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Original question:

TPS92640 ROFF formula

TPS92641EVM: IADJ to IOUT ratio not constant

Martin Sulzinger1
Prodigy 10 points
Part Number: TPS92641EVM
Other Parts Discussed in Thread: TPS92640

Dear TI-support,

I'm in the process of evaluating the TPS92640 for an application.

In the EVM I've changed Rsense to 333 mOhm and R6 to 47 K to make V_IADJ 2.5 V for an nominal output current of 750 mA.

In my testing I noticed that the ratio between the voltage across the sense resistor and V_IADJ is not 10 and not constant over the analog dimming range.

Here are some measured voltages, measured directly on the IC Pins:
V_CS = 251 mV
V_IADJ = 2.428 V
Ratio: 9.64

V_CS = 38.75 mV
V_IADJ = 330.5 mV
Ratio: 8.53

Best regards,

Martin

  • 0
    TI__Genius 15776 points

    Robin, please help in this case.

  • 0
    TI__Genius 13001 points

    Hi Martin,

    For low IADJ, the TPS92640 dims well but may have more variation between circuits. So I suggest you use PWM diming to achieve small output current.

    Besides, for analog dimming, it's most accurate when device is in CCM mode. BTW, would you mind introducing your application simply? Thanks!

    Robin

  • 0 in reply to Yuting Shao
    Prodigy 10 points

    Hi Yuting Shao.

    PWM is not wanted because of SVM requirements, we want to achieve low analog dimming. This is why this device was recommended to me.

    Regardless of CCM or DCM, the average inductor current across Rsns should be proportional to the IADJ setpoint.

    Our application is residential lighting. We want to dim down to about 50 ... 100 mA current.
    Technical Data:
    Vin 48...50 V
    I,out 750 - 100...50 mA
    VLED appr. 36 V at 750 mA, 32 V at 100 mA

  • 0 in reply to Martin Sulzinger1
    TI__Genius 13001 points

    Hi Martin,

    The inductor current ripple is related with the inductor value, switching frequency, the voltage difference between input and output, the on-time setting and so on. Pls keep the half of inductor current ripple is lower than average current to maintain CCM mode.

    http://proddmsvm.itg.ti.com/viewdoc/slva419b.pdf

    I think you can refer this AN and try with the third method with TPS92640 to achieve deeper analog dimming.

    Robin

  • 0 in reply to Yuting Shao
    Prodigy 10 points



    Yuting Shao said:
    The inductor current ripple is related with the inductor value, switching frequency, the voltage difference between input and output, the on-time setting and so on. Pls keep the half of inductor current ripple is lower than average current to maintain CCM mod


    Of course you are right, that CCM is dependent on those factors. However the set point for the LED current is given by VADJ / 10 / Rsns. But in my application it seems that the 10 factor is not constant and  not 10.

    Thank you for the application note. This shows interesting ways to analog dim the LED current.
    What are the advantages over using IADJ of the TPS do influence the current?

    Martin

  • +1 in reply to Martin Sulzinger1
    TI__Genius 13001 points

    Hi Martin,

    The method introduced in the AN can guarantee the linearity even in small current.

    Robin