06_Step-down constant current driver LED_C336597

①Requirements analysis

In the design, in order to ensure the brightness consistency of the two lamp beads, constant current drive was selected.

According to the LED manual, the operating voltage of each lamp bead is in the range of 2.8-3.3V, and two lamps connected in series are about 6V. If a 12V power supply is used, the voltage must be reduced, so this step-down constant current driver chip was chosen.

②Calculate current

The driving current is determined according to the formula: Iout=0.1/Rs.

③Test situation

In the experiment, three 1206 package resistors were used, with resistance values ​​of 0.3Ω, 0.15, and 0.14Ω respectively to test the size of the driving current. Remove the jumper cap on the test circuit board and connect an ammeter to measure the current. When Rs=0.15Ω, the theoretical value Iout=667mA, but the measured Iout is 500mA; when Rs=0.3Ω, the theoretical value Iout=333mA, but the measured Iout is 200mA; as for the existence of such a large deviation, the problem has not been found yet reason. . .

④About dimming control

Use the PWM function of the STM32 microcontroller to adjust the brightness. Test the two lower left pins of the circuit board, connect the first GND pin to the common ground of the STM32 microcontroller, and connect the second dimming pin to the PWM pin of the STM32 microcontroller.

When the PWM output frequency is 100Hz, increase the duty cycle and the light gradually becomes brighter. When the duty cycle = 0% (or the dimming pin ADJ is grounded) the light is completely turned off; when the duty cycle = 100%, the light appears brightest.

Figure 1: Full brightness (duty cycle = 100%)

  Figure 2: Weak brightness (duty cycle = 10%)

Supplement: Considering the heat dissipation problem of high-power lamp beads, the heat dissipation effect will be better if the circuit is made of aluminum substrate.