Efficient white LED driver circuit

Since the adoption of color LCD displays in cellular phones and portable game consoles, there has been a large demand for high-brightness solid-color background light sources. White light diodes are considered to be the best background light sources currently available on the market. This article introduces the characteristics of two series of commonly used white LED driver ICs, XC9103 and XC6367, and the design of the driver circuit. Introduction to commonly used drive circuits Figure 1 is a schematic diagram of a commonly used series LED drive circuit. The drive IC in the figure is an XC9103 series DC converter. This circuit has the following characteristics: 1. White LEDs can be used in series, regardless of the number of connected LEDs. Only one resistor is needed to provide a constant current to them; 2. The number of components is reduced, further reducing the overall power consumption of the circuit; 3. The difference in the forward voltage of each white LED will not affect the working current of the white LED; 4. You can adjust the working current of the white LED with a resistor; 5. You can change the number of white LED connections at will without making any other changes; 6. Use ceramic capacitors to obtain low ripple noise and extend the service life of the circuit ; 7. Can greatly save circuit space: both XC9105 and FET+SD are packaged in SOT23, the maximum thickness of the coil is only 1.2 mm, and the input and output capacitors are ceramic capacitors. When using XC9103 series devices, ceramic capacitors can be used as CL capacitors to suppress harmful signal radiation. The boost DC/DC converter in the figure is used to output a constant drive current to drive the LED. The LED drive current value is equal to the FB control voltage divided by the resistance of the connected resistor. The FB control voltage of XC9103 is 0.9V, and the FB control voltage of XC6367 is 1.0V. Therefore, by changing the resistor value, the LED driving current can be adjusted to the required value. The output voltage of the DC/DC converter is equal to the forward voltage of the LED plus the FB terminal voltage. If the number of LEDs is greater than 1, the output voltage is the product of the FB terminal voltage plus the forward voltage of a single LED and the number of LEDs. The driving current of the LED in the figure can be calculated by the following formula: ILED=VFB /RFB2 In the above formula, ILED is the LED driving current, and VFB is the FB pin control voltage. The FB control voltage of XC9103 is 0.9V, and the FB control voltage of SC6367 is 1.0V. The RFB2 resistor value in the picture is 47 ohms, so the LED drive current is 19mA. The efficiency is the percentage of the product of the voltage drop on the LED and the driving current relative to the product of the input voltage and the input current: VLED×ILED/(VIN×IIN)×100% where VLED is the voltage drop on the LED. OUT). " hspace=12 src="../news_uploadimg/20041228234914389.gif" width=250 align=right vspace=12> When using XC6367 series devices, the RSEN resistor is not required and a 221μF tantalum capacitor is used as the output capacitor CL. Drive circuit settings The external device ratings listed in the figure only allow 4 white LEDs to be connected when VIN=3.0V. If 5 or more white LEDs are to be lit, the VIN voltage must be increased, or the rated current value must be selected. Higher inductance coil or field effect transistor, and the DC resistance of the selected device should be smaller. When the LED front-end voltage Vf=3.5V, VDD and VCE can be driven from VOUT when the number of white LEDs is no more than two. Obtained, the capacitor CDD can be removed at this time; when the number of white LEDs is 3 or more, since VOUT will exceed 10V, the efficiency calculation formula of the drive circuit can only be obtained from VIN: efficiency (EFFI%) =. The voltage on the white LED × the current on the white LED × 100 / input voltage × input current. The efficiency of the circuit will also be very different depending on the LED connection method. Table 1 shows the comparison of the efficiency values ​​of LEDs in series and parallel. It is found that as the number of LEDs increases, the efficiency of the circuit becomes higher. With the same number of LEDs, the efficiency of series connection is much higher than that of parallel connection. As mentioned above, if more than 4 white LEDs and resistors are used in series, it is necessary to use them in series. For white LEDs, you need to choose a coil with a higher rated current value.