Design of multi-channel LED integrated driver application circuit based on LT3598

LED has become the mainstream application solution for energy-saving and environmentally friendly lighting systems with its advantages of low operating voltage, low power consumption, high luminous efficiency and long life. Based on the topological structure of LT3598, we designed its peripheral circuit. According to the design requirements of LT3598 multi-channel LED driver circuit, we rationally selected key components such as inductors, capacitors, diodes, etc., optimized the setting of switching frequency, and designed overvoltage, overcurrent and thermal protection. Finally, through specific circuit experiments, the stability and reliability of this circuit were confirmed, and the effects of analog dimming and PWM dimming experiments were compared to explain their respective advantages and disadvantages.

Preface

LED is the abbreviation of light emitting diode. As the fourth generation of semiconductor lighting source, LED has the advantages of low operating voltage, low power consumption, high luminous efficiency and long life. Compared with traditional incandescent lamps and fluorescent lamps, LED lamps can save more than 90% of electricity. Not only is it more efficient than tungsten filament bulbs, but it also does not contain harmful chemicals like cold cathode fluorescent lamps (CCFL). Therefore, it has gradually become popular and has become a lighting system solution that helps protect the environment.

1 LT3598 Topology Features

The internal topology of LT3598 is shown in Figure 1. It adopts fixed frequency, peak current mode control scheme and has excellent line and load regulation capability. Its internal 6 current sources provide 6 channels, which can drive 6 strings of LEDs, with up to 10 white LEDs in each string. The driving current of each string can be up to 30mA, the efficiency can reach 90%, and the current accuracy between each string can be guaranteed to be within 115% to ensure the brightness of each string of LEDs is consistent. The built-in boost converter uses an adaptive feedback loop to adjust the output voltage to slightly higher than the required LED voltage to ensure the highest efficiency. If any LED string has an open circuit, it will not affect its normal operation. LT3589 can continue to adjust the existing LED string and send an alarm signal to the OPENLED pin.

Figure 1 LT3598 internal topology block diagram

2. Select the inductor

In order to ensure the stability of the power supply, there are several issues to be solved when selecting the inductor: First, the inductance must be large enough to ensure that the switch tube Q1 can supply enough energy to the load during the cut-off period. Second, the inductor must be able to withstand a certain peak current without saturation or even damage; third, the DC resistance of the inductor should be as small as possible to minimize the power loss (I2R) of the inductor itself. Here, for the LT3598 integrated circuit, the best efficiency can be obtained by using a ferrite core inductor, and its inductance value of 417~22μH can meet the needs of most applications.

3 Output capacitor selection

Apply low equivalent series resistance (ESR) ceramic capacitors at the output to minimize output ripple voltage. For the LT3598 application circuit, an output capacitor of 417 to 10μF can meet most high output current design requirements.

4. Selection of rectifier diodes

When choosing a high-frequency rectifier diode, you need to consider the following aspects. First, the forward voltage drop should be low; second, the switching speed should be fast, so Schottky diodes are the best choice; third, the average rated current of the rectifier diode must be greater than the total average output current of the application; fourth, the reverse breakdown voltage of the rectifier diode must be higher than the maximum output voltage.

5. Optimizing switching frequency

Choosing the best switching frequency depends on several factors: First, although reducing the high-frequency inductor can get a higher switching frequency, the switching loss will also increase, so the efficiency is slightly reduced; second, in some applications, if the power supply is insufficient, a very high duty cycle is required to drive a large number of LEDs. If necessary, the switching frequency must be reduced, because a low switching frequency can not only obtain a higher duty cycle, but also allow the high duty cycle to be maintained for a longer time, so that more LEDs can be driven.

LT3598 itself has the function of boost DC2DC converter. Its normal working high-frequency switching frequency is set between 200kHz and 215MHz, and it can work well. The high-frequency switching frequency is regulated by the resistance of the external resistor connected to the ground of its RT pin. In this example, the resistance of the RT external resistor R9 is 5111kΩ, and the switching frequency of the DC2DC converter is 1MHz. The RT pin cannot be left floating. Figure 2 shows the relationship curve between the switching frequency and the RT external resistor.

Figure 2 Relationship between switching frequency and RT external resistance

LT3598 can also work in an external synchronous mode. If the SYNC pin is connected to an external synchronous signal, the signal frequency must be slightly higher than the switching frequency of the DC2DC converter, generally between 240kHz and 3MHz, with a duty cycle between 20% and 80%, and an amplitude between 0.14 and 1.15V. At this time, the switching frequency controlled by the RT external resistor should be 20% lower than the external SYNC synchronous pulse frequency. The SYNC pin cannot be left vacant and must be grounded when not in use.

6 Overvoltage protection

The maximum output voltage of the LT3598 application circuit can be obtained using the following formula:

The maximum output voltage can be determined by setting the resistance of external resistors R1 and R2. The output voltage should be slightly higher than the normal working voltage of the LED string. When the LED working voltage exceeds the set Vout (max), the overvoltage protection circuit is activated so that the DC2DC converter can reduce the output voltage.

7 Set the maximum LED current

According to the actual application needs, by setting the resistance value of the external resistor R4 of the Iset pin (the resistance range of 10~100kΩ can meet the normal operation of this drive circuit), the current flowing through the LED string can be set. The experimentally measured current estimation formula for the LED string is:

, where ILED represents the current flowing through the LED string. In this example, R4 is 1417kΩ, so the maximum current is 20mA.

The larger the ILED setting, the higher the power consumption of LT3598 itself. If ILED = 30mA and the PWM dimming duty cycle is 100%, the internal power consumption of LT3598 is at least 144mW.

