Nexperia – LED driver design techniques

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The replacement of incandescent and halogen lighting by LEDs is driven by various advantages of LED lighting devices, such as:

• Higher luminous efficiency
• Smaller size
• Faster turn on/off times
• Shock resistance
• Longer lifetime
• Fixed colour

Disadvantages also exist:
• Higher initial costs, although the price difference continues to shrink
• Sensitivity of temperature and voltage polarity
• Different white balance and blue hazard but can be overcome by semiconductor control circuitry

Most lighting systems require three basic elements: Power supply: An LED needs DC current, thus in AC power environments an AC/DC conversion is required and in DC environments a voltage level conversion is often needed as well Controller: Required for switching and dimming LEDs Driver: The final interface to the LED needs to provide the required energy with controlled current and voltage

Nexperia provides discrete solutions for the power supply, around then controller ICs and especially for the driving circuits.

There are various advantages when using discrete devices either in addition to integrated solutions or as a standalone solution:

• Cost reasons: Discount control ICs and reuse them with discrete devices rather than spending several control ICs
• Allowing second source supply, often preferred in large volume applications
• Design control: Fine-tuning the performance can be improved using discrete devices
• Discrete devices are used as peripheral extensions for integrated solutions

Basic principles of LED control
In comparison to conventional incandescent light bulbs, the control of LEDs is more complicated: the current control, and in most cases the low voltage, require some control circuitry. Figure 1 shows an overview of components. The tasks can be described as follows:

1. The control signal switches the supply (control to PMOS on top) on and off.
2. The boost signal switches the NMOS to regulate the supply voltage level in the boost circuit, consisting of the inductor, L, the diode, D, and the capacitor, C, to keep the DC voltage at the right level for the LED strings. The sense input feedbacks the voltage level for correction, the control method is duty cycle for the NMOS.
3. The CTRL signal switches the series transistor in the LED string PWM mode and by doing so, dimming is achieved. The sense input is used to verify the current through the LEDs.

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Fig. 1: LED string with control circuit

Application areas for LED driving devices
LED drivers are categorised according to power consumption, and can be classified in the following ways:

1. Low-power LEDs: Commonly used in status indicators and typically less than 20mA
2. Mid-power LEDs: Often used for high-brightness indicators (car turn signals, etc.) or LED illumination (LED flashlights, etc.) with current up to about 100mA and voltage up to 30V
3. High-power LEDs: For high-power illumination applications such as automotive headlights and room lighting where voltages are higher than 50V and currents can reach several amps
4. Sophisticated control: LED PWM dimming, constant current, colour mixing, ‘heartbeat’ blinking, etc.

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Fig. 2: 74LVC8T595 shift register used as LED driver

Solutions for the application areas
1. For a small number of LEDs, solutions include direct drive from a simple logic gate or microcontroller I/O pin. For larger numbers of LEDs, (i.e. more than 8) a device in the 595-function series (74HC595, 74AHC595, 74LVC595, 74LVC8T595) is a better solution. An example is the NEXPERIA 74LVC595: an 8-bit serial-in/serial or parallel-out shift register with a storage register and 3-state outputs. The block diagram of the 595-function series is shown in Figure 2. Both the shift and storage register have separate clocks. Data is shifted on the positive-going transitions of the shift register clock input. The data in the shift register is transferred to the storage register on a positivegoing transition of the storage register clock input.

The serial output is typically used to concatenate several registers and thus expand the number of LED strings that can be controlled, whereas the parallel output is the actual driving output for the LEDs.

In an application, the bit stream shifted into the register decides which LED is switched on or off. The signals for the clocks and for data are received from a microcontroller. The maximum drive current of the device is 24mA per channel and the total amount of current is limited to 100mA, which allows direct driving of LEDs within the power limit. Dimming can be done by modulating the duty cycle of the data input, in effect switching the LED frequently on and off at a typical frequency of 100Hz to 400Hz.

Since the 74LVC595 is operating at >33MHz, this can easily be done by software PWM. A new product in the 595-function series is the 74LVC8T595. In addition to the shift register functionality, it offers voltage translation between storage and output registers. Translation from 1.2V to 5V in both directions is possible here.

2. Mid-power LEDs: Often used for high-brightness indicators (car turn signals, etc.) or LED illumination (LED flashlights, etc.) with current up to 100mA and voltage up to 30V. For a small number of LEDs, a discrete MOSFET or bipolar transistor is an adequate solution.

For a larger number of LEDs Nexperia’s HEF4794B/HEF4894B drivers are a good choice: they have a similar topology as the 595-function series, with serial input and parallel output but use open drain outputs instead of three-state outputs, operate over a voltage range of 3V to 15V, and can handle up to 20mA continuous current per output channel. Higher loads can be achieved using external transistors.

3. MOSFETs can provide the necessary drive current but require microcontroller and software management. Dedicated LED driver devices, such as the ASLx500 family from NXP Semiconductors, are a cost-effective solution.

4. For a few LEDs, the above solutions can be utilised with the PWM/ CC/blinking done via software in the microcontroller. For a larger number of LEDs, a sophisticated controller such as an I2C controller from NXP is the most appropriate.

Discrete constant-current drivers: integrated bipolar transistors
Bipolar transistors are ideal to drive a constant current since they can operate well in linear mode. For the extension of current drive capabilities of shift register drivers, a bipolar transistor, in collaboration with diodes and resistors, can be used. The diodes keep the base voltage of the transistor at a defined level and control the point of operation. These kinds of circuits are available as integrated solutions. An example is the NCR405U series from Nexperia, which is shown in Figure 3. It can drive up to 50mA through the transistor to the LED at the output. For switching, a Resistor Equipped Transistor (RET) has been added, consisting of a bipolar transistor with two integrated resistors.

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