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LP8861-Q1: System efficiency measurement configuration

Ryuji
Guru 20090 points
Part Number: LP8861-Q1

Hello,

Could you please let me know the system efficiency measurement configuration? ( Input and Output voltage/current measurement point)

Is the output voltage measured between lowest OUTx pin and output voltage(anode of the LED)?
Is the boost efficiency output voltage measured between GND and the anode of the LED?

Best Regards,
Ryuji Asaka

  • 0
    TI__Intellectual 1235 points
    Hello Mr. Asaka,

    The system efficiency could be computed by using the following equation [OUT1 x (Vboost - Vout1) + OUT2 x (Vboost - Vout2) + OUT3 x (Vboost - Vout3)] / [Vin x Ivin] / 10;
    Where,
    (i) Vout1, Vout2, and Vout3 are the voltages as measured on pin # 15, 14, and 13 respectively (not DC average).
    (ii) OUT1, OUT2, and OUT3 are the currents as measured on each one of the LED strings
    (iii) Vin and Ivin are the input voltage and current.
    (iv) Vboost is the boost voltage measured right after output capacitance Cout

    In the above equation voltages are given in V, Ivin in A, and OUTx in mA

    The preferred method to measure current would be to measure voltage across a known low resistance (sense resistor).

    Kind Regards,

    David
  • 0 in reply to David Diaz Quintana1
    Guru 20090 points

    Hello David san,

    Thank you. 
    Why are you measuring in 3 strings rather than 4 strings?

    Best Regards,
    Ryuji Asaka

  • 0 in reply to Ryuji
    TI__Intellectual 1235 points
    Hello Asaka-san,

    The above explanation was given for the example of application using only three strings.

    The same above explanation would work for any given number of strings selected by just adjusting the number of OUTX x (Vboost - VoutX) terms.

    In the case of using four strings the above expression would just simply be expanded to include the 4th string as:

    [OUT1 x (Vboost - Vout1) + OUT2 x (Vboost - Vout2) + OUT3 x (Vboost - Vout3) + OUT4 x (Vboost - Vout4)] / [Vin x Ivin] / 10;

    Kind Regards,

    David