Figure 6-2 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) is used for real-time photo
feedback. Includes support for 0.75V to 288V/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 establish appropriate current levels in the inductor prior to
enabling LED during gaps between light pulses.