Provide PWM brightness control for LED drive circuit

introduction

In a typical application, a command is sent to the LED driver through a serial port to change the register value of the corresponding LED to adjust the brightness. The data used for brightness control is usually 4 to 8 bits, corresponding to 16 to 256 brightness levels; the brightness control of some Maxim LED drivers is achieved by adjusting the constant current sucked into the open-drain LED port.

This application note discusses how to add PWM brightness control to LED constant current drivers, and adjust the brightness by controlling the on and off of the LED power supply. It is also possible to simulate external PWM brightness control by refreshing the data bits. LED drivers with built-in PWM can also achieve brightness control through external PWM, as long as the external clock of the PWM signal can be synchronized.

PWM Simulation

Sending an on/off control signal to the LED driver at a certain period can simulate the effect of PWM brightness adjustment. Because the transmission rate of the LED data interface is much higher than the frequency of the PWM signal, the PWM dimming method 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 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 simulation data is continuously sent by the controller to each port of the LED, 1 bit per port. One update of all ports is one step of PWM. The simulation of PWM steps is repeated starting from index value 1 until the index value is equal to the set brightness level, forming a PWM cycle. For example, if the brightness level is 256, each port refreshes the data 256 times to form a PWM cycle. 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. As long as the LED remains lit, the PWM simulation cycle is repeated.

The PWM simulation control can be realized by the following C program:

PWM simulation technology is applicable to MAX6968 and MAX6969. MAX6968 is an 8-port LED constant current driver with a data interface transmission rate of up to 25Mbps; MAX6969 is a 16-port version of MAX6968. This method can be used to achieve 16-bit or 65,536-level brightness control. The PWM frequency of MAX6968 can be set to 47Hz, and the PWM frequency of MAX6969 can be set to 24Hz. If only 12-bit brightness control resolution is required, the corresponding PWM frequencies can be set to 752Hz and 376Hz respectively. PWM simulation technology can achieve brightness control for each driver port without any modification to the circuit.

introduction

In a typical application, a command is sent to the LED driver through a serial port to change the register value of the corresponding LED to adjust the brightness. The data used for brightness control is usually 4 to 8 bits, corresponding to 16 to 256 brightness levels; the brightness control of some Maxim LED drivers is achieved by adjusting the constant current sucked into the open-drain LED port.

This application note discusses how to add PWM brightness control to LED constant current drivers, and adjust the brightness by controlling the on and off of the LED power supply. It is also possible to simulate external PWM brightness control by refreshing the data bits. LED drivers with built-in PWM can also achieve brightness control through external PWM, as long as the external clock of the PWM signal can be synchronized.

PWM Simulation

Sending an on/off control signal to the LED driver at a certain period can simulate the effect of PWM brightness adjustment. Because the transmission rate of the LED data interface is much higher than the frequency of the PWM signal, the PWM dimming method 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 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 simulation data is continuously sent by the controller to each port of the LED, 1 bit per port. One update of all ports is one step of PWM. The simulation of PWM steps is repeated starting from index value 1 until the index value is equal to the set brightness level, forming a PWM cycle. For example, if the brightness level is 256, each port refreshes the data 256 times to form a PWM cycle. 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. As long as the LED remains lit, the PWM simulation cycle is repeated.

The PWM simulation control can be realized by the following C program:

PWM simulation technology is applicable to MAX6968 and MAX6969. MAX6968 is an 8-port LED constant current driver with a data interface transmission rate of up to 25Mbps; MAX6969 is a 16-port version of MAX6968. This method can be used to achieve 16-bit or 65,536-level brightness control. The PWM frequency of MAX6968 can be set to 47Hz, and the PWM frequency of MAX6969 can be set to 24Hz. If only 12-bit brightness control resolution is required, the corresponding PWM frequencies can be set to 752Hz and 376Hz respectively. PWM simulation technology can achieve brightness control for each driver port without any modification to the circuit.

