White Paper (R) Implementing LED Drivers In MAX Devices Introduction Discrete light-emitting diode (LED) driver chips are common on many system boards. Altera(R) MAX(R) 7000B, MAX 7000A, MAX 3000A, and MAX 7000S devices offer unique capabilities that allow designers to integrate single or multiple LED driver chips into a single device. This white paper explains how to implement LED drivers in MAX devices. Commercial LED Driver Chips Many LEDs, like the 7-segment display, are common-anode LEDs. The LED's anode connects to VCC and the cathodes are each connected to an output pin of the current-sinking LED driver chip. The driver chip sinks the DC current required to drive the display, and the LED is turned on when the driver chip's output pins are pulled low. Current-regulating circuits are implemented inside the LED driver chips. Current-sinking LED drivers are more common than current-sourcing drivers. Table 1 lists some common LED driver chips manufactured by Texas Instruments, National Semiconductor, and Toshiba. More information about specific LED driver chips can be found in the datasheets provided by the respective manufacturers. Table 1. Current-Sinking LED Driver Chips LED Driver Chip Tl TLC5905 TI TLC5910 Tl TLC5911 TI TLC5921 National DS8874 National DS8863 National DS8963 Toshiba TB62701AN Toshiba TB62705 Toshiba TB62706 Toshiba TB62707 Description LED driver with shift registers, data latch, and constant current circuitry LED driver with shift registers, data latch, on-chip PLL for gray scale generation, and constant current LED driver with shift registers, data latch, on-chip PLL for gray scale generation, and constant current LED driver with shift register, data latch, and current-sink constant current circuitry 9-digit shift input LED driver MOS-to-LED 8-digit driver MOS-to-LED 8-digit driver 16-bit constant current LED driver with shift register and latch functions 8-bit constant current LED driver with shift register and latch functions 16-bit constant current LED driver with shift register and latch functions 8-bit constant current LED driver with latch functions Implementing LED Drivers in MAX Devices When a MAX device is used as an LED driver chip, a current-limiting resistor is placed between the cathode side of the LED's diode and the MAX device's I/O. The LED is tied to the VCC, and is turned on when the MAX device's I/O is pulled low. The most important aspect of an LED driver chip is the amount of current it has to sink. Many LED applications call for a current sink specification of 5 to 15 mA. Because MAX 7000B, MAX 7000A, MAX 3000A, and MAX 7000S devices can sink up to 50 mA per pin, these MAX device families can directly integrate commercial current-sinking LED driver chips. Table 2 shows the maximum sink current per pin for MAX devices. M-WP-LEDMAX-01 May 2001, ver. 1.0 1 Implementing LED Drivers In MAX Devices White Paper Altera Corporation Table 2. Maximum Sink Current for MAX Devices MAX Device Maximum Sink Current Per Pin Unit 25 25 50 25 mA mA mA mA MAX 7000S MAX 7000A MAX 7000B MAX 3000A Even though a single pin from a MAX 7000B device can sink up to 50 mA of DC current, each IOGND group can concurrently sink up to 200 mA of current due to the support of advanced I/O standards. The Device Pin-Outs section of the MAX 7000B Programmable Logic Device Family Data Sheet specifies the IOGND groupings of I/Os. For more information about the current sinking capabilities of the MAX devices, refer to the respective data sheets. Implementing LED Driver Chips Figure 1 shows an example of an application circuit with the TB62701AN, Toshiba's 16-bit constant current LED driver with shift registers and latch functions. The 16 outputs of the circuit sink current for two 7-segment displays. A designer can implement the LED driver chip in the circuitry using only one MAX device, provided the device has enough register and pin capabilities to replace the functionality of the entire LED driver chip. Figure 1. Application Circuit Example VCC VDD Scan 15 7 0 CLOCK OUT0 S-IN 10 TB62701AN OUT7 LATCH 17 CPU ENABLE OUT15 R-EXT GND 115 S-OUT 10 = 17 = 1 15 Figure 2 shows a block diagram of the TB62701AN. To emulate the functioning of the TBN62701AN, a designer needs 32 registers for the latches, flip-flops, and at least three input pins (clock, serial input, and latch) and 17 output pins (the serial output and the 16 LED outputs) for a total of 20 I/O pins. An extra input pin and 16 extra registers for the AND gates are also required for implementing the enable function. 2 Altera Corporation Implementing LED Drivers In MAX DevicesWhite Paper Figure 2. Block Diagram of TB62701AN OUT0 OUT1 OUT15 * * * * Constant Current Driver R-EXT I-REG. ENABLE Q ST Q D ST Q D ST D LATCH SERIAL-IN D Q D Q D Q SERIAL OUT CK CK CK CLOCK To integrate the entire circuit in one MAX device, a designer must choose a device that has at least 20 I/O pins and 32 registers. The smallest MAX 7000A device that can satisfy the requirements is the EPM7032AE device, with 32 registers (macrocells) and a maximum of 34 user I/O pins. However, to also implement the enable function, the smallest MAX 7000A device required would be the EPM7064AE device with 64 registers (macrocells) and a maximum of 68 I/O pins. The external resistor(R-EXT) and the current-regulating circuit have to be replaced with individual current-limiting resistors placed between the cathode side of the LED's diodes and the I/Os of the MAX device. Figure 3 shows the implementation of the LED driver using a MAX device. Figure 3. Implementing the LED Driver Using a MAX Device +3.3 V +3.3 V Discrete Discrete LED LED +3.3 V Resistor Array Seven Segment MAX Device LED Display The right hand side of Figure 3 shows the connection between discrete LEDs and the I/Os of a MAX device, while the left hand side shows the connection between a 7-segment LED and the MAX device. The output pins of the MAX device connected to the LEDs are pulled low to turn on the LEDs. 3 Implementing LED Drivers In MAX Devices White Paper Altera Corporation Advantages The major advantage of implementing LED drivers with MAX devices is that MAX devices can also integrate other user logic using their programmable logic. If user logic has to be implemented on the same board as the LED driver, additional devices are required if a commercial LED driver chip is used. If a MAX device is used, however, additional chips would not be required, saving valuable board space and reducing the overall system cost. Conclusion Altera's MAX devices not only provide solutions to the communications and industrial fields, but also offer simple solutions to integrate commodity products such as LED drivers. MAX devices can integrate LED drivers and provide user logic, which saves on board space and reduces overall system cost. (R) 101 Innovation Drive San Jose, CA 95134 (408) 544-7000 http://www.altera.com Copyright 2001 Altera Corporation. Altera, MAX, MAX 7000A, MAX 7000B, MAX 7000S and MAX 3000A devices are trademarks and/or service marks of Altera Corporation in the United States and other countries. Other brands or products are trademarks of their respective holders. The specifications contained herein are subject to change without notice. Altera assumes no responsibility or liability arising out of the application or use of any information, product, or service described herein except as expressly agreed to in writing by Altera Corporation. Altera customers are advised to obtain the latest version of device specifications before relying on any published information and before placing orders for products or services. All rights reserved. 4