Using the MAX6958/MAX6959 LED Drivers to Implement a Single LED

Abstract: This application note details how to implement pixel-level (individual LED) brightness control using the MAX6958 and MAX6959 LED display drivers. This technique enhances the 64-step global (all LEDs at once) brightness control capability built into these drivers.

introduce

The MAX6958 and MAX6959 LED display drivers use 6-bit PWM (64-step) brightness control to simultaneously adjust the average current of all lit LEDs. This application note discusses how to extend the functionality of the MAX6958 and MAX6959 to implement individual pixel-level (LED) control.

How to connect multiplexed LED drivers in average LED current mode

The MAX6958 and MAX6959 are 4-digit, 9-segment LED drivers that use a reduced pin-count multiplexing technique (Charlieplexing - see Charlieplexing - Reduced Pin-Count LED Display Multiplexing for technical details) to drive 36 segments using only 10 driver pins (Table 1). As shown in Table 1, the MAX6958/MAX6959 pin and LED bit connections are different from the standard connection method. Depending on the time slot of the multiplexing cycle, pins 4 to 7 act as common cathode drivers or anode drivers.

Table 1. MAX6958/MAX6959 Standard Driver Connection Method

	DIG0/SEG0	DIG1/SEG1	DIG2/SEG2	DIG3/SEG3	SEG 4	SEG 5	SEG 6	SEG 7	SEG 8	SEG 9/IRQ
LED Digit 0	CC0	SEG 0	SEG	SEG	SEG	SEG	SEG	SEG	SEG	SEG 4
LED Digit 1	SEG 1	CC1	SEG	SEG	SEG	SEG	SEG	SEG	SEG	SEG 5
LED Digit 2	SEG	SEG	CC2	SEG 2	SEG	SEG	SEG	SEG	SEG	SEG 6
LED Digit 3	SEG	SEG	SEG 3	CC3	SEG	SEG	SEG	SEG	SEG	SEG 7

The MAX6958/MAX6959 use a quad-multiplexed drive scheme that alternately drives the cathodes of four groups of nine LEDs each. The four groups of LEDs are referred to as Bit 0, Bit 1, Bit 2, and Bit 3. The four cathode connection points for LEDs Bits 0 through 3 correspond to CC0 through CC3 in Table 1. The LED anode connection points are prefixed with SEG. In normal multiplexed operation, the CC0 through CC3 cathode outputs go low in turn, with each bit enabled sequentially for a quarter cycle.

The 6-bit (64-step) PWM overall brightness control function adjusts the average current (and thus the brightness) of the LEDs. This is accomplished by adjusting the actual amount of time that the CC0 through CC3 cathode outputs are on during their multiplexed time slots. The PWM values ​​applied to the CC0 through CC3 cathode outputs are identical, and no individual constant current source adjustment is made to each anode driver. Therefore, the brightness of all LEDs is uniformly adjusted.

Realize individual LED brightness control

In many applications, it is useful to highlight specific LEDs. One way to achieve this is by blinking them; cursors often blink to highlight their position. Another good way to achieve this is to make the LED segments brighter than the others. The latter approach is not possible with the standard multiplexer configuration of the MAX6958/MAX6959 described above.

A simple way to achieve individual intensity control of the LED segments is to allow the LEDs to be driven for more than one multiplexer cycle, i.e., to double drive them. Figure 1 illustrates this, with the usual two-bit control signals for LED 0 and LED 1 driving a single, 7-segment plus decimal point digit. Note that Figure 1 is incomplete.


LED 2 and LED 3 are not shown; they could be another single digit, or two digits in the standard configuration. Figure 1. Connections for the MAX6958/MAX6959 to drive a single digit with two levels of independent intensity control

The double-drive digit shown in Figure 1 is driven for two multiplexer cycles instead of the usual one cycle. Now, diodes D1 and D2 control two cathode drivers, and the same LED is operated whether CC0 or CC1 goes low as the cathode driver. Because a single dual-drive bit replaces the two standard drive bits, it is controlled by two bit registers (bits 0 and 1, corresponding to 0x20 and 0x21, respectively) instead of just one bit register. Each LED is represented by two bits (one in each bit register) instead of one. If both bits are cleared, the LED is off. If only one bit is set and the other is cleared, the LED turns on at standard brightness. If both bits are set, the LED turns on at twice the standard brightness. Therefore, each LED has two brightness settings instead of one. LEDs connected in the standard manner, such as SEG0 and SEG1, can only be set to standard brightness. Because diodes D1 and D2 carry 20mA current for each of the eight LEDs a quarter of the time, the diodes must be rated for 160mA peak current and 40mA average current.

The connection of Figure 1 has some advantages and disadvantages. The number of bits driven by the MAX6958/MAX6959 is reduced here because each LED with two brightness settings requires the drive capability of two "standard" LEDs. To achieve two brightness settings per pixel, the MAX6958/MAX6959 can only drive two 7-segment digits, rather than four digits with standard on/off control. However, this technique is still very effective and economical when only a few LEDs need brightness control. Figure 2 shows a dual-drive solution for a single LED, sacrificing the drive capability of only one LED. Diode pair D3 only needs to carry 20mA of current for one LED, with a peak current of 20mA. Therefore, a low-cost signal diode pair can be used.


Figure 2. Connection of the MAX6958/MAX6959 with two independent brightness levels for only one LED

Another connection method to achieve independent LED brightness control

It is also possible to achieve 2-step brightness control by replacing the back-to-back LEDs SEG0 and SEG1 with a single diode. As shown in Figure 3, the diode bridge rectifies the back-to-back drive current. Since two diodes are now connected in series in the LED current path, a small-signal Schottky diode is used to minimize the voltage drop.


Figure 3. Alternative MAX6958/MAX6959 connection for 2-step independent brightness control of only one LED