Tool/software:
Hi,
I'm designing a circuit which can be summarized as LP5861 driving 16 LED brightness via analog dimming. Each LED connected to each channel of L5861. Only 8 LED is turned on at the same time, it's either channel 0-7 or channel 8-15 at the same time. In addition, I setup a common-emitter phototransistor circuit to measure the brightness of each LED. The phototransistor is 1541021NCA170.
The analog dimming seems working, except that it seems there is a behavior that I'm not sure intended or not. So, I measured the "perceived brightness" via phototransistor voltage. I turned off the LED by setting the analog current scale to 0, then I turn it on by setting the analog current scale to non-zero. The LP5861 is set to mode 1, max current is 20 mA, and current scale is changed via DC register. The result is shown below (high voltage is LED off. The brighter is LED the lower is the voltage).
It seems like there is some "stepping" when the LED is transitioning from off to intended brightness. It also takes some time to reach intended brightness. The response time is usually around 180 - 220 ms, but there are some cases where it even goes up to ~800 ms. I'm not really sure whether this response time comes from the LED driver, so I have several questions.
1. What is the response time of analog dimming whenever the current scale is changed via DC register?
2. The datasheet mentioned analog dimming is performed via current gain control. But how does it exactly work? Is it using some sort of current feedback control that's why there is "stepping" in the brightness?
Thank you!
