Here’s help understanding the basics and for selecting driver ICs to design today’s LED driving circuits
BY AKIRA TAKAHASHI
National Semiconductor
Tokyo, Japan
http://www.national.com
Featuring small size, long battery life, and low power consumption, LEDs are expected to be used in a variety of new applications as a next-generation lighting source. With remarkable improvements in performance made in recent years, new types of products, such as highly efficient LEDs with greater than 100 lm/W, or high-power LEDs over 5 W are available, accelerating the move to replace existing lighting sources with LEDs.
Today, LEDs are in the spotlight, but there are still a number of issues to be considered to obtain uniform light emission, and some design engineers may be facing challenges to solve those issues. It would be preposterous if power consumption increases as a result of adopting very complex control circuits, trying to obtain the desired lighting performance.
To maximize the benefits of LEDs, it is critical to understand the characteristics of LEDs and select appropriate driving circuits according to the applications.
The first part of this article provides an overview of methods to drive LEDs to effectively deliver uniform lighting performance, and the second part introduces an example of application circuits using National Semiconductor’s switching regulators.
Voltage/current characteristics of LEDs
The LED is a diode that requires a voltage above a certain threshold to be applied for bias current. This voltage is called forward voltage (Vf ), which ranges from about 3 to 4 V for white LEDs. The value of Vf fluctuates due to varying quality in manufacturing process and changes in temperature so it is not appropriate to drive LEDs at a constant voltage.
If the applied voltage exceeds the Vf , LED current will suddenly increase. Each LED has a maximum current rating, and supplying current above the maximum value may cause damage and shorten LED lifetime. Therefore constant-current circuits should be used for drive control to maintain the LEDs current rating.
Adjusting brightness of LEDs
When LEDs are on, the brightness varies according to the current. When comparing an example of LED current to relative luminous flux characteristics, the brightness rises as current increases, but are not always changing proportionally. One of the characteristics of LEDs is that the spectrum (color) changes depending on the current being supplied (see Fig. 1 ). Especially in white LEDs, with white being generated by blending blue light emitted from a chip and yellow light emitted by phosphor covering the chip, the color balance varies as the current changes this is prominent in white LEDs compared to mono-color LEDs. In applications where the color of emitted light must be kept constant, changing the current cannot be used to adjust brightness.
Fig. 1. Forward current-color/degree characteristics.
PWM method
Pulse-width modulation (PWM) dimming is a well-known method to adjust brightness while keeping the color balance constant. When using PWM, it takes advantage of the natural averaging of the human eye. It allows a high-speed cyclical change of light brightness to appear as continuously on light at the cycle’s average brightness.
Dimming is done by cyclically turning the LEDs on and off at high speed, in sync with the PWM signal. The on/off time ratio (duty cycle) determines the brightness actually perceived by observers, where the larger the duty cycle, the higher the brightness observers sense.
Since current supplied through the LEDs is always constant when the LEDs are turned on, colors generated by using this method are not affected by the brightness. Typically 200 Hz of PWM dimming frequency is sufficient to get these dimming benefits but if something else such as cameras or sensors, it is necessary to select the dimming frequency considering the response characteristics of the capture device.
Optimal circuit to drive LEDs
The ideal LED driving circuit is optimized when the LED characteristics explained above are taken into account. LEDs are electronic devices that emit light by the delivery of current, so appropriate power sources are required to drive them.
Supply voltage is typically predetermined in each application system. As explained, forward drop voltage (Vf ), which is predetermined for each LED, must be provided to LEDs to turn them on. In many cases, supply voltage to drive LEDs is not identical with the LED’s Vf , so it is obvious that some appropriate voltage conversion circuit would be required.
A constant-current source circuit is required to enable safe and stable lighting, designed to keep the delivery of current to LEDs at a constant rate. To enable PWM dimming, a constant current circuit should have external signal control. From these requirements, an optimum LED driving circuit can be configured. One example is shown in Fig. 2 .
Fig. 2. Optimum LED driving circuit. The initial block (1) is a voltage conversion circuit, which converts the voltage from the power supply source to an appropriate voltage according to the LED’s Vf. Block 2 is the current source that supplies constant current to the LED, with control from external circuits, that can turn the current to the LED on and off through PWM signal inputs.
LED driver circuits are designed mainly to turn on LEDs, so ideally all the power provided from inputs is consumed by LEDs. Although this is not practical, it is necessary nevertheless to reduce power consumed in circuit blocks other than the LED.
Switching regulators are best to obtain higher power efficiency in voltage conversion circuits. Switching regulators use inductors to transmit energy that enables them to convert voltage at an overwhelmingly high efficiency. Although it depends on conditions, the efficiency can reach up to 95%.
LED driving circuit using a buck regulator
Figure 3 shows an example of ways to configure the LED driving circuit in Fig. 2, using the LM2734 buck switching regulator. The LED is placed between the inductor’s output end and the voltage feedback pin (FB pin). The LM2734 regulates the FB pin voltage, keeping it at 0.8 V.
Since the input impedance at this pin is very high, the current flowing into the pin is nearly zero. Therefore, the current flowing into the LED (If) can be expressed by using resistor R1 inserted between the FB pin and GND in the following equation:
If = 0.8 / R1
This shows that the current provided to the LED is controlled to be kept constant. Inductor L1 supplies current to the LED, so the constant -urrent source is configured by inductor L1 and resistor R1 . Output voltage Vout is automatically determined by the LED’s Vf .
Fig. 3. Example of how to configure the LED driving circuit.
The EN pin on the LM2734 uses external signals to shut down outputs. When the EN pin is set low, the output is shut down, reducing the current flowing to the LED to zero. A PWM signal to the EN pin turns the LED on only when the pin is set high, enabling PWM dimming. It takes about 200 µs to get the desired output when the EN pin setting changes from low to high, because of the soft-start function, which prevents rush current when the power source is turned on.
Since an output capacitor (Co) is required for stable operation, the time required to turn off the LED current, when the EN pin setting changes from high to low, is limited by the discharge time of the capacitor. When width modulation is used for brightness adjustments, there are some timing limitations based on duty cycles. ■
For more on LED drivers, visit http://electronicproducts-com-develop.go-vip.net/linear.asp.
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