LM3402 Applied schematic diagram（As shown in figure 1）
figure 1 schematic diagram
When DIM= 3.3v, the oscilloscope is used to measure the signals of SW, CS and A respectively：
Figure2 CS（red）and SW（yellow）
As shown in the figure, SW frequency is 1MHz and amplitude is 11.64v
Figure3 A point(red) and SW （yellow）
As shown in the figure, A point V = 8.2V。
According to the current formula in the manual：
calculated If = 212mA , But by measuring the actual current flowing through the LED serials Current value is 30mA 。
Figure 4 shows the LED voltage ampere curve：
Figure 4 LED voltage-ampere curve
My question:
- When DIM= 3.3v, what determines the frequency of SW point? What's the formula?
- The CS end is described as 200mV in the manual, but the actual measurement is an approximate triangular wave with the same frequency as SW. It should be abnormal according to the manual, so what is the problem?
- The actual measured current flowing through the LED is only 30mA, which should be calculated as 212mA according to the formula. What's wrong with me?
I hope you can provide help as soon as possible, thank you!
Meanwhile, I hope USA will be healthy and good luck!
