To control their brightness, LEDs need a constant current; this can be done with a resistor placed in series with the LED string. Both the LED-string voltage and the supply voltage can vary, so a dedicated LED driver is a must to guarantee the current accuracy. Two solutions – each with advantages and disadvantages – are widely used: a linear constant-current LED driver or a step-down switching converter.
Linear drivers are simple solutions requiring few components and are essentially noise-free, but they dissipate heat proportionally to the difference between the supply voltage and the LED forward voltage. To protect against overheating, the driver package may require an additional heat-spreading area on the PCB, adding to the cost and amount of PCB real estate required and increasing the risk that the driver IC will enter thermal shutdown and turn off the LEDs. If the driver is located next to the LEDs, the additional heat can cause the LEDs to operate at an elevated temperature, shortening their lifetimes.
Step-down, or buck, converters are efficient and generate little heat, but switching solutions require an inductor and a Schottky diode. The solutions also create noise, especially when the supply voltage drops and approaches the LED forward voltage. In automotive applications, RFI (radio-frequency interference) is a major concern. EMI/RFI filters are recommended in front of the switching converters to prevent high-frequency-switching conducted noise from going back into the supply, as it may interfere with other equipment, such as the AM/FM-band radio.
Linear-driver operation is at its optimum when the buck converter behaves poorly, running out of headroom. To benefit from both approaches without the disadvantages, you can adopt a combined linear/buck solution, which minimizes the switching noise without compromising efficiency.
Ideally, a battery voltage varies across a wide range, such as in automotive (8 to 17 V) applications, where the linear/buck driver provides the desirable lower-noise operation and higher efficiency. Linear LED drivers convert to buck mode once the supply voltage increases above a limit, thereby protecting the linear driver from overheating.
The circuit described here selects each LED driver independently with adjustable threshold voltages when transferring between the switching and linear modes, with additional hysteresis for a smooth changeover. Figure 1 shows the schematic using CAT4201 350-mA buck and CAT4101 1 A, constant-current LED drivers; the comparator logic is also shown. Unlike the more common buck topology, with a high-side switch and a low-side diode, the CAT4201 swaps those devices.
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| Figure 1. | The LM393 comparator monitors the LED string’s low-side voltage and enables either the buck regulator (CAT4201) or the linear regulator (CAT4101). |
As with a typical buck switcher, when the switch turns on, the current increases through the inductor, L, and the LEDs until it reaches a peak value equal to twice the average LED current; then the switch turns off. The charged inductor forces the current to continue to flow through the Schottky diode, D1, and the LEDs until it drops to zero; the cycle then repeats. This switching operation is referred to as boundary conduction mode.
The R1/R2 resistor divider produces V+ at a fraction of the cathode voltage. If the comparator (LM393) input voltage is greater than a fixed reference voltage of 2.5 V, then the output is high; OUT is low, disabling the linear driver and enabling the buck converter. If V+ is lower than the reference voltage, the comparator output is low and the linear driver is enabled, while the buck converter is disabled. The feedback resistor, R5, adds some 0.6 V hysteresis, such that once the cathode voltage rises above 3.6 V, the buck turns on; as the cathode voltage falls below 3 V, the linear driver takes over. Note that if the other half of the LM393 is not used for another LED power supply, good design practice dictates that all unused input and output pins on the LM393 be tied to ground.
Figure 2 shows the LED current regulation for the buck alone and the combination linear/buck driver. The linear/buck driver extends the LED current regulation down to a lower supply voltage below 8 V, compared with the buck alone, allowing the LEDs to remain on, even as the battery voltage drops further. For a supply voltage below 11 V, the buck alone loses its accuracy and also generates higher switching ripple current back into the supply. The lower-frequency ripple current is more difficult to suppress with an EMI filter. On the other hand, under the same supply-voltage range, the linear driver provides better regulation and noise-free operation.
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| Figure 2. | The linear/buck current sink extends the compliance range for current regulation down to a lower supply voltage (below 8 V), compared with the buck regulator alone, and reduces EMI with low battery. As a result, the LEDs can remain on under low-battery conditions. |
In spite of the additional components, the combined linear/buck solution is valuable in applications where low-noise performance and the extended supply-voltage range are desirable. Linear-to-buck transition voltages can be set to optimize the thermal dissipation.
