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TPS61161: Question of Rset Range Selection of the TPS61161

Wayne Chen (ex FAE)
Guru 15950 points
Part Number: TPS61161

Hi Sirs,

My customer is asking a question which needs product line's suggestion:

According to the equation in datasheet, VFB = Duty * 200mV, 
VFB is linear with four LEDs in series, LED current = 20mA with Rset = 20ohm or 10ohm.

However, VFB becomes non-leaner when Rst was reduced to 6.8ohm or 4.7ohm:

DUTY=100%, VFB=200 mV
DUTY=50%, VFB=160 mV
DUTY=20%, VFB=80 mV

Hence we would like to know the range of Rset to fit the equation of VFB = Duty * 200mV.




Thank you and Best regards,

Wayne Chen
05/06/2020

  • 0
    TI__Mastermind 22745 points

    Hi Wayne,

        Please check whether the circuit is stable or not when you have decreased the RFB since it will deliver a higher output current when you decrease the RFB.

    BR

    Sean

  • 0 in reply to Sean Zhou
    Guru 15950 points

    Hello Sean,

    Thank you for your advise. Should we measure the output voltage of the boost converter to check the stability of the circuit? 
    Do we have the specification of the minimum Rset range in terms of stability?



    Thank you and Best regards,

    Wayne Chen
    05/07/2020

  • 0 in reply to Wayne Chen (ex FAE)
    TI__Mastermind 22745 points

    Hi Wayne,

         You'd better measure the voltage waveform of the "FB" pin and the current through the inductor to check whether the loop is stable or not.

         There is no special spec for the RFB range and stability. 

    BR

    Sean

        

  • 0 in reply to Sean Zhou
    Guru 15950 points

    Hello Sean,

    Thank you for your advise. With customer's 5V power supply, four WLEDs in series, the output voltage of the boost converter is 3.4V*4+0.2V=13.8V.

    With 10ohm RSET, LED current = 0.2/10 = 20mA, output power is 13.8*20mA=0.276W. Consider the efficiency of the TPS61161 device, the inductor current >= 0.276W/5.0V = 0.0552A.
    With 6.2ohm RSET, LED current = 0.2/6.2 = 30mA,  output power is 13.8*30mA=0.414W. Consider the efficiency of the TPS61161 device, the inductor current >= 0.414W/5.0V = 0.0828A which can exceed 0.7A current limit specification of the TPS61161, hence 6.8ohm RSET is within specification in this case.



    Would you pls advise if my assumption correct?

    Thank you and Best regards,

    Wayne Chen
    05/07/2020





     

  • 0 in reply to Wayne Chen (ex FAE)
    TI__Mastermind 22745 points

    Hi Wayne,

        Yes, but the current you calculate for the inductor is the average current, you also need to consider the current ripple of the inductor. Ipeak_inductor=Iavg+Iripple/2.

    BR

    Sean

  • 0 in reply to Sean Zhou
    Guru 15950 points

    Hello Sean.

    Thank you for your confirmation, My customer got 20mA LED current with 10ohm and 6.8ohm of RSET, We assume that we should get higher output current with 6.8ohm RSET. We found the VFB was reducing with RSET, hence we would like to know the minimum requirement of the RSET.

    Would you pls advise if there is any missing or mistake with the configurations? The VF is 2.7V, we got 11.15V stable Vout in both conditions.

    Wayne Chen
    05/08/2020

  • 0 in reply to Wayne Chen (ex FAE)
    TI__Mastermind 22745 points

    HI Wayne,

         It is strange that the VFB will decrease, can you measure the waveform of the "SW" or inductor current? You need to check whether the circuit is stable or not.

    BR

    Sean