Efficient driving solution for multi-LED arrays

LEDs are increasingly used to generate light in electrical products, for example, in LCD TV backlight applications, LEDs have replaced traditional cold cathode fluorescent lamps (CCFLs). In industrial and general lighting applications, it has gradually replaced incandescent lamps, compact fluorescent lamps (CFLs), and high-intensity lamps. The light intensity provided by an LED is proportional to the current flowing through the LED; the power dissipated is proportional to the current and the LED forward voltage.

The light source and power dissipation are best shared using multiple low-current LEDs, rather than using a few high-power LEDs that may require a more expensive PCB and additional heat dissipation that will increase the total system cost. For example, in liquid crystal display (LCD) backlight applications, white LEDs are evenly distributed on one or several sides of the LCD; in LED lamps, a large number of LEDs provide more diffuse lighting and better heat dissipation on the PCB and heat sink. For these two reasons, the approach of using a large number of LEDs is very common.

Driving a string of LEDs instead of multiple LEDs in parallel provides inherent current matching and fewer interconnect terminals. In the event that a single string is disconnected, lighting will stop completely. Therefore, a good measure is to introduce some redundancy and connect at least two strings of LEDs in parallel. As a result, the anode voltage of the biased LED is reduced - this will improve safety.

On the other hand, providing consistent light output from multiple strings requires current matching. The forward voltage characteristics of different strings may differ slightly and need to be monitored, as this may result in excessive heat dissipation when using linear drivers. It is critical that the driver circuit detects open-LED and short-LED fault conditions and continues to operate.

24V DC power supply with boost converter and linear driver

Traditionally, TV sets contain a switch mode power supply (SMPS) that provides 24VDC. The same power supply can now be connected to a DC-DC boost converter to generate the high voltage required to forward bias long LED strings (ranging between 80V and 200V). The boost converter contains a pulse width modulation (PWM) controller, such as the UC3843 or NCP1252 from ON Semiconductor. The system then uses a linear LED controller to regulate the multiple string currents and provide feedback to the boost converter, which automatically adjusts the anode voltage to the lowest possible level to minimize the power dissipation of the linear LED driver (see Figure 1). Several faults can occur during operation that need to be handled by the controller. Common problems include open LED channels and short LED channels.

Figure 1: Block diagram of a DC-DC boost converter with a linear LED driver.

LED open circuit protection

In the event of an overvoltage or LED string becoming open circuit, the boost converter will increase the anode voltage (VOUT) as it attempts to regulate the current through the LED channel. The peak anode voltage must be limited to a safe value (i.e., the "LED open circuit voltage") to avoid damage to the boost circuit, its output capacitor, or the linear driver due to exceeding the rated voltage.

One method to detect an open LED fault is to connect the anode terminal to a voltage divider resistor (Ra, Rb) and connect the midpoint to a low voltage comparator (OCA input), as shown in Figure 1. Once the anode voltage reaches the LED open voltage, the LED driver can identify that the LED channel is open by detecting that the cathode is pulled down to ground by the current regulator circuit. The power supply can then shut down and ignore the open channel until the system is reset or the DC-DC boost circuit is shut down. The LED open fault can be reported to the system by using the open drain output flag (OPEN) (see waveform in Figure 2). The LED open voltage can be set to 10% or 20% higher than the highest anode voltage corresponding to the maximum forward voltage drop of the LED (under low ambient temperature conditions).

Figure 2: Power-on when LED is open circuit

LED short circuit protection

Under certain abnormal conditions, some LEDs may be shorted, or the cathode and anode terminals of the LED string may be accidentally connected together. This will cause the cathode voltage to increase (at the LEDx pin) and may cause excessive heat dissipation in the driver circuit. To reduce power dissipation, one option is to reduce the LED current until the short-circuit condition disappears. LED forward voltage drop mismatch can also trigger an LED short-circuit fault. The criterion for judging this condition is an increased cathode voltage: once the cathode voltage exceeds this limit, the fault is triggered. This fault can be reported to the system by using the open-drain output flag (SHORT). The power dissipation (PD) of the driver circuit can be calculated as follows: PD=∑VLEDxxILEDx, where VLEDx is the cathode voltage and ILEDx is the LED current of each channel.

Dimming method

Dimming is an important function that allows users to adjust the brightness or luminous flux by changing the LED current. There are two dimming methods: either using PWM or using an analog input signal. PWM dimming adjusts the LED current by repeatedly turning the LED channel on and off so that the average LED current is proportional to the duty cycle. For example, a 5% duty cycle for a nominal current of 100mA is equivalent to an average current of 5mA. In order not to cause visible flicker, the PWM frequency should be at least 100Hz (usually around 300Hz). Lower PWM frequencies allow for higher dimming resolution, especially at low duty cycles. One advantage of PWM dimming is that it maintains the color of the LED. Analog dimming controls dimming through an analog voltage (ANLG input) that sets the current value proportional to its voltage. If desired, both methods can be used simultaneously.

Solution

The CAT4026 6-channel LED controller from ON Semiconductor provides an integrated solution for regulating multiple high-voltage strings and monitoring them, as well as supporting fault diagnostics. Each LED channel is regulated by an external bipolar power transistor (Q1 to Q6) (see the application circuit in Figure 3). The transistor is connected to ground through a series resistor so that the current can be controlled by regulating its voltage (RSET pin) to 1V. The transistor is also connected to the LED cathode: the LED cathode may experience high voltage and should be able to handle the maximum anode voltage in the event of an LED short-circuit fault. The voltage rating of the transistor is very important. A good way to roughly estimate the power dissipated by the transistor is to multiply the collector-to-emitter voltage by the LED current. The worst-case scenario should be considered when choosing the package type and designing the PCB layout for heat dissipation.

The anode voltage is automatically determined by a closed-loop system where the LED controller provides feedback to the DC-DC converter. The CAT4026 identifies the highest forward voltage drop string or the lowest cathode voltage through the VCS pin. Once the lowest cathode voltage is at the nominal headroom voltage, the boost converter reaches normal operation. Since all channels are connected to the same anode voltage, other strings will experience higher cathode voltages, causing the transistors to dissipate some power. The current of all active LED channels is set individually by external resistors (R1 to R6) as shown in Figure 3, and ILEDx=1V/Rx.

Figure 3: A boost converter with a CAT4026 LED controller.

The CAT4026 VCS pin detects the lowest cathode voltage through a diode or network (LO-SENSE, adding 0.6V to the diode voltage drop). The CAT4026 starts LED open circuit detection when the OCA pin voltage reaches 1V and the open channel cathode is pulled down to ground potential. The LED short circuit event is detected by the SCA pin detecting the highest cathode voltage through a diode or network (HI-SENSE). The Zener diode (ZvSCA) supports LED short circuit threshold voltage adjustment.

The CAT4026 controls and ensures tight matching of up to 6 LED strings. For applications with less than 6 strings, unused channel pins remain unconnected; for applications with more than 6 strings, multiple CAT4026s can be connected in parallel - a master controller provides feedback to the power supply. The CAT4026 uses a SOIC 28-pin package for easy mounting on a single-sided PCB.

Conclusion

Linear controllers provide a flexible solution for driving multiple LED strings, where power is dissipated in discrete transistors rather than in the controller IC. The CAT4026 controller does not come in direct contact with the high voltage applied to the LED string, making it ideal for applications with long LED strings with voltages up to 200V or more and output power of 100W and more.