Programmable logic devices (PLDs) are increasingly popular on system boards, where product-term based PLDs are used to integrate control and glue logic as well as other common functions such as LED drivers. Pin and register capabilities of a PLD are very important when integrating a common function. By choosing a device with sufficient pin and register capabilities, one can integrate single or multiple LED driver chips along with the required user logic into a single product-term based PLD.

Many LEDs, in particular, the 7-segment display, are common-anode LEDs. The anode of such LEDs is connected to Vcc and the cathodes are each connected to an output pin of a current-sinking LED driver chip that sinks the DC current required to drive the display. Current-regulating circuits are implemented inside the LED driver chips. The LED is turned on when the output pins of the LED driver chip are pulled low.

Many chip manufacturers such as Texas Instruments, National Semiconductor and Toshiba manufacture current-sinking LED driver chips.

Table 1. Some of the common current-sinking LED driver chips by Texas Instruments, National Semiconductor and Toshiba

Suppose we have an application circuit with a generic 16-bit constant current LED driver with shift registers and latch functions, and it looks like the following:

Figure 1. Example of an application circuit for a 16-bit LED driver
(click to enlarge)

There are 16 outputs in the above circuitry, which sink current for 2 seven-segment displays. To implement the LED driver chip in the above circuitry into a PLD, it is important to choose one with enough register and pin capabilities in order to replace the functionality of the entire LED driver chip.

The block diagram of a typical 16-bit constant current LED driver with shift registers and latch functions could look like the following:

Figure 2. Block diagram of a 16-bit LED driver.
(click to enlarge)

To emulate the functioning of the LED driver as pictured above, we need 32 registers for the latches and flip-flops and at least three inputs pins (clock, serial input, 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 would also be required for implementing the enable function.

To integrate the entire circuit in just one PLD, we must choose a device that has at least 20 I/O pins and 32 registers in its programmable logic. For example, a 32-macrocell MAX 7000AE device by Altera with 32 registers and a maximum of 34 I/O pins can satisfy the requirements. If it is necessary to implement the enable function, we could use a 64-macrocell MAX 7000AE device with 64 registers 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 PLD.

The implementation of the LED driver using a product-term based PLD such as a 64-macrocell MAX 7000AE would look like the following:

Figure 3. Using a product-term based PLD to implement a current-sinking LED driver
(click to enlarge)

The right hand side of the diagram shows the connection between discrete LEDs and the I/Os of the PLD, while the left hand side shows the connection between a 7-segment LED and the PLD. The output pins of the PLD connected to the LEDs are simply pulled low to turn on the LEDs.

When a product-term based PLD is used as an LED driver chip, a current-limiting resistor is placed between the cathode side of the LED’s diode and the I/O of the PLD, as shown above. The LED is tied to VCC, and the I/O of the PLD is pulled low when the LED is to be turned on. Registers in the programmable logic of the PLD emulate the latches and flip-flops found in a commercial LED driver chip.

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 spec of 5 to 15mA. PLD vendors normally publish the current sink and source specifications in their datasheets. As an example, Altera has the following current sink specifications for its product-term based PLDs, which makes them possible to integrate LED driver designs:

Table 2. Maximum sink current per pin for Altera’s MAX Devices

Product-term based PLDs provide solutions not only to communications and industrial fields, but also provide simple solutions to integrate commodity products such as LED drivers. The major advantage of implementing LED drivers using product-term based PLDs is that product-term based PLDs can also integrate other user logic using the programmable logic. If user logic needs to be implemented on the same board as the LED driver, additional devices would normally be required if a commercial LED driver chip is used. On the other hand, if a PLD is used, the additional chip would not be required, saving on valuable board space and reducing the overall system cost.

		Click here to get your listing up.