ZHCSBB4B July 2013 – June 2017 TPS61197
PRODUCTION DATA.
LED brightness dimming is set by applying an external PWM signal of 90 Hz to 22 kHz to the PWM pin. Varying the PWM duty cycle from 0% to 100% adjusts the LED from minimum to maximum brightness, respectively. The recommended minimum on-time of the LED string is 10 µs. Thus, the TPS61197 has a minimum dimming ratio of 5000:1 at 200 Hz.
When the PWM voltage is pulled low during dimming off, the TPS61197 turns off the LED string and keeps the boost converter running in pulse frequency modulation (PFM) mode. In PFM mode, the output voltage is kept at a level which is a little bit lower than that when the PWM voltage is high. Thus, the TPS61197 limits the output ripple due to the load transient that occurs during PWM dimming.
When the PWM voltages are pulled low for more than 20 ms, to avoid the REF pin voltage dropping due to the leakage current, the voltage of the REF pin is held by an internal reference voltage, which is a little bit lower than the REF pin voltage in normal dimming operation. Thus, the output voltage is kept unchanged during the long dimming off time.
Because the output voltage in long-time dimming off status is almost the same as the normal voltage for turning the LED on, the TPS61197 turns on the LED very fast without any flicker when recovering from long-time dimming off to normal dimming operation.