SLAAEA6 February   2025 CSD19538Q3A , LM25148 , MSP430FR2355 , TLV5624

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
    1. 1.1 LED Lighting in Medical Systems
    2. 1.2 Key Challenges of LED Drivers in Medical Applications
  5. 2Proposed LED Drive Designs With Analog Dimming
    1. 2.1 Drive LED With Linear Constant-Current Source
    2. 2.2 Drive LED With a DC-DC Regulator
    3. 2.3 Drive LED With TPS92640 or TPS92641
  6. 3Summary
  7. 4References

2.2 Drive LED With a DC-DC Regulator

Figure 2-3 shows how designers can also use a buck regulator or buck module to build an LED driver with analog dimming. The Zener diode is used to clamp the output voltage to Vz + VREF if the LED is opened. The Zener diode is removable since the maximum output voltage is equal to the input voltage in the worst case for a buck regulator; therefore, there is no danger to humans in most cases. Assume the output of the DAC is VDAC and the reference voltage of the buck regulator is VREF. Then the designer can determine the function between LED and DAC, as Equation 4 shows.

Equation 4. I L E D = 1 + R T R B + R T R C × V R E F R s × G - R T × V D A C R C × R S × G
TLV5624,CSD19538Q3A,LM25148,MSP430FR2355 Drive LED Using Buck Regulator With Dimming Figure 2-3 Drive LED Using Buck Regulator With Dimming

Equation 4 is complicated and the current of the LED is inversely proportional with the DAC voltage. To simplify the function between the DAC voltage and the LED current, the designer can use an external error amplifier (for example, OPA863A) as Figure 2-4 shows. The LED current and DAC voltage can now follow Equation 5 which is quite simple. The function of the diode in this design is to block current from the Zener diode and removable.

TLV5624,CSD19538Q3A,LM25148,MSP430FR2355 Dimming LED With a Simple Function Figure 2-4 Dimming LED With a Simple Function
Equation 5. I L E D = V D A C R S × G I N A 241

This design has a higher cost than the design in Section 2.3. Also, this design cannot to do fast PWM dimming. The transient response is not faster than the design in Section 2.3 since the buck regulator has a low bandwidth in most cases. The designer also must keep the minimum output voltage of buck regulator in worst case less than the LED forward voltage under LED lighting off mode. In this condition, the LED has no current and the buck regulator outputs the minimum voltage since most buck regulators have a minimum turn on time. The designer needs to keep output voltage in this condition smaller than the LED forward voltage. Otherwise, the LED has forward current and is out of control.