LP8869C-Q1: -40℃ startup output waveform has a small step

Hi Alan,

The LP8867-Q1 has an adaptive boost voltage to adjust the output voltage in certain steps. It seems that the adaptive control takes effect and triggers one increase action at the beginning. This may cause by that the LED has higher forward voltage in low temperature.

Best regards,

Suchen

Hi Suchen，

The customer agreed that the LED VF would be higher at lower temperatures, but the voltage wasn't as high as the one shown here. Even after headroom adaptation was activated, the voltage still dropped. Furthermore, the LED wasn't turned on at this point; it only turned on after headroom adaptation was activated.

Hi Alan,

Could you please ask the customer whether the waveform of the chip under low temperatures is stable/reproducible or only occurs once? If it's a single occurrence, this is possible for a single low compare event is mis-detected. If it's stable and reproducible, could the customer test it together with the LED current? This would allow us to conduct further analysis.

Best regards,

Suchen

Hi Suchen，

Refer to the waveform below. CH4 is the output current of channel 1. You can see that there is still a small step. Could you help me figure out what triggers the boost action? Thank you.

Hi Alan,

Thank you for your feedback. For LP8869, the action is triggered by a low voltage on the OUTx pin. Since the increase occurred before the LED current was sunk, it is recommended to check if the voltage of any OUTx pin is below LOW_COMP (0.9V) just before the action to identify the root cause.

Best regards,

Suchen

Hi Suchen，

The customer believes that leakage should be smaller at low temperatures, the sink circuit has high resistance, and the OUT voltage should be relatively high. Furthermore, the LED is slightly on, and VF is very low, so VOUTx shouldn't be as low as 0.9V. Also, since headroom adaptation starts after boost start, does the Vlow_comp function get checked during boost start?

Hi Alan,

Just checked that Adapive boost will not work until the LED Current Sink works.

Because in the initial voltage start-up phase, the Boost will generate a higher voltage than expected, so at each temperature, the Boost will be adjusted to no longer work due to a slightly higher voltage, so that the Boost voltage will gradually decrease. However, in the case of low temperature, various components in the system will cause changes in actual parameters due to low temperature, resulting in waiting time less than 50ms, which causes the Boost to continue to work for a while, resulting in a small voltage increase.

Best regards,

Suchen