High-power LED driver chip XLT604 and its circuit application

LEDs are widely used in indicator lights, large billboards, scanners, fax machines, mobile phones, car lights, traffic lights, etc. due to their long life and low power consumption. However, in terms of lighting sources, LEDs are not yet able to replace other light sources due to their brightness and price. However, as the brightness continues to increase, LEDs will replace incandescent lamps and fluorescent lamps in the near future, and their prices will also decrease due to advances in mass production technology, and the demand for applications will increase significantly.

1. Structure and function of XLT604 chip

XLT604 is a PWM high-efficiency LED driver control chip designed with BICMOS process. It can effectively drive high-brightness LEDs in the input voltage range from 8V (DC) to 450V (DC). The chip can drive external MOS-FETs at a fixed frequency of up to 300 kHz. And its frequency can be programmed by external resistors. The external high-brightness LED string can be controlled by constant current to maintain constant brightness and enhance the reliability of the LED. Its constant current value can be determined by the external sampling resistor value, and its range is from a few milliamperes to 1 ampere.

The brightness of the LED driven by XLT604 can be adjusted linearly through an external control voltage, and the brightness of the LED string can also be adjusted through an external low-frequency PWM method.

The functional block diagram of XLT604 is shown in Figure 1.

Figure 1: Functional block diagram of XLT604

The main functions of each pin are shown in Table 1.

Table 1: Main functions of each pin

2. Application circuit of XLT604

XLT604 is a control chip that can step down, step up, and step up and down to drive high-power LED strings. This chip is suitable for both AC input and 8-450 V DC input. When AC input is used, a passive power factor correction circuit can be added to the circuit to improve the power factor. XLT604 can drive hundreds of LEDs in series or several series in parallel, and can ensure the brightness of the LED and extend its life by adjusting the constant current value. The PWM_D terminal can use low-frequency pulse width modulation to adjust the LED brightness, and also serves as an enable terminal. When this terminal is left floating, the chip has no output control. In fact, the chip can also adjust the brightness of the LED through the linear voltage regulation method of the LD terminal. Figure 2 shows the typical application circuit of XLT604 in AC and DC input.

Figure 2: XLT604 buck drive circuit for AC and DC input

3. Parameter design of circuit components

3.1 Calculation of circuit switching frequency

The switching frequency determines the size of the inductance in the circuit. A larger frequency can use a smaller inductance, but this will increase the loss of the circuit. The typical frequency should be around 20 to 150 kHz. The voltage in Europe is 230 V, so a smaller frequency can be used; the voltage in North America is 120 V, so choosing 100 kHz is a good compromise. The oscillation resistance in the circuit can be calculated by the following formula:

fosc=22000／(Rosc+22)

Where, the unit of Rosc is kΩ

3.2 Design of AC input inductor

Assume that the input effective value is 120 V, Iled is 350 mA, fosc is 50 kHz, and the forward voltage drop Vleds of 10 LEDs is 30 V; then:

Vin = 120 × 1.41 = 169 V

Then, the switching duty cycle is:

D = Vleds / Vin = 30 / 169 = 0.177

Ton=D/fosc=3.5 ms

L=(Vin-Vleds)Ton／(0.3Iled)=4.6 mH

3.3 Design of input filter large capacitor

The input filter capacitor should ensure that the rectified voltage value is always greater than twice the LED string voltage. Assuming that there is a 15% ripple voltage across the capacitor, the simple calculation method for its capacitance is as follows:

Cmin=0.06IledVleds／Vin2=22μF

Therefore, a capacitor with a value of 22μF/250V is selected as the input filter capacitor.

4. Application Control

4.1 LED drive control

XLT604 can be used to control various types of converters including isolated/non-isolated, continuous/discontinuous, etc. When the GATE terminal outputs a high level, the energy stored in the inductor or the primary inductor of the transformer will be directly transferred to the LED string, and when the power MOSFET is turned off, the energy stored in the inductor will be converted into the driving current of the LED.

When the VDD voltage is greater than UVLO, the GATE terminal can output a high level, and the circuit will work by limiting the peak current of the power tube. When the external current sampling resistor is connected in series with the source of the power tube, the power tube will be turned off when the voltage value of the external sampling resistor exceeds the set value (internal setting value 250 mV, can also be set externally by LD). If you want the system to soft-start, you can connect a capacitor to the ground at the LD terminal to make the voltage at the LD terminal rise at the desired rate, thereby controlling the LED current to rise slowly.

4.2 Dimming

The dimming of this circuit has two modes: linear adjustment and PWM adjustment. The two modes can be adjusted separately or in combination.

Linear dimming can be achieved by adjusting the voltage of the LD port (from 0 to 250 mV), which takes precedence over the internal setting value of 250mV. The voltage of the CS terminal can be changed by adjusting the variable resistor connected to the power ground. When the voltage of the LD terminal is higher than 250 mV, its voltage change will not affect the output current. If a larger output current is desired, a smaller sampling resistor can be selected.

P. WM dimming is achieved by adding a PWM signal of several hundred Hz to the PWM_D terminal. The high level time length of the PWM signal is proportional to the brightness of the LED lamp. In this mode, the LED current can be 0 or one of the set values. The PWM adjustment method can be used to dim the light in the range of 0 to 100%. However, the current cannot be adjusted higher than the set value. The accuracy of PWM dimming is only limited by GAT.