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TPS61169: About ILED setting at Low PWM Duty

Hide
Guru 21045 points
Part Number: TPS61169

Hi Team,

 

We are evaluating TPS61169EVM at the following conditions.

ILED of measurement result does not accord with the calculated value at low PWM duty.

Obviously, we doubt our measurement environment but please let me ask one more question.

 

I got advice from you about VFB (Please refer to the following E2E).

https://e2e.ti.com/support/power_management/led_driver/f/192/p/554429/2030636#2030636

 

Therefore, I understand that I can control ILED exactly at PWM duty 1% to 100%.

(I understand that ILED accuracy depend on VFB and RSET so it doesn’t depend on other parameters)

 Is my understanding correct?

If no, could you please let us know if you have any concern?

 

According to our customer, they would like to control ILED from around 1mA to 30mA using PWM signal.

So, this is an extremely important point.

 

------------------

[Calculation]

ILED = (DUTY * VFB) / RSET = (0.01 * 204mV) / 6.75ohm ≒ 0.3mA

ILED = (DUTY * VFB) / RSET = (0.02 * 204mV) / 6.75ohm ≒ 0.6mA

ILED = (DUTY * VFB) / RSET = (0.03 * 204mV) / 6.75ohm ≒ 0.9mA

-----

 

[Measurement result]

ILED = 1.818mA at DUTY=0.01 , RSET=6.75ohm

ILED = 2.091mA at DUTY=0.02 , RSET=6.75ohm

ILED = 2.455mA at DUTY=0.03 , RSET=6.75ohm

-----

 

[EVM conditions]

-VL=8.4V

-VIN=3.15V

-ILED = around 1.818mA to 30mA (Measurement result)

-PWM 3.3V, 50kHz, 1% to 97%

-LED 7strings

-RSET= around 6.75ohm


 

Regards,

Kanemaru

  • 0
    TI__Intellectual 2090 points
    Hello Kanemaru-san

    The TPS61169 does not specify absolute accuracy.
    There is a minimum EA output of ~1.2 mV which equates to 1.2 mA output current minimum.
    The minimum EA output results in higher absolute error for low duty cycle but the linearity is maintained down to 1% DC.
    Let me know if you need anything else.

    Best regards,
    Randy
  • 0 in reply to Randy Dahl
    Guru 21045 points

    Hi Randy-san,

     

    Thank you for the information.

    I have one more question.

    According to your comment, “There is a minimum EA output of ~1.2 mV which equates to 1.2 mA output current minimum”.

    However, I understand that ILED(minimum)=around 0.48mA when EA output of ~1.2 mV , RSET=6.75ohm , Duty=1%.

    Is my understanding correct?

     

    [Calculation]

    ILED = ((DUTY * VFB)+1.2mV) / RSET = ((0.01 * 204mV) +1.2mV)/ 6.75ohm ≒ 0.48mA

     

    Regards,

    Kanemaru

  • 0 in reply to Hide
    TI__Intellectual 2090 points
    Hello Kanemaru-san

    Yes, your calculation is correct.

    Best regards,
    Randy
  • 0 in reply to Randy Dahl
    Guru 21045 points

    Hi Randy-san,

     

    Thank you for your prompt reply.

    So, I understand that TPS61169 can control ILED from around 1mA to 30mA using PWM signal.

    Is my understanding correct?

     

    And, I have one more question.

    Is “EA output of ~1.2 mV” spec at TA=-40℃ to 85℃?

     

    Regards,

    Kanemaru