause increased core loss. Care
should be exercised to insure all the inductor’s maximum ratings are