The MAX6968 is an 8-port constant-current LED driver with a 25Mbps transmission rate data interface. The PWM emulation technique can be applied to MAX6968 and MAX6969. This PWM emulation procedure can be implemented using the following C code: The emulated PWM period is repeated as long as the LED is on. The data bit is 1, if the corresponding port's intensity level is higher than the index of the emulated PWM step otherwise the data bit is zero. For example, if the intensity level is 256, data refresh for every port is repeated 256 times to form a PWM period. This emulated PWM step is repeated with a start index of 1 and an end index equal to the number of intensity levels to form a PWM period. A complete data update for all ports is considered a PWM step. In this technique PWM emulation data is sent continuously from a controller to the LED driver with 1 bit for each port. Where f CLOCK is the clock frequency for the data interface f PWM is the PWM frequency n PORT is the number of ports and n LEVEL is the number of intensity level. The relationship between PWM frequency, data transmission clock frequency, and PWM intensity level can be described by Equation 1: Since the transmission rate of the LED data interface is usually much faster than a PWM period, this technique can be easily implemented with a microcontroller or an FPGA (field-programmable gate array). On/off data can be sent to an LED driver frequently to emulate the effect of PWM intensity control.
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LED drivers with built-in PWM can also benefit from additional, external PWM intensity control if clocks regulating these PWM signals can be synchronized. External PWM intensity control can also be emulated by refreshing data bits. This application note discusses how additional PWM intensity control can be applied to constant-current LED drivers by using a switched-LED power supply. Intensity control for other Maxim LED drivers is carried out by adjusting the amount of the constant-current sinking through an open-drain LED port. The number of bits for intensity control ranges from 4 to 8 bits, which corresponds to 16 to 256 intensity levels.
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The intensity is normally controlled by using the serial interface to send the LED driver a command to change the register bits for the corresponding LED. Maxim Integrated provides many LED (light-emitting diode) drivers with PWM (pulse-width modulation) intensity control for a wide range of applications. Several constant-current LED drivers, with or without built-in PWM, serve as examples of the techniques.Ī similar article appeared in the May 2010 edition of Electronic Products magazine. This application note explains several methods for applying additional PWM-based intensity control to LED drivers. Maxim Integrated provides many LED (light-emitting diode) drivers, with PWM (pulse-width modulation) intensity control.
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