Combination of linear and switching LED power supply design

　　To control brightness, light emitting diodes (LEDs) require a constant current. This is achieved by placing a resistor in series with a group of LEDs. Since both the voltage across a group of LEDs and the supply voltage may vary, a dedicated LED driver must be used to ensure accurate current. Two solutions are widely used: linear constant current LED drivers and step-down switching converters, each with their own advantages and disadvantages.

　　Linear drive is a simple solution that requires very few components and is essentially noiseless. However, the heat it dissipates is proportional to the difference between the supply voltage and the LED forward voltage. To prevent overheating, its package may require an additional heat sink on the PCB, which increases the cost and number of PCBs required, and also increases the risk of the driver IC shutting down due to thermal shutdown, thereby turning off the LED.

Figure 1 The LM393 comparator monitors the low-side voltage of the LED string and enables either a buck regulator (CAT4201) or a linear regulator (CAT4101).

　　If the driver is placed next to the LED, the extra heat will cause the LED to run at a higher temperature, reducing its life. Step-down (or buck) converters are efficient and generate little heat, but the switching solution requires an inductor and a Schottky diode. This solution also generates noise, especially when the supply voltage drops quickly to the LED forward voltage. In automotive applications, radio frequency interference (RFI) is an important consideration. It is recommended to place an EMI/RFI filter in front of the switch converter to prevent the noise generated by the high-frequency conversion from returning to the power supply , as it may interfere with equipment such as AM/FM band radios.

　　When the buck converter performs poorly and runs out of headroom, the linear driver operates optimally. To avoid the disadvantages and take advantage of both options, a combination of linear and buck can be used to minimize conversion noise while maintaining efficiency.

　　Ideally, the battery voltage fluctuates widely, such as in automotive applications (8V to 17V), and linear/buck drivers provide the required low noise operation and high efficiency. When the application voltage rises above the limit, the LED driver switches to buck mode to prevent the linear driver from overheating.

　　The circuit in this article allows for individually selectable adjustable voltage thresholds for each LED driver when switching between switch mode and linear mode, with additional hysteresis to ensure smooth transitions. The schematic shown in Figure 1 uses the CAT4201 350-mA buck driver from ON Semiconductor, along with the CAT4101 1A constant-current LED driver, with the logic comparator also shown. Unlike the more common buck topology, which has a high-side switch and a low-side diode, the CAT4201 swaps these devices.

　　As with a typical buck switcher, when the switch is turned on, the current through the inductor L and the LED increases until it reaches a peak value that is twice the average LED current. The switch is then turned off. The charged inductor forces the current to continue flowing through the Schottky diode D1 and the LED until its value reaches zero. The cycle then repeats. This switching operation is called critical conduction mode.

　　The R1/R2 resistor divider produces V+, which is a fraction of the cathode voltage. If the input voltage to the comparator (LM393) is above the fixed reference voltage of 2.5V, the output is high; OUT is low, disabling the linear driver and enabling the buck converter. If V+ is below the reference voltage, the comparator output is low, enabling the linear driver and disabling the buck converter. The feedback resistor R5 adds 0.6V of hysteresis, meaning that once the cathode voltage exceeds 3.6V, the buck converter starts; when the cathode voltage drops below 3V, the linear driver takes over. Note that if the other half of the LM393 is not used for other LED power supplies , a better design approach is to ground all unused input and output leads on the LM393.

Figure 2 shows LED current regulation when using a buck converter alone and a linear/buck driver together. Compared to a buck converter alone, a linear/buck driver extends LED current regulation to supply voltages below 8V, allowing the LED to remain lit even as the battery voltage continues to drop. When the supply voltage is below 11V, using only a buck converter loses its accuracy and generates more switching ripple current back to the supply. EMI filters have a harder time suppressing ripple current at lower frequencies. On the other hand, a linear driver provides higher regulation and noise-free operation over the same supply voltage range.

　　Despite the increased component count, the linear/buck combination is valuable for applications that require low noise performance and extended power range. The linear-to-buck transition voltage can be set to achieve optimal thermal performance.

　　Figure 2. Compared to using only a buck converter, a linear/buck current sink extends the current regulation range to lower supply voltages (below 8V) and reduces EMI in low-battery conditions. As a result, the LED can remain lit even when the battery voltage is low.