LED power switch control

PWM brightness adjustment of LEDs can also be achieved by turning the LED power supply on and off. The circuit shown in Figure 1 uses PWM to control the power supply to provide additional brightness adjustment for the LED. The microprocessor sends I²C commands to the LED driver to generate a PWM signal, and the PWM waveform can be controlled by software. This method is suitable for the MAX6969 with a constant current LED port but no internal brightness adjustment function, and the MAX6956 with an adjustable constant current LED port. This solution controls the duty cycle of the PWM signal through a transistor to achieve the purpose of brightness adjustment. The LED brightness can be controlled indirectly by the microprocessor through the LED driver, or directly by the transistor. Taking the MAX6956 as an example, constant current drive is combined with PWM duty cycle adjustment without the intervention of any other circuit.

Figure 1. Using PWM to control LED power to achieve brightness adjustment

The circuit shown in Figure 2 uses MOSFET transistors as switching devices, which helps improve efficiency.

Figure 2. Power MOSFET as a switching device.

The power dissipation in the external transistor is calculated using the following formula:

Among them, tRISE is the rise time of the transistor, tFALL is the fall time of the transistor, T is the PWM period, tON/T is the PWM brightness level, I is the total LED current, and RON is the on-resistance of the transistor.

Equation 2 gives the sum of the transistor switching loss and conduction loss. The switching loss is determined by the on/off time. When the transistor is closed or open, almost all the current flows through the transistor during the process of the voltage across the transistor rising from zero to VLED, or in the reverse direction.

When using high-speed switching transistors, the rise time and fall time are typically 50ns. For a PWM period (T) of 1/1000 seconds, an LED voltage (VLED) of 5.5V, and a total LED drive current of 200mA, the total transistor power dissipation is:

If the transistor on-resistance is 0.1Ω, the on-power consumption of the transistor at maximum brightness is:

From Equation 4, we can see that the loss can be minimized by properly selecting high-speed switching transistors.

Master control and hierarchical control of each port

Some LED drivers can achieve PWM brightness control through hierarchical control between the master and each port. For example, MAX6964, MAX7313, MAX7314, MAX6965, MAX7315, and MAX7316. As shown in Figure 3, the PWM brightness control waveform of each port is repeated multiple times. Each repetition is equivalent to one host control. Therefore, if the host controls 15 levels of brightness adjustment, the control waveform is repeated 15 times. The control signal of each port of the LED driver determines the duty cycle of the waveform. The master control signal determines the number of repetitions of the control waveform. For example: the duty cycle of a port is 3/16, and the master control is set to 4/15. The on-time of the waveform accounts for 3/16 of the entire cycle, and the waveform is repeated in the first 4 time slots of all 15 time slots.

Figure 3. PWM brightness hierarchical control of the master and each port

Unfortunately, the master control signal of one MAX6964 cannot be combined with the port signal of another MAX6964 to form a multi-chip chain mechanism. This is because multiple MAX6964s cannot achieve clock synchronization; the PWM control on-time of each port cannot be consistent with the on/off time window of the master controller's brightness adjustment signal. If the edges of the clock signals cannot be aligned, the brightness cannot be controlled synchronously, and the LED will dim. Due to the phase deviation between the clocks, the LED will also flash periodically (on and off).

The hierarchical PWM brightness adjustment scheme can avoid flickering problems through LED drivers and is suitable for devices such as MAX7302 that have clock synchronization mechanisms and a wide clock frequency range. Figure 4 shows a typical circuit for implementing PWM brightness hierarchical control using two MAX7302s and switching transistors.

Figure 4. Using two MAX7302s to implement PWM brightness control.

The output port of one MAX7302 is connected to the cathode of the LED, and each output port serves as an independent brightness control port. The output of another MAX7302 is connected to the anode of the LED through an external transistor, and this MAX7302 serves as the brightness master controller. The brightness control of each port is driven by an external 1MHz high-frequency clock, which is the upper limit of the MAX7302 working clock. For example, when the brightness level of a port is set to 15/33, the output of the P2 brightness control port serves as the clock input of the master controller. The equivalent clock frequency of the master controller is approximately 1000000/33 = 30kHz. In this application example, each brightness control port can be used to adjust the color of the RGB LED, while the master controller is used to adjust the brightness.