In order to maintain strict hysteresis current control when designing the LED constant current source, the inductance must be large enough to ensure that energy can be supplied to the load during HO and ON, and the load current is not significantly reduced, resulting in the average current falling below the expected value.
First, let's look at the effect of the inductor. Suppose there is no output capacitor (COUT), so the load current and the inductor current are exactly the same, which can more clearly explain the effect of the inductor. The figure below shows the effect of the inductance value on the frequency over the range of the input voltage. It can be seen that the input voltage has a great influence on the frequency, and the inductance value has a great influence on the frequency reduction when the input low voltage. (Note: It is not necessarily the same as the reference picture, but it is only a description of the problem)
![[Support] Correct selection of LED constant current source peripheral components](http://i.bosscdn.com/blog/lazy.gif)
The figure above shows the frequency response at different inductance values. The figure below shows that the load current changes significantly in the range of input voltage variation when the inductance is reduced.
The figure below shows the variation of the frequency according to different output voltages and different inductance values.
The figure below shows that the fluctuation of the load current is significantly increased within the variation range of the output voltage when the inductance is reduced.
The LED driver circuit produces audible noise (audible noise, or microphonic noise). Usually white LED drivers are all switching power supply devices (buck, boost, chargepump, etc.), and their switching frequency is around 1MHz, so there is no audible noise in the typical application of the driver. However, when the driver performs switching adjustment, if the frequency of the PWM signal falls exactly between 200 Hz and 20 kHz, the inductance and output capacitance around the white LED driver will produce noise that is audible to the human ear. Therefore, avoid using low frequency bands below 20 kHz during design.
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