introduction
In a typical application, a serial port is sent to the LED driver to change the register value of the corresponding LED for brightness adjustment. The data used for brightness control is typically 4 to 8 bits, corresponding to 16 to 256 brightness levels; some of Maxim's LED driver brightness control is achieved by adjusting the constant sink current of the open-drain LED port.
This application note discusses how to add PWM dimming to the LED constant current driver to adjust the brightness by controlling the on and off of the LED power supply. External PWM brightness control can also be simulated by refreshing the data bits. The LED driver with built-in PWM can also be adjusted by external PWM, as long as the external clock of the PWM signal can be synchronized.
PWM simulation
The effect of PWM brightness adjustment can be simulated by sending an on/off control signal to the LED driver at a certain period. Because the transmission rate of the LED data interface is much higher than the frequency of the PWM signal, PWM dimming can be easily simulated using a microcontroller or FPGA (Field Programmable Gate Array). The relationship between the PWM switching frequency, the clock frequency of data transmission, and the PWM brightness level is as shown in Equation 1:
Among them, fCLOCK is the clock frequency of the data interface, fPWM is the PWM frequency, nPORT is the number of control ports, and nLEVEL is the brightness level.
In this technique, PWM emulation data is continuously sent by the controller to each port of the LED, one bit per port. All ports are updated once and are a step of the PWM. The PWM step is repeated from the index value 1 until the index value is equal to the set brightness level, forming a PWM period. For example, if the brightness level is 256, each port refreshes the data 256 times to form a PWM period. If the brightness level of the corresponding port is higher than the index value of the PWM simulation step, the data is 1; otherwise the data is zero. The PWM simulation cycle is always repeated as long as the LED remains lit.
The PWM simulation control can be implemented by the following C program:
PWM simulation technology is available for the MAX6968 and MAX6969. The MAX6968 is an 8-port LED constant-current driver with a data interface transfer rate of up to 25Mbps; the MAX6969 is a 16-port version of the MAX6968. With this method, 16-bit or 65,536-level brightness control can be achieved. The PWM frequency of the MAX6968 can be set at 47 Hz, and the PWM frequency of the MAX6969 can be set at 24 Hz. If only 12-bit brightness control resolution is required, the corresponding PWM frequency can be set at 752 Hz and 376 Hz, respectively. The PWM simulation technology achieves brightness control of each drive port without any modification to the circuit.
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