7.3.1.2 Illumination Control Loop
Figure 7 shows the illumination control loop. This loop consists of the following features:
- An external buck controller (LM3409) and related discrete components which control the main LED drive PFET and controls and limits peak current using a high side sense circuit. This circuit creates a controlled current source that drives the LED high side connection (LED_ANODE).
- A 10-bit peak current limit (ILIM) adjustment DAC included in the TPS99000-Q1.
- Synchronization logic for external LED drive buck. SYNC pin to override the controlled off time pin of external device, and DRV_EN to control enable of external device.
- High speed comparator, used to compare photo feedback signal to programmable reference.
- 12-bit photo feedback comparison DAC. Sets reference for LED light pulse peak threshold in both continuous and discontinuous operating modes.
- A high speed, low noise, wide dynamic range transimpedance amplifier (TIA1) used for real time photo feedback. Includes support for 0.75 V to 288 V/mA gains, with 14 discrete gain steps and additional RGB specific trim of 1.0 to 0.2 gain. (Two TIAs included. TIA1 is dedicated to illumination control function).
- Negative LDO for cost effective reverse bias of photodiodes.
- 12-bit ADC, with differential input dedicated to low side current measurements.
- External FET gate drivers for RGB channel selection and two shunt path selections. Shunt paths provide a conduction path around the LEDs. These paths are used to control inductor current while LEDs are not emitting light. Control logic and firmware establishes appropriate current levels in inductor prior to enabling of LED during gaps between light pulses.