TPS61165: Analog dimming schematic

Hi Shuji

     For TPS61165, there are two ways to realize analog dimming. One is directly apply PWM signal on Ctrl pin, it is not needed to add a zener for this method. The other is what shown in the pic. above by external filter. The reason is that the LED open protection cannot be triggered in this configuration as the open protection metric is to detect if FB voltage is less than half of the reference voltage, typically 0.1V. The external PWM signal will always add a voltage level higher than 0.1V on FB pin no matter the operation mode. 

Hi Mason,

Thank you for your reply.

Overvoltage protection also works when the LED is open.
In this case, I think that open protection is possible by overvoltage protection even without a Zener diode.
Is this understanding correct?

Best Regards,
Ishiwata

hi Shuji

    do you use the method shown in the pic.?  What is the voltage level and duty cycle for the external PWM? 

   Because in the analysis, LED open protection cannot be realized in this configuration. 

Hi Mason,

Thank you for your reply.

I confirmed the PWM and voltage level from the customer.
We will contact you. It works fine.

- Voltage level of input PWM signal
   High: 3.3V Low: GND (0V)

- PWM frequency :100Hz

- PWM on Duty ratio
  Control from 100% to 0% in 10% increments

Best Regards,
Ishiwata

hi Shuji

    thanks for the feedback. Could you further share the values of R1, R2 and Rset. Just want to confirm the reason that LED open protection works in this configuration. Anyway, the only concern to add this zener diode is open protection. If the open protection can be ensured, that would be fine without zener diode. 

Hi Mason,

Thank you for your reply.
We have received an excel design tool for peripheral parts, so please check it.

Design Tool for Analog Dimming using a PWM Signal.xls

Best Regards,
Ishiwata

hi Shuji

   Thanks for the information. This is the design tool that you can use for better design on analog dimming by external filter. It would be fine that you design based on this.

Hi Mason,

Is it okay to understand that the protection function works without the Zener diode in the case of the excel file circuit I sent earlier?

Best Regards,
Ishiwata

hi Shuji

    I think there is no difference between the circuit in the excel file and the one shown in this thread, both of them use external filter to realize analog dimming. It is recommended to add zener diode for open protection concern. You mentioned the protection can work well under this configuration, I think it would be better to make it clear why it still works by checking (1) if the SW voltage exceeds the VOVP threshold, and (2) if the FB voltage is less than half of regulation voltage.

  If you have concern on zener diode, it is also available to realize analog dimming by applying PWM signal on Ctrl Pin.