8 Design of thermal protection circuit

For a single boost converter with 6 linear current sources, powering 6 strings of LEDs, any voltage mismatch of the LED strings will cause additional power dissipation, causing excessive heating of the drive circuit. In addition, the increase in ambient temperature will also cause the IC temperature to rise. Therefore, the circuit design needs to consider the impact of the heating factor.

The operation process of the thermal loop is very simple. When the ambient temperature rises, the junction temperature inside the driver IC also rises. Once the temperature rise reaches the set maximum junction temperature, the LT3598 begins to linearly reduce the LED current and try to maintain this temperature level as needed. If the ambient temperature continues to rise after exceeding the set maximum junction temperature, the LED current will be reduced to about 5% of the total LED current. Therefore, the circuit design should take into account the specific use environment to avoid excessive ambient temperature rise that affects the normal operation of the circuit. As shown in Figure 1, a resistor divider network R8 and R5 is connected to the Tset pin of the IC. The ratio of R8 and R5 is appropriately selected to determine the maximum junction temperature value that needs to be set. In practical applications, several sets of commonly used data are obtained based on the ratio of R8 and R5 and the measured temperature, as shown in Table 1.

Table 1 Relationship between Tset junction temperature and external resistor divider network resistance

A more intuitive method is to set the voltage value of the pin by changing the ratio of the resistor divider network connected to the Tset pin, which determines the maximum junction temperature value that needs to be set. The relationship between the junction temperature and the voltage value of the Tset pin obtained through experiments is shown in Figure 3. From Figure 3, it can be concluded that as the voltage VTset of the pin increases, the maximum junction temperature that the IC can tolerate for normal operation also increases linearly.

Figure 3 Relationship between junction temperature and Tset pin voltage

9 Practical Application Circuit

The typical practical application circuit is shown in Figure 4. There are 6 strings in total, 10 LEDs in each string. The maximum normal working current of each string is designed to be 20mA. The power supply uses the boost DC2DC conversion circuit built into the IC, the boost inductor L1, the built-in power switch tube, the Schottky rectifier diode D7, and the high-frequency switching frequency is designed to be 1MHz. The maximum supply voltage of the LED string is designed to be 41V. The voltage within this range can make the circuit work normally. Otherwise, the overvoltage protection circuit is activated to restore the LED supply voltage to normal. Using a PWM dimming pulse with an amplitude of 313V, a rising edge and a falling edge of 10ns, and a frequency of 1kHz, a PWM true color dimming range of 3000:1 can be achieved. In the dimming process, the maximum current of the LED string is always stable at 20mA. What changes is only the duty cycle of the PWM dimming pulse, which means that the average current of the LED string is changed, thereby achieving the purpose of LED dimming. Any unused LED string cannot be left empty and should be connected to Vout. The internal fault detection loop will ignore this string and will not affect the open circuit LED detection of other strings.

Figure 4 Practical application circuit diagram

10 LED current dimming control

Two different types of dimming modes can be used for dimming with LT3598. In some cases, the preferred solution is to use a variable DC voltage to adjust the LED current and then control the brightness. The CTRL pin voltage of LT3598 can be used to adjust the LED string current to achieve dimming. When the pin voltage changes from 0V to 1V, the LED string current will rise from 0 to the set maximum current (set to 20mA in this example). When the CTRL pin voltage exceeds 1V, it has no effect on the LED string current. We call this dimming technology analog dimming. Its biggest advantage is to avoid the audible noise generated by the PWM dimming pulse; its disadvantages are twofold. First, it increases the energy consumption of the entire system and the system efficiency is low, because the LED drive circuit is always in the working mode at this time, and the power conversion efficiency drops sharply as the output current decreases; second, the LED light quality is not high, because the LED light color changes with the change of the forward current, and it directly changes the current of the white light LED string.

For true color dimming, PWM dimming technology is commonly used, which uses digital pulses with different duty cycles to drive a PMOS field effect tube (as shown in Figure 1) to change the output current and thus adjust the brightness of the white light LED. This dimming technology has higher efficiency than analog dimming technology and can produce higher quality white light.

A higher switching frequency and a lower PWM frequency can provide a wider PWM dimming range. However, when the driver performs PWM dimming, if the frequency of the PWM signal happens to fall between 20Hz and 20kHz, the inductance, parasitic inductance and output capacitor around the white light LED driver will generate audible noise. Therefore, in order to avoid noise during design, the PWM signal frequency should not be lower than 20kHz.

For true color dimming applications, the current driving the LED must remain constant so that the advantages of the LED can be fully utilized, because any slight fluctuation in the current driving the LED will cause the LED light color to be unstable. By only changing the PWM pulse duty cycle on the PWM pin without directly changing the LED drive current, the LT3598 can achieve a PWM dimming range of 3000:1. When the PWM pulse duty cycle changes from 100% to 0.11%, the LED can maintain a constant color.

FIG5 shows the waveforms of various points when dimming is performed using a 1kHz PWM dimming pulse, a pulse period of 1ms (indicated by the horizontal axis), and a duty cycle of 90%.

Figure 5 Waveforms at various points after PWM duty cycle stabilizes to 90% (1msPDIV)

11 Summary

Channels can be connected in parallel to provide higher current for each LED string. For example, if there are 2 LED strings, each string needs 90mA current, then 3 channels can be connected in parallel to form two channels, which can provide up to 90mA current for each of the 2 strings of LEDs.

If the forward voltage drop of each LED string is very different, it will generate huge power consumption and reduce the power supply efficiency. In order to obtain high efficiency, when selecting LEDs, firstly, the number of LEDs in each string must be the same, and secondly, the voltage drop through each LED string must be as consistent as possible.

The input voltage range of LT3598 is from 312 to 30V, and its multi-channel capability makes it an ideal choice for applications such as notebook computer displays, medium-sized displays and automotive LCD displays.