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Original question:

[FAQ] List of FAQs for TPS922055 / TPS922054 / TPS922053 / TPS922052 LED driver

[FAQ] How to get a quick transition of TPS922055 / TPS922054 / TPS922053 / TPS922052 from shutdown into flexible dimming mode?

Steven Li
Genius 14815 points
Other Parts Discussed in Thread: TPS922052, TPS922055, TPS922054, TPS922053

I want to strobe the LED current and output nearly square-wave-shape current output. I do not want the soft start to influence my output current. How can I get a quick transition of TPS922055 / TPS922054 / TPS922053 / TPS922052 from shutdown into flexible dimming mode? 

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    TI__Genius 14815 points

    The TPS922055 / TPS922054 / TPS922053 / TPS922052 implements soft start feature to avoid inrush current during startup. In flexible dimming mode, if you do not want the soft start to influence your inductor current (which is then filtered to become the output current), you should input your EN/PWM signal after the soft start ends. There is method to accelerate the process of soft start, which is mentioned below.

    The device will undergo two steps during the transition from shutdown to flexible dimming mode.

    Firstly, you need to follow the startup sequence shown in Section 8.3.4 and Figure 8-2 from the datasheet so that the device will be ready for dimming mode selection ("Dimming Mode Start" = high). This step cannot be accelerated.

    Secondly, normally you need time for the device to identify and select the flexible dimming mode with ADIM/HD pin = high for at least 1.5us. Then the device will undergo a soft-start time to let the reference voltage V_REF rise to the target value. This step can be accelerated by following the below method:

    • After the device is ready for dimming mode selection, you need to input 1.5% Duty, 10kHz PWM for at least 4 periods to the ADIM/HD pin. Then the soft start will end and the device will be ready to get your ADIM/HD and EN/PWM input. Please refer to the below figure for an illustration.