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)
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 frequency varies according to different output voltages and different inductance values.
When the inductance is reduced, the fluctuation of the load current is significantly increased within the variation range of the output voltage.
The LED driver circuit produces audible noise (audible noise, or microphonic noise). Usually white LED driver is a switching power supply device (buck, boost, chargepump, etc.), and its switching frequency is about 1MHz, so in the typical application of the driver, there is no noise that can be heard by human ears. 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.
We all know that a low-frequency switching signal acts on a common wire-winding coil, which causes mechanical vibrations between the coils in the inductor. The frequency of the mechanical vibration falls just at the above frequency, and the noise emitted by the inductor can He was heard by the human ear. The inductor produces some noise and the other part comes from the output capacitor.
Selecting the inductor inductance value is the most important value in the reference design range. The proper selection of the sense value mainly needs to consider the condition that the line works in the proper frequency range, the appropriate switching frequency reduces the number of MOS switches, and reduces the heat of the mos. Avoid the same frequency interference with the PCB line; choose the appropriate internal resistance of the inductor, the internal resistance is the main factor of the inductor heating, thus improving the line efficiency; choose the appropriate current value, sometimes the volume and cost are the main factors, but still greater than 2 times the peak current (usually 65%), even if the board space is very precious, it should also ensure 30% reserved space, which can effectively reduce the internal resistance and reduce the heat; the quality is not good. Winding loose inductors can also be noisy; unshielded inductors will change the line oscillation frequency when the metal casing is installed, resulting in noise. In this case, the inductor needs to be shielded; in addition, when the wavelength of the shielded interference signal is exactly the same as the metal When a certain size of the casing is close, the metal casing can easily become a large cavity, that is, electromagnetic waves will be on the metal machine. Reflected back and forth inside the shell and will overlap each other.
For best efficiency, a ferrite core inductor should be used. An inductor that can handle the necessary peak current without causing saturation should be selected to ensure a low DCR (copper resistance) for the inductor copper. In order to reduce the I2R power consumption. Remember that the inductive copper wire insulation layer can't withstand 160 degrees or long time high temperature environment. SMT sometimes has an effect, which will cause serious changes in the inductance value. It is necessary to carefully understand the supplier's product temperature tolerance limit requirements.
EMC inductance selection:
EMC inductors are used in input and output filters to reduce conducted interference and are used in designs that are below the EMC standard. All inductors require a ferromagnetic core instead of a ferrite. Before it saturates, it can handle more current, and you need to choose the appropriate current value according to the load.
To make the filter inductor, which core material is used, in addition to the prevention of core saturation, the constant magnetic permeability of the core must also be considered. It should be pointed out that some designers often only pay attention to the index of inductance, choose materials with high magnetic permeability to reduce the number of turns of the coil, and when the rated current of the inductor is large, whether the inductance is reduced or not, to what extent, will it not? Will reach saturation, considering less. This should be avoided.
Since the iron powder core has high saturation magnetic flux density, good constant magnetic permeability and low price, it has been widely used.
Output capacitor parts selection:
The output can use the output capacitor simultaneously to achieve precise control of the target frequency and current. The capacitor reduces the frequency over the entire input voltage range, and a small 4.7μF capacitor can significantly reduce the frequency. The current adjustment can also be improved due to an increase in the capacitance value. It can be easily seen from the picture below, there is an inflection point on the graph, and then increase the capacitance value, which has little effect on the adjustment of the operating frequency and output current.
Increasing the output capacitance (COUT) essentially increases the amount of energy that can be stored in the output stage, which means that the time it takes to supply current is longer. Therefore, by slowing down the di/dt transient of the load, the frequency is significantly reduced. With the output capacitor (COUT), the inductor current will no longer match the current seen on the load. The inductor current will still be the shape of a perfect triangle, and the load current will have the same trend, except that all sharp corners become rounded and all peaks are significantly reduced.
The application design uses a low ESR (Equivalent Series Resistance) ceramic capacitor on the output to minimize output ripple. X5R or X7R material dielectrics are used, which maintain their capacity over a wide range of voltages and temperatures compared to other dielectrics. For most high current designs, a 4.7 to 10uF output capacitor is sufficient. Converters with lower output currents only require a 1 to 2.2uF output capacitor.
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