Illumination Control Loop shows the
illumination control loop. This loop consists of the following features:
- An external buck controller
(LM3409) and related discrete components control the main LED drive PFET and
control and limit 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 is included in the TPS99000S-Q1.
- Synchronization logic for
external LED drive buck. SYNC pin to override the controlled off time
pin of an external device, and DRV_EN to control enabling an 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.