“Controlling LED brightness requires a driver capable of delivering constant, regulated current. To achieve this, the driver topology must produce enough output voltage to forward bias the LEDs. So how does a designer choose when the input and output voltage ranges overlap? Converters may sometimes need to gradually reduce the input voltage, but may also need to increase the output voltage at other times. The above conditions are typically found in applications with a wide range of “dirty” input power sources, such as in-vehicle systems.In this buck/boost operation, several topologies work well, such as SEPIC or quadruple switching.
Controlling LED brightness requires a driver capable of delivering constant, regulated current. To achieve this, the driver topology must produce enough output voltage to forward bias the LEDs. So how does a designer choose when the input and output voltage ranges overlap? Converters may sometimes need to gradually reduce the input voltage, but may also need to increase the output voltage at other times. The above conditions are typically found in applications with a wide range of “dirty” input power sources, such as in-vehicle systems. In this buck/boost operation, several topologies can achieve good results, such as SEPIC or a quadruple switching buck-boost topology. These topologies typically require a large number of components, which increases the material cost of the design. But because they provide a positive output voltage, designers often see them as an acceptable solution. But a negative output voltage converter is another alternative solution that should not be ignored.
Figure 1 shows a schematic diagram of an inverting buck-boost circuit driving 3 LEDs in a constant current configuration. This circuit has many advantages. First, it uses a standard buck controller, which minimizes cost and facilitates reuse at all system levels. Designers can also easily adapt the circuit to improve efficiency with an integrated FET buck controller or synchronous buck topology if desired. This topology uses the same number of power stage components as a simple buck converter, thus minimizing the component count of the switching regulator while achieving the lowest overall cost relative to other topologies. Since the output of the LED itself is light, the bias of the LED due to negative voltage has no effect on the system level, unlike the case of positive voltage, which makes it a circuit design worth considering.
Figure 1 Regulating Constant LED Current in Buck-Boost Topology Using Negative Output Voltage
The LED current is regulated by sensing the voltage across the sense resistor R1 and using it as feedback to the control circuit. The controller ground pin must be referenced to the negative output voltage for this direct feedback to function properly. If the controller is a reference voltage to system ground, a level shift circuit is required. This “negative ground” imposes some limitations on the circuit. The voltage rating of the power MOSFETs, diodes and controllers must be higher than the sum of the input and output voltages.
Second, connecting the controller externally (eg enable) requires level shifting the signal from system ground to controller ground, thus requiring more components. For this reason alone, eliminating or minimizing unnecessary external controls is the best approach.
Finally, compared to the four-switch buck-boost topology, the power devices in the inverting buck-boost topology are subject to additional voltage and current stress, which reduces the associated efficiency, but the efficiency is comparable to SEPIC. Even so, the circuit was able to achieve an efficiency of 89 percent. With the full synchronization of the circuit, the efficiency can be improved by another 2% to 3%.
Quickly turning the converter on/off by shorting the soft-start capacitor C5 is an easy way to adjust the LED brightness. Figure 2 shows the PWM input signal and the actual LED current. This PWM dimming method is more efficient because the converter is turned off and consumes very little power when the SS pin is shorted. But this method is also relatively slow because the converter must gradually increase the output current in a controllable manner each time it is turned on, creating a nonlinear, finite dead-time before the output current increases. . At the same time, this also reduces the minimum duty cycle of the turn-on time to 10%-20%. In some LED applications that do not require high speed and 100% PWM regulation, this approach may be sufficient.
This inverting buck-boost circuit gives engineers another way to drive LEDs. The use of low-cost buck controllers and low component count make it an ideal alternative to high-complexity topologies.
Figure 2 PWM drive (top) efficiently controls LED current (bottom)