DLPS202A October 2020 – August 2024 TPS99000S-Q1
PRODUCTION DATA
A constant off time feature (see COMPOUT_LOW signal in Figure 6-13) is included in continuous mode operation. During continuous mode operation, when the desired light level is achieved, the PFET gate drive is turned off by control of the DRV_EN signal and the light level begins to decrease as the inductor current begins to decrease. In a typical hysteretic controller, when a turn on threshold is reached, the PFET is turned on and the light/inductor current increases again. The frequency of switching is dependent on the difference between the turn on and turn off thresholds, loop delay and discrete component values (with the inductor inductance and voltage being most dominant factors).
In the TPS99000S-Q1, the control is modified to regulate the operating frequency. A constant off timer is included in the TPS99000S-Q1 control loop. When the photo feedback comparator threshold is achieved, a counter is started. The length of the counter is adjustable. While this counter is active, the output of the photo feedback comparator is ignored and the PFET drive (via DRV_EN output from TPS99000S-Q1) is disabled. Once the constant off time period counter has expired, the output of the photo feedback comparator is once again used to control the LED current drive. The minimum off-time establishes an upper limit on the hysteretic control loop switching frequency, separate from the natural frequency of the circuit. This feature is useful for assuring the circuit will not operate in the AM radio frequency band, and can also enable the usage of lower inductance value inductors (which can result in system cost savings and power efficiency improvements).