Hello, I have used your company's TPS922054 chip to make an LED driver board. The schematic diagram is as follows:
The input voltage range is set at 24 to 28V. I have tried the switching frequency at 2MHz and 680KHz (the calculation in this diagram is approximately 680KHz). The LED current is set at 0.8A (due to the sampling resistor accuracy issue, the actual measured LED current is 0.76A).
Then I used an electronic load instrument to measure the following data (R = 25Ω):
It can be seen that when the input voltage is 24V, it is closest to the current value I set. At this time, The Fsw fluctuates around 680KHz by a few KHz (possibly due to measurement error). My idea is that when the input voltage is reduced, the chip should theoretically reduce the off-time to increase the ratio of the output voltage to the input voltage (duty cycle D) to stabilize the output voltage and output current. However, this is not the case in reality: there is always a voltage difference of approximately 5.5V between the output voltage and the input voltage, resulting in the output current not being the set value, and the Fsw fluctuates greatly, sometimes around 500KHz and sometimes around 800KHz.
My questions are as follows:
1. I want to know if the actual test results are normal or if my idea is correct?
2. Why does this phenomenon occur as I measured?
3. When the input voltage increases from 24V, the inductor starts to make a whistling sound and the sound gets louder as the input voltage increases, but the output voltage decreases as the input voltage increases. Why is this?
4. When I connect my LED (operating voltage is 20 to 25V, corresponding operating current is 0 to 0.8A), the switching frequency is 2MHz. Even if the input voltage is adjusted to the maximum of 28V, due to the minimum off-time of the chip, the output voltage is approximately 5.5V lower than the input voltage, and the LED cannot output at maximum power. Only when the input voltage is 31V can the LED reach 25V. Even when the switching frequency is reduced to 680KHz, there is still a 5.5V voltage difference. What should I do?
