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[FAQ] How will TPS923655 / TPS923654 / TPS923653 / TPS923652 transform from shutdown into PWM dimming mode?

Other Parts Discussed in Thread: TPS923652, TPS923653, TPS923654, TPS923655

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, so I need to know when the soft starts ends and then I will input my EN/PWM signal. Is there any illustration about the transition of TPS923655 / TPS923654 / TPS923653 / TPS923652 from shutdown into PWM dimming mode?

  • The TPS923655 / TPS923654 / TPS923653 / TPS923652 implements soft start feature to avoid inrush current during startup. In PWM dimming mode, if you do not want the soft start to influence your inductor current (which is then periodically filtered to become the output current), you should input your EN/PWM signal after the soft start ends.

    The device will undergo two steps during the transition from shutdown to PWM 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).

    Secondly, the device will undergo a soft-start time (around 6ms) to let the reference voltage V_REF rise to the target value. During this soft start period, it is ok to input a pulse to the EN/PWM pin, however the inductor current will not be square-wave-shape as you expected as the reference voltage V_REF is still ramping up.

    When the soft start ends, you can input the EN/PWM signal and the inductor current waveform will be nearly square-wave-shape. You can refer to the below figure for an illustration.