DLPS039F December 2015 – April 2019 TPS99000-Q1
System designs should consider the amount of additional capacitance placed in parallel with the photodiode, and the capacitance of the photodiode itself. While the TPS99000-Q1 is designed to function with a very wide range of total capacitance, the lowest light level brightness performance is directly impacted by this capacitance. Higher TIA1 input capacitance will result in a brighter minimum brightness achievable by the system due to this light pulse overrun phenomenon. This results in a reduction of dimming range. (For highest performance, system designer should minimize total capacitance of the photodiode, photodiode cable and connector system).
The leading edge of the light pulse in discontinuous mode is controlled by the charging rate of the capacitance in parallel with the LED. The photo feedback DAC sets the threshold to turn on the shunt FET which shunts the current away from the LED. Latency in the photo feedback loop will result in the light climbing higher than the threshold as shown in Figure 45. The amount of light that occurs after the threshold is reached (shown as hashed green area) is the majority of the light at the lowest discontinuous mode brightness levels. Figure 45 also shows that a reduction in photo feedback DAC level by a factor of two does not reduce the total light pulse power by a factor of two because of the light that occurs after the threshold. The amount of light overrun after the threshold is a function of the photo feedback latency, inductor initial current, capacitance in parallel with the LED, LED voltage to current characteristics and shunt FET timing.