High-efficiency LED driver LTC3219 and its application

　　The LTC3219 is an inductorless, low noise, high efficiency LED driver that provides nine independently configurable current sources for the main, sub and RGB display/display devices, and can perform 64 levels of brightness adjustment control. These universal current sources can be digitally controlled for independent dimming, brightness, blinking and grayscale control, and can be programmed through a simple two-wire I2C serial interface.

　　The LTC3219 has 1X, 1.5X and 2.0X low noise charge pump operation modes, operates at a low noise constant frequency, and can automatically optimize efficiency based on the voltage on the LED current source. The LTC3219 also has short circuit and overheat protection circuits, and its soft start circuit prevents surge current and excessive input noise during startup and mode switching. The device is mainly used in cellular phone displays, lighting equipment, and video phones with QVGA displays.

　　1 Pin Function

　　The LTC3219 uses a flat QFN-20 package of 3 mm × 3 mm x 0.75 mm, and its pin arrangement is shown in Figure 1. The functions of its pins are as follows:

　　CPO: LED power supply charging pump output terminal. This pin needs to be connected to the ground with a 2.2μF ceramic capacitor;

　　ULED1~ULED9: LED drive current source output. The current range is 0~28 mA. The current can be adjusted and controlled by software and internal 6-bit linear DAC in 64 levels. Each output can be activated by setting the data register (REG1~REG9) corresponding to each LED segment to 0. In addition, they can also be used as open-drain outputs of I2C. They can be grounded when not in use;
　　DVcc: Data I/O power supply. This pin can be used to set the logic reference level of LTC3219. When the voltage of this pin is lower than the undervoltage lockout threshold, the data register will be restarted. This restart method can usually be used after the device is powered on. This pin needs to be connected to a 0.1μF ceramic capacitor;
　　SCL: I2C clock input. The logic level of this pin needs to refer to DVcc;
　　SDA: Serial port data input. The device can be controlled by moving one bit of serial data in each clock cycle. Its logic level can refer to DVcc;
　　ENU: Enable input. Used to pre-select the ULED output to be turned on or off. When this pin switches from low (disconnected) to high (connected), the selected LED lights up. If the output controlled by ENU is valid and other outputs are also valid, the LTC3219 will reset to 1X charge pump mode at the falling edge of ENU; when the output controlled by ENU is valid and other outputs are invalid, the charge pump mode of the LTC3219 is turned off. The logic level of ENU can refer to DVcc. It can be grounded when not in use;
　　GND: system ground. When in use, this pin and the welding pad (pin 21) must be connected to the ground plane;
　　ClP, C2P, ClM, C2M: charge pump fast capacitor pins. A 1μF ceramic capacitor should be connected between C1P and C2P, and between ClM and C2M respectively.
　　VBAT: device power supply terminal. A 2.2μF low ESR ceramic capacitor should be connected when in use.

　　2 Working principle

　　The LTC3219 is a highly integrated multi-channel display LED driver. It uses an efficient, low-noise charge pump to power 9 general-purpose LED drivers, and can set its maximum display current by an internal precise current reference source. Each current source can be independently adjusted for gradient, on/off, flashing, and grayscale, and can be implemented through the I2C serial interface. Its LED current source has a 6-bit linear DAC for brightness adjustment control.

　　LTC3219 can change the boost ratio according to the voltage on the LED current source and through automatic or manual switching to optimize its efficiency. At startup, the boost ratio is 1X mode. When the effective current source approaches the shutdown (Dropout) state, LTC3219 will automatically switch to 1.5 times boost mode, and when shutdown occurs again, it will switch to 2 times boost mode. When new data is received from I2C, LTC3219 will automatically reset to 1X mode. Figure 2 shows the structural functional block diagram of LTC3219.

　　2.1 Soft Start

　　The soft start of LTC3219 mainly works at the beginning of 1.5X and 2X mode conversion. During initialization, the soft start circuit is in the off state. When power is on, VBAT slowly charges the output capacitor of CPO through a weak current switch between VBAT and CPO (the current on the CPO pin increases linearly with a cycle of 125μs) to prevent large charging current. When the device switches to the enhanced mode, this soft start feature of the charge pump will further limit the inrush current and power supply deviation.

　　2.2 Mode Conversion

　　When the effective current source voltage of the LTC3219 drops below the power programming voltage, the LED terminal will experience voltage dropout, and the charge pump will automatically switch from 1X mode to 1.5X mode. The time from voltage dropout to automatic mode switching is about 400μs, during which the LED will warm up and eventually reach the required forward voltage.

　　By writing different words to CPO, you can manually switch to 1X, 1.5X or 2.0X mode, which is often used in power loads where the automatic mode of the CPO terminal is invalid. In addition, the non-programmed current source will not affect the shutdown. When the ENU of the current source control is low, it will not affect the shutdown.

　　2.3 External enable control (ENU)

　　Programming is done through the ENU pin, that is, by setting the corresponding data bits of REG1~REG9 or the corresponding control bits of REG10 and REG11, all selected displays can be adjusted independently. When there is no need to repeatedly access the I2C port, ENU can be used to turn the LTC3219 on and off, so that it will indicate an incoming call without the control of the microcontroller.

　　The ENU pin can only be used if the I2C interface is set to the output of the desired LED. When ENU is high, the selected display can be turned on and REGl0 and REGll are set; when ENU is low, the selected display is turned off. If no display is programmed to turn on, the LTC3219 device will be in the off state. The

　　ENU pin can also be used for pre-programmed control of grayscale. In such applications, the register can be programmed according to the corresponding grayscale requirements, and the UP bit can be ignored during programming. When ENU is low, the register is allowed to be programmed; when ENU is high, the register is turned on and the output of the selected LED is raised. In addition, when ENU is low, the output current of the selected LED is also reduced to 0 and the output is turned off. If the off state is required in the design, the charge pump does not need to be placed in manual mode. When the ENU pin is applied and the other ULED outputs are valid, ENU will put the device into 1X charge pump mode on its falling edge. When the ENU pin is not used, it can be grounded.

         2.4 I2C Serial Bus Interface

　　LTC3219 can communicate with the host through a standard two-wire I2C interface. The timing relationship of each signal on its bus is shown in Figure 3. The microcontroller connected to the I2C serial interface can provide all command and control input signals. It has a total of 12 data registers, an address register and a secondary address register. The data at the SDA input can be loaded on the rising edge of SCL, D7 first and DO later, and the data register should be written first, then the secondary address register. When all address bits are written to the address register, LTC3219 will send an acknowledgment signal to the host. Each data register has a secondary address. After the data is written, a load pulse will be generated immediately after the stop bit. This load pulse can convert the data in the data register to the DAC register, and the stop bit will be delayed until all the data is written. However, the LED current will change at this time. Table 1 shows the address bits and functions of the secondary address register. The serial interface uses static logic registers. When the bus is not in use, SDA and SCL can be set high. In addition, an external pull-up resistor or current source is required during design.

　　3 Application Circuit

　　The LTC3219 requires only five small ceramic capacitors to implement a tiny, complete LED power regulator and current controller solution. Figure 4 shows a typical application circuit diagram of the LTC3219. The corresponding parameters of the external components given in Figure 4 can be used as a reference in specific design.

　　4 Conclusion

　　The LTC3219 is a non-inductive, low noise, high efficiency LED driver. The device has multiple optional charge pump modes and can perform automatic or manual mode switching, so the efficiency can be further optimized, making it convenient for use in videophones and lighting facilities.