Precise current control is essential for efficient LED lighting solutions, and a wide range of LED drivers can help engineers optimize these LED lighting designs . So what should you pay attention to when designing high-power LED driver circuits based on these buck or boost converters? This article explores design issues to be aware of when using Zetex's ZXSC310, National's LM3410, and Texas Instruments' TPS61160/1 boost converters.
From small flash to stage lighting systems, high-brightness LEDs offer higher efficiency, longer life and smaller size than traditional light sources, as well as special effects such as dimming, sequencing and flicker. Precise current control is essential for efficient LED lighting solutions, and a wide range of LED drivers can help engineers optimize these LED lighting designs.
High-power LEDs for lighting produce enough light flux to be used in flash, room lighting, outdoor lighting, and electronic signage when these devices are properly sized. A range of driver devices maintain drive currents in high-power white LEDs well above 1A.
The drive current determines the light output of the LED, so all LEDs in the array must be driven with a constant current to ensure that the end product emits light and is uniform. To ensure uniform current, the LEDs are often connected in series. The LED forward voltage drop must also be considered simultaneously. Each LED in the LED string has its own forward voltage drop VF. The VF is usually around 3.4V and the minimum variation range is 2.5~4V (or higher).
Boost converter for LED drivers
To obtain the sufficient voltage to drive a certain number of series LEDs, a boost converter can be used to increase the voltage of the battery-powered application. These applications include flashlights or portable devices with LED-backlit displays. On the other hand, in applications such as electronic signage or traffic signs that require a large number of LEDs, the output voltage of the driver topology can be as high as 40V.
In addition, one or more multi-channel driver ICs can also be used. In these devices, the current matching between the channels must be very close to prevent different brightness between the different LED strings. For example, the latest multi-output drivers have very good current matching within modern high-power LED light output tolerances.
Many designers who want to design small-size, high-efficiency LED lighting applications are concerned about products with very low operating voltages. Zetex's ZXSC310 driver is a constant current boost converter with an input voltage as low as 0.8V, which provides constant current to the LEDs in the event of a drop in battery voltage. The device can be used to boost low voltage power supplies to drive high power LEDs with a nominal VF of 3.4V.
The ZXSC3 is very useful in controlling LED backlight applications in flash and small portable devices. One of its external pins controls normal operation or 5μA off mode, or it is connected to a pulse width modulation (PWM) signal to control LED dimming.
National Semiconductor (NS)'s LM3410 is also a boost converter for low voltage equipment. The converter converts an input voltage of 2.7~5.5V to an output voltage of 3~24V, which can be used in display backlights and other portable applications.
In non-battery-powered applications, a boost driver can be used to replace a halogen lamp in a room with an LED lamp. For example, Texas Instruments' TPS61160/1 boost converter can boost the standard low-voltage halogen lamp's nominal 12V DC supply voltage to an output voltage of up to 18V that can drive six or ten white LEDs.
Figure (a) is a circuit diagram of driving 10 LEDs using the TPS61161. The TPS61161 integrates a 40V/0.7AN channel MOSFET switch. Its dimming control pin can be used as a single-wire digital interface or as a PWM input, allowing designers to implement various control modes.
Figure: (a) Circuit diagram using Texas Instruments' TPS61161 boost converter to drive 10 LEDs
Figure: (b) National Semiconductor's LM3406 buck converter provides constant current LED control
Buck converter for use as an LED driver
Most general lighting drivers are designed to operate at higher DC voltages that are converted from the mains voltage and range from approximately 5V to 30V or higher. For example, NS's LM3406 is a self-contained buck regulator with an output range of 6 to 32V and a forward constant current of up to 1.5A. A typical application circuit as shown in Figure (b) has an external resistor that sets the LED current and a dedicated input pin for PWM dimming.
The Zetex ZXLD1350 is also a similar buck regulator with an input voltage of 7 to 30V and can deliver up to 350mA to the LED. It provides an external pin for current regulation. This pin supports a PWM dimming signal or a simple DC voltage to adjust the output up and down by the voltage set by the external sense resistor.
Multi-channel and multi-driver circuit design
The number of LEDs that can be driven with a buck regulator depends on the maximum output voltage. For large LED arrays such as electronic signage or stage lighting, multiple drivers may be required, each controlling a string of LEDs.
Electronic system design
Another option is to use a multi-channel driver. TI's 8-channel TLC5917 driver delivers up to 120mA per channel. The device also provides eight regulated current ports for operation up to 17V. The output current accuracy between the channels is better than ±3% to ensure uniform light output of different LED strings.
In addition, the accuracy between ICs is ±6%, enabling designers to drive larger LED arrays using multiple TLC5917 drivers. The driver incorporates an open-circuit load, a short-circuit load, and over-temperature protection to indicate device failure in large LED arrays.
As high-power LEDs continue to enter a variety of lighting applications, more advanced drivers will emerge to achieve higher drive currents and simpler control, allowing designers to freely choose the optimal number of applications for their target applications. LED.
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