®
White Paper
Implementing LED Drivers In MAX Devices
May 2001, ver. 1.0 1
M-WP-LEDMAX-01
Introduction
Discrete light-emitting diode (LED) driver chips are common on many system boards. Altera
®
MAX
®
7000B,
MAX 7000A, MAX 3000A, and MAX 7000S devices offer unique capabilities that allow designers to integrate
single or multiple LED dri ver chips into a single device. This white paper explains how to implement LED dri vers in
MAX devices.
Commercial LED Driver Chips
Many LEDs, like the 7-segment display, are common-anode LEDs. The LED’s anode connects to V
CC
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 dri vers are more common than current-sourcing dri vers. Table 1 lists some common LED dri ver
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
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 V
CC
, and is turned on when the MAX device’s
I/O is pulled low.
The most important aspect of an LED dri ver 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.
LED Driver Chip Description
Tl TLC5905 LED driver with shift registers, data latch, and constant current circuitry
TI TLC5910 LED driver with shift registers, data latch, on-chip PLL for gray scale generation, and constant current
Tl TLC5911 LED driver with shift registers, data latch, on-chip PLL for gray scale generation, and constant current
TI TLC5921 LED driver with shift register, data latch, and current-sink constant current circuitry
National DS8874 9-digit shift input LED driver
National DS8863 MOS-to-LED 8-digit driver
National DS8963 MOS-to-LED 8-digit driver
Toshiba TB62701AN 16-bit constant current LED driver with shift register and latch functions
Toshiba TB62705 8-bit constant current LED driver with shift register and latch functions
Toshiba TB62706 16-bit constant current LED driver with shift register and latch functions
Toshiba TB62707 8-bit constant current LED driver with latch functions
2
Implementing LED Drivers In MAX Devices White Paper Altera Corporation
Table 2. Maximum Sink Current for MAX Devices
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 adv anced I/O standards. The De vice Pin-Outs section
of the
MAX 7000B Programmable Logic De vice 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
dri ver with shift re gisters and latch functions. The 16 outputs of the circuit sink current for two 7-se gment 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
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 e xtra re gisters for
the AND gates are also required for implementing the enable function.
MAX Device Maximum Sink Current Per Pin Unit
MAX 7000S 25 mA
MAX 7000A 25 mA
MAX 7000B 50 mA
MAX 3000A 25 mA
CPU
CLOCK
S-IN
LATCH
ENABLE
Scan
TB62701AN
OUT0
OUT7
OUT15
VDD VCC
0715
10
17
115
10= 17 = 115
R-EXT GND S-OUT
Altera Corporation Implementing LED Drivers In MAX DevicesWhite Paper
3
Figure 2. Block Diagram of TB62701AN
To integrate the entire circuit in one MAX de vice, a designer must choose a de vice 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 reg-
isters (macrocells) and a maximum of 34 user I/O pins. Ho wever, 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
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 sho ws the connection between a 7-segment LED and the MAX de vice. The output pins of the MAX
device connected to the LEDs are pulled low to turn on the LEDs.
QD
CK
Q
ST D
Q
ST D
Q
ST D
QD
CK SERIAL OUT
QD
CK
R-EXT
ENABLE
CLOCK
I-REG.
OUT0 OUT1 OUT15
LATCH
SERIAL-IN
Constant Current Driver
*
*
**
MAX Device
Resistor
Array
Seven
Segment
LED
Display
LED
+3.3 V +3.3 V
+3.3 V
Discrete LED
Discrete
4
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.
®
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San Jose, CA 95134
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http://www.altera.com
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 de vices are required if a commercial LED driv er chip is used. If a MAX de vice is used, howe v er , 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 inte grate LED drivers and provide
user logic, which saves on board space and reduces overall system cost.
