TPS922055: PWM input frequency - Flexible dimming mode

Hi Darren,

Below explanations apply to flexible dimming / analog dimming / PWM dimming mode.

For EN/PWM pin,

• A higher input PWM frequency means you can turn on/off the current output more quickly (higher-frequency PWM dimming). In such condition, you can avoid the visible (or being able to be captured by high-speed cameras) flickering issue with those low-frequency PWM dimming. Also, with a PWM dimming frequency higher than 20kHz, you can avoid the audible noise issue commonly seen in those low-frequency PWM dimming as >20kHz is out of the generally accepted standard audible range for human.
• A lower input PWM frequency means you can achieve higher dimming ratio as there is a minimum on time that the device can recognize - 150ns. Please refer to Section 7.5 Electrical Characteristics from the datasheet. A screenshot is shown below. For example, if you are using 20kHz PWM frequency, the theoretical maximum PWM dimming ratio you can achieve is around (1/20kHz):150ns = 333:1 (less than 9-bit resolution). However, if you are using 120Hz PWM frequency, the theoretical maximum PWM dimming ratio you can achieve is around (1/120Hz):150ns = 55555:1 (more than 15-bit resolution). Please be noticed that the 150ns is only the minimum on time of the device, but not the minimum on time of your system (including the inductor and capacitor, etc., which will slow down then turning-on/off of the current output). Therefore, the actual maximum dimming ratio you can achieve will be lower than the theoretical values above.

For ADIM/HD pin,

• A higher input PWM frequency means you can transit between different brightness level more quickly as the device needs at least one PWM cycle to recognize the new duty input. (Here is a graphical illustration: [FAQ] How to get a quick transition of TPS922055 / TPS922054 / TPS922053 / TPS922052 from shutdown into flexible dimming mode?).
• A lower input PWM frequency means you can get higher ADIM resolution and therefore higher dimming ratio. Please refer to Section 7.5 Electrical Characteristics from the datasheet. A screenshot is shown below. 

I only list some advantages of using higher / lower PWM frequency here. The disadvantages are vice versa.

Best Regards,

Steven