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3A, 1MHz Buck-Mode LED Driver with Integrated Voltage Limiting

Posted by: Yoyokuo 2022-05-25 Comments Off on 3A, 1MHz Buck-Mode LED Driver with Integrated Voltage Limiting

The LT3952 monolithic LED driver includes a 4A, 60V DMOS power switch ideal for driving high current LEDs in buck mode. An input current sense amplifier is one of its many features that can be used to provide built-in LED voltage limiting in buck mode.

introduction

The LT3952 monolithic LED driver includes a 4A, 60V DMOS power switch ideal for driving high current LEDs in buck mode. An input current sense amplifier is one of its many features that can be used to provide built-in LED voltage limiting in buck mode.

3A, 1MHz Buck-Mode LED Driver with Integrated Voltage Limiting

In buck mode, the positive pole (LED+) of the LED string is connected to the input voltage and the converter draws current from the negative pole (LEDC) of the string. In the open circuit situation, a buck mode converter drives LEDC close to GND. The total output voltage should be limited in this open fault condition.

One way to limit the voltage is to use an external PNP transistor as a level shifter. While this approach suffices, a more sophisticated solution can be achieved by utilizing the internal resources of the LT3952.

The key is to repurpose the input current sense amplifier on the IVINP/IVINN pins to act as a high-side voltage regulator, as shown in Figure 2.

A resistor divider across the LED string allows the IVINP/IVINN pins to sense the output voltage. When the IVINP/IVINN voltage reaches 60mV, the IVINCOMP output reaches 1.2V and the output is limited. As shown in Figure 2, connecting IVINCOMP to FB adds the benefits of output overvoltage protection and open LED protection.

For those applications utilizing PWM dimming, connecting a large value resistor between FB and GND prevents the FB pin from floating during the PWM off time.

3A, 1MHz Buck-Mode LED Driver with Integrated Voltage Limiting

application circuit

To test the design, an application circuit was built for a 40W, 1MHz buck mode LED driver. When using the R1, R2 and R4 values ​​shown, the voltage limit across the LED string is approximately 22V.

Figure 3 shows the LEDC voltage and LEDC open limit measurements using the circuit in Figure 4 with VIN swept from 0V to 40V. The voltage limits track the input well over the entire operating range.

3A, 1MHz Buck-Mode LED Driver with Integrated Voltage Limiting

3A, 1MHz Buck-Mode LED Driver with Integrated Voltage Limiting

Figure 5 compares the transient response for an open circuit condition with and without the output limiting circuit: VIN = 36V, ILED = 3A, four LEDs in series.

3A, 1MHz Buck-Mode LED Driver with Integrated Voltage Limiting

It can be seen that an open circuit fault without the limiter causes LEDC to be pulled all the way from its 23V nominal value to ground, resulting in a near full 36V input potential between LED+ and LEDC.

However, when a limiter is used, the output voltage is quickly limited to a more reasonable value. The connection of FB to IVINCOMP enables fault indication on the OPENLED pin.

The overall efficiency of this 40W solution is over 92% at 24V input voltage and higher than 90% over the entire input voltage range from 14V to 40V.

When using any output limiting method, remember to allow some margin between the limiting voltage and the normal operating voltage.

3A, 1MHz Buck-Mode LED Driver with Integrated Voltage Limiting

in conclusion

The LT3952 is a general-purpose, high-performance platform that can drive LED strings in a variety of topologies. In addition to input and output current regulation, a range of features such as spread spectrum modulation, fully built-in PWM generators and excellent fault protection simplify the design of advanced lighting solutions.

The Links:   G150XGE-L04 SKKH92-16E